Author name code: kosovichev
ADS astronomy entries on 2022-09-14
author:"Kosovichev, Alexander G."
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Title: Implicit large eddy simulations of global solar convection:
effects of numerical resolution in non-rotating and rotating cases
Authors: Guerrero, G.; Stejko, A. M.; Kosovichev, A. G.; Smolarkiewicz,
P. K; Strugarek, A.
Bibcode: 2022arXiv220805738G
Altcode:
Simulating deep solar convection and its coupled mean-field motions is
a formidable challenge where few observational results constrain models
that suffer from the non-physical influence of the grid resolution. We
present hydrodynamic global Implicit Large-Eddy simulations (ILES) of
deep solar convection performed with the EULAG-MHD code, and explore
the effects of grid resolution on the properties of rotating and
non-rotating convection. The results, based on low-order moments
and turbulent spectra reveal that convergence could be achieved
in non-rotating simulations provided sufficient resolution in the
radial direction. The flow is highly anisotropic, with the energy
contained in horizontal divergent motions exceeding by more than
three orders of magnitude their radial counterpart. By contrast, in
rotating simulations the largest energy is in the toroidal part of the
horizontal motions. As the grid resolution increases, the turbulent
correlations change in such a way that a solar-like differential
rotation, obtained in the simulation with the coarser grid, transitions
to the anti-solar differential rotation. The reason for this change
is the contribution of the effective viscosity to the balance of the
forces driving large-scale flows. As the effective viscosity decreases,
the angular momentum balance improves, yet the force balance in the
meridional direction lessens, favoring a strong meridional flow that
advects angular momentum towards the poles. The results suggest that
obtaining the correct distribution of angular momentum may not be a
mere issue of numerical resolution. Accounting for additional physics,
such as magnetism or the near-surface shear layer, may be necessary
in simulating the solar interior.
Title: Spatial Scales and Time Variation of Solar Subsurface
Convection
Authors: Getling, Alexander V.; Kosovichev, Alexander G.
Bibcode: 2022arXiv220804642G
Altcode:
Spectral analysis of the spatial structure of solar subphotospheric
convection is carried out for subsurface flow maps constructed
using the time--distance helioseismological technique. The source
data are obtained from the Helioseismic and Magnetic Imager
(HMI) onboard Solar Dynamics Observatory (SDO) from 2010 May to
2020 September. A spherical-harmonic transform is applied to the
horizontal-velocity-divergence field at depths from 0 to 19~Mm. The
range of flow scales is fairly broad in shallow layers and narrows
as the depth increases. The horizontal flow scales rapidly increase
with depth, from supergranulation to giant-cell values, and indicate
the existence of large-scale convective motions in the near-surface
shear layer. The results can naturally be interpreted in terms of
a superposition of differently scaled flows localized in different
depth intervals. There is some tendency toward the emergence of
meridionally elongated (banana-shaped) convection structures in the
deep layers. The total power of convective flows is anticorrelated
with the sunspot-number variation over the solar activity cycle in
shallow subsurface layers and positively correlated at larger depths,
which is suggestive of the depth redistribution of the convective-flow
energy due to the action of magnetic fields.
Title: Constraining Global Solar Models through Helioseismic Analysis
Authors: Stejko, Andrey M.; Kosovichev, Alexander G.; Featherstone,
Nicholas A.; Guerrero, Gustavo; Hindman, Bradley W.; Matilsky, Loren
I.; Warnecke, Jörn
Bibcode: 2022ApJ...934..161S
Altcode: 2022arXiv220405207S
Global hydrodynamic simulations of internal solar dynamics have focused
on replicating the conditions for solar-like (equator rotating faster
than the poles) differential rotation and meridional circulation using
the results of helioseismic inversions as a constraint. Inferences
of meridional circulation, however, have provided controversial
results showing the possibility of one, two, or multiple cells along
the radius. To help address this controversy and develop a more
robust understanding of global flow regimes in the solar interior,
we apply a "forward-modeling" approach to the analysis of helioseismic
signatures of meridional circulation profiles obtained from numerical
simulations. We employ the global acoustic modeling code GALE to
simulate the propagation of acoustic waves through regimes of mean
mass-flows generated by global hydrodynamic and magnetohydrodynamic
models: EULAG, the Pencil code, and the Rayleigh code. These models are
used to create synthetic Dopplergram data products, used as inputs for
local time-distance helioseismology techniques. Helioseismic travel-time
signals from solutions obtained through global numerical simulations
are compared directly with inferences from solar observations, in
order to set additional constraints on global model parameters in a
direct way. We show that even though these models are able to replicate
solar-like differential rotation, the resulting rotationally constrained
convection develops a multicell global meridional circulation profile
that is measurably inconsistent with local time-distance inferences
of solar observations. However, we find that the development of
rotationally unconstrained convection close to the model surface is able
to maintain solar-like differential rotation, while having a significant
impact on the helioseismic travel-time signal, replicating solar
observations within one standard deviation of the error due to noise.
Title: Physics-based Modeling of Multiscale Solar Dynamics for
Understanding Origins of Space Weather Disturbances
Authors: Kitiashvili, Irina; Sadykov, Viacheslav; Wray, Alan;
Kosovichev, Alexander
Bibcode: 2022cosp...44.3232K
Altcode:
Forecasting space weather on different temporal scales is a problem that
requires the development of advanced physics-based models, algorithms,
and data analysis approaches for a variety of observations and their
inferences. We take advantage of currently available computational
capabilities to model solar dynamics from the deep interior to
the corona and investigate mechanisms that may drive space weather
conditions. Comparison of the synthetic observables obtained from
numerical simulations and actual observations allows us to uncover
physical processes associated with observed phenomena. To facilitate a
transition from modeling short-term physical phenomena to developing a
reliable forecast-oriented model, we suggest using the data assimilation
approach. It allows us to cross-analyze dynamo model solutions and
observations and to consider possible uncertainties and errors. Our
results demonstrate promising potential for modeling upcoming solar
activity combined with observations. In this presentation, we briefly
summarize current multi-scale modeling capabilities and results
and discuss ongoing developments to build a reliable physics-based
forecast-oriented model of solar activity.
Title: Helioseismic Monitoring of Solar Subsurface Dynamics and
Activity
Authors: Kosovichev, Alexander; Pipin, Valery; Getling, Alexander;
Stejko, Andrey; Stefan, John; Guerrero, Gustavo
Bibcode: 2022cosp...44.3215K
Altcode:
Uninterrupted helioseismic data from Michelson Doppler Imager (MDI)
onboard Solar and Heliospheric Observatory (SoHO) and from Helioseismic
and Magnetic Imager (HMI) onboard Solar Dynamics Observatory (SDO)
have provided unique information about flows and structures evolving
on various temporal and spatial scales inside the Sun. The data
cover the past two solar cycles and the rising phase of the current
solar cycle. In particular, our analysis of variations of the internal
differential rotation reveals "extended" cyclic variations of migrating
zonal flows ("torsional oscillations") through the whole convection
zone. The observed patterns of subsurface flow acceleration provide
evidence of hydromagnetic dynamo waves, which control the strength
of sunspot cycles, and potentially carry information about the future
solar cycles. Similarly, "extended" cyclic variations of the subsurface
meridional circulation, detected by local helioseismic techniques,
reflect the evolution of subsurface magnetic fields and emerging
magnetic flux. Furthermore, to monitor emerging active regions, we
develop a "deep-focus" helioseismic diagnostics, which allows us to
detect large emerging active regions before they become visible on
the surface. We present recent advances in helioseismic monitoring
of solar activity and discuss helioseismic constraints on models of
global solar variability and space weather forecasting.
Title: Characterization of Stellar Jitter Using 3D Realistic Modeling
of Solar-Type Stars
Authors: Kitiashvili, Irina; Granovsky, Samuel; Wray, Alan; Kosovichev,
Alexander
Bibcode: 2022AAS...24041702K
Altcode:
Recent progress in the ab-initio modeling of solar magnetoconvection
makes it possible to simulate the surface dynamics of solar-type
stars with a high degree of realism. These simulations can be used
to characterize stellar photospheric disturbances which contaminate
the radial velocity signal and limit our capabilities to detect
Earth-mass exoplanets. We use the 3D radiative MHD "StellarBox" code
to obtain realistic stellar (magneto)convection models and characterize
photospheric noise. The initial conditions for the numerical simulations
are generated using the MESA stellar evolution code. The computational
domain covers the upper layers of the convection zone and the lower
atmosphere. We present numerical models of solar-type planet-host stars,
disk-integrated synthetic observations, and observables such as line
profiles, Doppler shift, etc. Synthesis of observations performed for
a list of FeI lines using the radiative transfer "Spinor" code. We
present the stellar jitter modeling results for selected target stars,
discuss the origin of the noise and compare with high-resolution
HARPS observations.
Title: Simulating Solar Near-surface Rossby Waves by Inverse Cascade
from Supergranule Energy
Authors: Dikpati, Mausumi; Gilman, Peter A.; Guerrero, Gustavo
A.; Kosovichev, Alexander G.; McIntosh, Scott W.; Sreenivasan,
Katepalli. R.; Warnecke, Jörn; Zaqarashvili, Teimuraz V.
Bibcode: 2022ApJ...931..117D
Altcode:
Rossby waves are found at several levels in the Sun, most recently in
its supergranule layer. We show that Rossby waves in the supergranule
layer can be excited by an inverse cascade of kinetic energy from the
nearly horizontal motions in supergranules. We illustrate how this
excitation occurs using a hydrodynamic shallow-water model for a 3D
thin rotating spherical shell. We find that initial kinetic energy
at small spatial scales inverse cascades quickly to global scales,
exciting Rossby waves whose phase velocities are similar to linear
Rossby waves on the sphere originally derived by Haurwitz. Modest
departures from the Haurwitz formula originate from nonlinear finite
amplitude effects and/or the presence of differential rotation. Like
supergranules, the initial small-scale motions in our model contain
very little vorticity compared to their horizontal divergence, but the
resulting Rossby waves are almost all vortical motions. Supergranule
kinetic energy could have mainly gone into gravity waves, but we find
that most energy inverse cascades to global Rossby waves. Since kinetic
energy in supergranules is three or four orders of magnitude larger
than that of the observed Rossby waves in the supergranule layer,
there is plenty of energy available to drive the inverse-cascade
mechanism. Tachocline Rossby waves have previously been shown to
play crucial roles in causing seasons of space weather through their
nonlinear interactions with global flows and magnetic fields. We briefly
discuss how various Rossby waves in the tachocline, convection zone,
supergranule layer, and corona can be reconciled in a unified framework.
Title: Detection of Phase Travel Time Anisotropy from Subsurface
Horizontal Magnetic Fields in the Sun
Authors: Stefan, John; Kosovichev, Alexander
Bibcode: 2022AAS...24035014S
Altcode:
We derive a helioseismic measurement technique which isolates phase
travel time deviations caused by horizontal magnetic fields from
other contributions such as sound speed perturbations and flows. The
technique also allows for the azimuth of the subsurface magnetic field
to be determined in addition to a direct proxy for the horizontal
field's magnitude. We validate this technique using two independent
MHD simulations. The first model simulates propagation of acoustic
waves from a point source in a uniform, inclined magnetic field, and
we recover the correct azimuth and the expected travel time anisotropy
from ray theory. The second model simulates a realistic sunspot with
non-uniform magnetic field, where we find the anisotropy and azimuth
of the subsurface magnetic field. Finally, we apply the technique to
several active regions observed by HMI (Helioseismic and Magnetic
Imager) and examine characteristics of the subsurface horizontal
magnetic field at depths z=-5 to -7 Mm, -7 to -10 Mm, and -10 to -13 Mm.
Title: Detection of Travel Time Anisotropy from Subsurface Horizontal
Magnetic Fields
Authors: Stefan, John T.; Kosovichev, Alexander G.
Bibcode: 2022ApJ...930...10S
Altcode: 2022arXiv220303495S
A time-distance measurement technique is derived to isolate phase
travel time anisotropy caused by subsurface horizontal magnetic
fields; a method that uses the measured anisotropy to estimate the
field's orientation is also derived. A simulation of acoustic waves
propagating in a uniform, inclined magnetic field with solar background
structure is used to verify the derived technique. Then the procedure is
applied to a numerical simulation of a sunspot for which the subsurface
state is known to provide context for the results obtained from the
study of several sunspots observed by the Helioseismic and Magnetic
Imager. Significant anisotropies are detected, on the order of 1 minute,
and the subsurface field's azimuth is estimated and compared with the
azimuth of the surface magnetic field. In all cases, the subsurface
azimuth is found to be well aligned with that of the surface, and the
results from the numerical simulation are used to interpret features
in the detected travel time anisotropy.
Title: Numerical Convergence of 2D Solar Convection in Implicit
Large-eddy Simulations
Authors: Nogueira, H. D.; Guerrero, G.; Smolarkiewicz, P. K.;
Kosovichev, A. G.
Bibcode: 2022ApJ...928..148N
Altcode: 2022arXiv220202767N
Large-eddy simulations (LES) and implicit LES (ILES) are wise
and affordable alternatives to the unfeasible direct numerical
simulations of turbulent flows at high Reynolds (Re) numbers. However,
for systems with few observational constraints, it is a formidable
challenge to determine if these strategies adequately capture the
physics of the system. Here, we address this problem by analyzing
numerical convergence of ILES of turbulent convection in 2D, with
resolutions between 642 and 20482 grid points,
along with the estimation of their effective viscosities, resulting
in effective Reynolds numbers between 1 and ~104. The
thermodynamic structure of our model resembles the solar interior,
including a fraction of the radiative zone and the convection zone. In
the convective layer, the ILES solutions converge for the simulations
with ≥5122 grid points, as evidenced by the integral
properties of the flow and its power spectra. Most importantly,
we found that even a resolution of 1282 grid points,
$\mathrm{Re}\,\sim \,10$ , is sufficient to capture the dynamics
of the large scales accurately. This is a consequence of the ILES
method allowing the energy contained in these scales to be the same in
simulations with low and high resolution. Special attention is needed
in regions with a small density scale height driving the formation of
fine structures unresolved by the numerical grid. In the stable layer,
we found the excitation of internal gravity waves, yet high resolution
is needed to capture their development and interaction.
Title: Advances and Challenges in Observations and Modeling of the
Global-Sun Dynamics and Dynamo
Authors: Kosovichev, A. G.; Guerrero, G.; Stejko, A. M.; Pipin, V. V.;
Getling, A. V.
Bibcode: 2022arXiv220310721K
Altcode:
Computational heliophysics has shed light on the fundamental physical
processes inside the Sun, such as the differential rotation, meridional
circulation, and dynamo-generation of magnetic fields. However,
despite the substantial advances, the current results of 3D MHD
simulations are still far from reproducing helioseismic inferences
and surface observations. The reason is the multi-scale nature of
the solar dynamics, covering a vast range of scales, which cannot be
solved with the current computational resources. In such a situation,
significant progress has been achieved by the mean-field approach,
based on the separation of small-scale turbulence and large-scale
dynamics. The mean-field simulations can reproduce solar observations,
qualitatively and quantitatively, and uncover new phenomena. However,
they do not reveal the complex physics of large-scale convection,
solar magnetic cycles, and the magnetic self-organization that causes
sunspots and solar eruptions. Thus, developing a synergy of these
approaches seems to be a necessary but very challenging task.
Title: Leptocline as a Shallow Substructure of Near-Surface Shear
Layer in 3D Radiative Hydrodynamic Simulations
Authors: Kitiashvili, Irina N.; Kosovichev, Alexander G.; Wray,
Alan A.; Sadykov, Viacheslav M.; Guerrero, Gustavo
Bibcode: 2022arXiv220301484K
Altcode:
Understanding effects driven by rotation in the solar convection zone
is essential for many problems related to solar activity, such as
the formation of differential rotation, meridional circulation, and
others. We analyze realistic 3D radiative hydrodynamics simulations
of solar subsurface dynamics in the presence of rotation in a local
domain 80 Mm wide and 25 Mm deep, located at 30 degrees latitude. The
simulation results reveal the development of a shallow 10-Mm deep
substructure of the Near-Surface Shear Layer (NSSL), characterized
by a strong radial rotational gradient and self-organized meridional
flows. This shallow layer ("leptocline") is located in the hydrogen
ionization zone associated with enhanced anisotropic overshooting-type
flows into a less unstable layer between the H and HeII ionization
zones. We discuss current observational evidence of the presence of
the leptocline and show that the radial variations of the differential
rotation and meridional flow profiles obtained from the simulations
in this layer qualitatively agree with helioseismic observations.
Title: Revisiting the Solar Research Cyberinfrastructure Needs:
A White Paper of Findings and Recommendations
Authors: Nita, Gelu; Ahmadzadeh, Azim; Criscuoli, Serena;
Davey, Alisdair; Gary, Dale; Georgoulis, Manolis; Hurlburt, Neal;
Kitiashvili, Irina; Kempton, Dustin; Kosovichev, Alexander; Martens,
Piet; McGranaghan, Ryan; Oria, Vincent; Reardon, Kevin; Sadykov,
Viacheslav; Timmons, Ryan; Wang, Haimin; Wang, Jason T. L.
Bibcode: 2022arXiv220309544N
Altcode:
Solar and Heliosphere physics are areas of remarkable data-driven
discoveries. Recent advances in high-cadence, high-resolution
multiwavelength observations, growing amounts of data from realistic
modeling, and operational needs for uninterrupted science-quality data
coverage generate the demand for a solar metadata standardization and
overall healthy data infrastructure. This white paper is prepared as
an effort of the working group "Uniform Semantics and Syntax of Solar
Observations and Events" created within the "Towards Integration of
Heliophysics Data, Modeling, and Analysis Tools" EarthCube Research
Coordination Network (@HDMIEC RCN), with primary objectives to discuss
current advances and identify future needs for the solar research
cyberinfrastructure. The white paper summarizes presentations and
discussions held during the special working group session at the
EarthCube Annual Meeting on June 19th, 2020, as well as community
contribution gathered during a series of preceding workshops and
subsequent RCN working group sessions. The authors provide examples
of the current standing of the solar research cyberinfrastructure, and
describe the problems related to current data handling approaches. The
list of the top-level recommendations agreed by the authors of the
current white paper is presented at the beginning of the paper.
Title: Spatial Spectrum of Solar Convection from Helioseismic Data:
Flow Scales and Time Variations
Authors: Getling, Alexander V.; Kosovichev, Alexander G.
Bibcode: 2022arXiv220100638G
Altcode:
We analyze spectral properties of solar convection in the range of
depths from 0 to 19~Mm using subsurface flow maps obtained by the
time-distance heiioseismology analysis of solar-oscillation data from
the Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics
Observatory (SDO) from May 2010 to September 2020. The results reveal
a rapid increase of the horizontal flow scales with the depth, from
supergranulation to giant-cell scales, and support the evidence of
large-scale convection, previously detected by tracking the motion of
supergranular cells on the surface. The total power of convective flows
correlates with the solar activity cycle. During the solar maximum,
the total power decreases in shallow subsurface layers and increases
in the deeper layers.
Title: Large-Scale Dynamics of Solar Subsurface Shear Layer:
Theoretical Predictions and Helioseismic Inferences
Authors: Kosovichev, Alexander; Getling, Alexander; Guerrero, Gustavo;
Pipin, Valery; Stejko, Andrey
Bibcode: 2021AGUFMSH53C..03K
Altcode:
Helioseismic observations show that the solar rotation rate sharply
increases with depth in the outer 30 Mm-deep layers of the solar
convection zone. This subsurface shear layer (SSL) plays a critical role
in the formation of migrating activity belts (the butterfly diagram)
and the emergence of active regions. Measurements of the rotation
rate indicate that the magnetic field of sunspots and active regions
is anchored in the subsurface shear layer. The magnetic structure of
the SSL has not been directly measured by helioseismology, but it can
be inferred by comparing variations of subsurface flows and sound-speed
variations predicted by dynamo models with helioseismic measurements. To
establish the connections between the flows and fields, we construct
and analyze synoptic maps of subsurface flows in the SSL, obtained
from time-distance helioseismic inversion during the whole Solar Cycle
24 and the rising phase of Cycle 25. The results reveal the extended
solar-cycle pattern of variations of the meridional circulation,
predicted by the dynamo models and previously known from surface and
subsurface observations of zonal flows. We discuss the origin of the
observed variations and their links to the magnetism and dynamics of
the subsurface shear layer.
Title: Origin of Rossby waves observed near the solar surface
Authors: Gilman, Peter; Dikpati, Mausumi; Guerrero, Gustavo;
Kosovichev, Alexander; McIntosh, Scott; Sreenivasan, Katepalli;
Warnecke, Joern; Zaqarashvili, Teimuraz
Bibcode: 2021AGUFMSH53C..04G
Altcode:
Differential rotation and toroidal magnetic bands in the tachocline
are unstable to MHD Rossby waves and may be responsible for patterns
of solar activity seen in the photosphere. Helioseismic and surface
velocity measurements reveal energetically neutral Rossby waves in the
supergranulation layer. To explore plausible sources of energy for
these Rossby waves, we study nonlinear dynamics of horizontal flows
in the supergranular layer in thepresence of rotation and differential
rotation. With a shallow-water model we show that kinetic energy, put
into smallest resolved spatial scales, very quickly 'reverse cascades'
to largest scales, exciting energetically neutral Rossby-Haurwitz type
waves, as well as energetically active ones with low longitudinal
spectral modes, depending on differential rotation. Horizontal
velocities in supergranules are known to be much larger than their
vertical motions; our shallow-water system includes a similar ratio. If
supergranules are responsible for Rossby waves seen in photosphere, it
paradoxically follows that (i) stable stratification of a thin rotating
spherical shell may be a sufficient but not a necessary condition for
Rossby waves, and (ii) small-scale convection producing global Rossby
waves in a thin differentially rotating fluid may be the first ever
example found in a celestial body.
Title: Physical Properties of the Solar Atmosphere Derived from
Comparison of Spectro-Polarimetric SDO/HMI Observables with 3D
Radiative MHD Simulations
Authors: Sadykov, Viacheslav; Kitiashvili, Irina; Kosovichev,
Alexander; Wray, Alan
Bibcode: 2021AGUFMSH44A..06S
Altcode:
In this study, we compare the SDO/HMI line-of-sight observables
(magnetic field, velocity, continuum intensity, and line depth) with
the related physical properties for dynamo simulations performed using
the StellarBox 3D Radiative MHD code. The modeling of the Fe I 6173 A
Stokes profiles is performed using the SPINOR radiative transfer code in
the LTE approximation. The reproduced SDO/HMI line-of-sight pipeline is
applied to the modeled spectra, and the observables are synthesized with
high (numerical) and SDO/HMI (instrumental) resolutions. Correlations
between the observables and the physical properties at various
heights in the atmosphere are studied for a set of view angles (0,
30, 45, 60, 70, and 80 degrees away from the solar disk center). It
is found that SDO/HMI velocity and magnetic field (less prominently)
observables are correlated with physical parameters at certain heights
of the solar atmosphere. These heights increase from about 100-150 km
above the photosphere for the disk center case to 300-600 km above the
photosphere for the 80-degree case, however, are almost the same for
the 0-60 degree projection angles. The integrated unsigned magnetic
flux calculated from the observables underestimates the actual magnetic
flux at strongest correlation heights for about 40% on average. The
integrated continuum intensity as calculated from the observables
is about 4-8% larger with respect to its actual values. In addition,
we discuss a problem of contribution of unresolved magnetic elements
to solar brightness based on the modeling data. The results improve
physics-based interpretations of the SDO/HMI observables and provide a
better understanding of the physical properties of the solar atmosphere.
Title: 3D Radiative MHD Modeling of the Solar Atmospheric Dynamics
and Structure
Authors: Kitiashvili, Irina; Sadykov, Viacheslav; Wray, Alan;
Kosovichev, Alexander
Bibcode: 2021AGUFMSH45B2370K
Altcode:
Dramatic dynamical phenomena accompanied by strong thermodynamic
and magnetic structuring are the primary drivers of great interest
in studying the solar atmosphere with high spatial and temporal
resolutions. Using current computational capabilities, it became
possible to model the magnetized solar plasma in different regimes
with a high degree of realism. To study the fine structuring of the
solar atmosphere and dynamics, we use 3D MHD radiative models covering
all layers from the upper convection zone to the corona. Realistic 3D
radiative MHD modeling of the solar magnetoconvection and atmosphere
allows us to generate synthetic observables that directly link the
physical properties of the solar plasma to spectroscopic observables. We
calculate series of synthetic spectropolarimetric imaging data that
model observations from different space instruments: HMI and AIA (SDO),
SOT (Hinode), and IRIS, as well as for the upcoming DKIST ground
observations, and investigate how the observational data are linked
to physical processes in the solar atmosphere. In the presentation,
we discuss qualitative and quantitative changes of the atmospheric
structure and dynamics at different layers of the solar atmosphere,
properties of acoustic and surface gravity waves, sources of the local
heating in the chromosphere-corona transition region, formation of
shocks, and high-frequency oscillations in the corona, as well as
manifestation of these phenomena in the modeled observables.
Title: Dynamical Coupling of the Solar Subsurface Shear Layer and
the Atmosphere
Authors: Kitiashvili, Irina; Sadykov, Viacheslav; Kosovichev,
Alexander; Wray, Alan
Bibcode: 2021AGUFMSH53C..01K
Altcode:
The dynamical coupling of subsurface and surface layers of the Sun is
crucial for understanding how phenomena observed in the solar atmosphere
reflect the evolution of subsurface plasma flows in the present
global-scale rotation. In this work, we use long time-series (over
100-hours) of high-resolution 3D radiative hydrodynamic simulations
obtained for an 80-Mm wide and 25-Mm deep computational domain, using
the SolarBox code, to investigate the formation and dynamics of the
Subsurface Shear Layer (SSL) and observational manifestations. The
solar rotation is modeled in the f-plane approximation at 30 degrees
latitude. The simulation results reveal the formation of the SSL, and
meridional circulation. To compare the simulation results with the
SDO/HMI observations, we generate synthetic time series of the Fe I
(6173A) line profile for different locations on the solar disk, using
the SPINOR radiative transfer code. The line-profile data are converted
into the SDO/HMI observables using an HMI pipeline emulator and analyzed
for both the modeled and instrumental resolutions. The analysis results
reproduce the photospheric structure and dynamics as well as various
helioseismic properties such as rotational frequency splitting, ring-
and time-distance diagrams, and the center-to-limb effect. This work
provides a basis for a deeper understanding of the solar subsurface
dynamics and physical interpretation of observational data.
Title: Simulating Exoplanet Host Star -Horologii from the Surface
to the Bottom of the Convection Zone
Authors: Guerrero, Gustavo; Kitiashvili, Irina; Bonanno, Alfio;
Kosovichev, Alexander
Bibcode: 2021AGUFM.U44B..02G
Altcode:
The G0 type, planet-hosting, star -Horologii has been observed for
several years through different techniques. While there is still some
debate about its rotational period (4-8 days), it seems conclusive that
it exhibits a magnetic cycle of ~1.6 years. This short period allowed
for constructing the first butterfly diagram for a star different from
the Sun. The detailed study of this object provides unique opportunities
to understand the dynamo operating in solar-like stars. In this work,
we present realistic 3D radiative hydrodynamics simulations (RHD) of
surface and subsurface convection of this star. The depth reached by
these models connects with anelastic global MHD simulations (AMHD) of
the -Horologiis dynamo. The RHD models provide an understanding of the
structure and signal of surface convection and its spectra, allowing for
direct comparison with high-resolution spectroscopic observations. The
AMHD simulations enlighten the magnetic contribution to this signal. In
addition, we provide predictions of the star differential rotation,
meridional circulation and the alpha-Omega dynamo sustaining its
magnetic field.
Title: Multiscale Organization of Turbulent Convection in Global-Sun
Simulations
Authors: Guerrero, Gustavo; Stejko, Andrey; Kosovichev, Alexander;
Getling, Alexander; Smolarkiewicz, Piotr
Bibcode: 2021AGUFMSH55D1885G
Altcode:
Solar convection is at the core of fundamental phenomena such as
differential rotation and meridional circulation and, ultimately, the
solar dynamo. The governing mechanisms, amplitude, and dominant scales
of convection in the solar interior remain under debate. Furthermore,
the large Reynolds and Rayleigh numbers involved make it implausible
to resolve all relevant scales using direct numerical simulation. In
this work, we study solar convection through global, non-rotating, and
non-magnetic implicit large-eddy simulations (ILES), using the 3D global
hydrodynamic code EULAG. Our simulations exhibit a pattern of multiscale
convection, clearly visible on the domain surface (~0.96 R), generated
by a solar-like density and entropy stratification. Scale-splitting
is evident at various depths throughout the model, with the upper
boundary of convective cells penetrating surface layers, resembling
solar-like convection. A continuous and coherent multiscale structure
of convective cells is observed throughout the convective interior. The
power peak of convective scales continuously shrinks with height to a
maximum spherical harmonic degree of l = 40-50 on the model surface,
compatible with reports of giant cell observations. Simulations are
performed from low to high resolution to explore whether integral
properties of convection, such as the RMS velocity, temperature
profiles, and turbulent spectra, become independent of the mesh
size. Results are compared with current observations of surface and
sub-surface solar convection.
Title: The Observational Uncertainty of Coronal Hole Boundaries in
Automated Detection Schemes
Authors: Reiss, Martin; Muglach, Karin; Moestl, Christian; Arge,
Charles; Bailey, Rachel; Delouille, Veronique; Garton, Tadhg; Hamada,
Amr; Hofmeister, Stefan; Illarionov, Egor; Jarolim, Robert; Kirk,
Michael; Kosovichev, Alexander; Krista, Larisza; Lee, Sangwoo; Lowder,
Chris; MacNeice, Peter; Veronig, Astrid
Bibcode: 2021AGUFMSH15G2083R
Altcode:
Solar coronal holes are the observational manifestation of the solar
magnetic field open to the heliosphere and are of pivotal importance
for understanding the origin and acceleration of the solar wind. Space
missions such as the Solar Dynamics Observatory now allow us to observe
coronal holes in unprecedented detail. Instrumental effects and other
factors, however, pose a challenge to automated detection of coronal
holes in solar imagery. The science community addresses these challenges
with a variety of detection schemes. Until now, scant attention has
been paid to assessing the disagreement between these schemes. Here
we present the first comprehensive comparison of widely-applied
automated detection schemes in solar and space science. By tying
together scientific expertise worldwide, we study a coronal hole
observed by the Atmospheric Imaging Assembly instrument on 2018 May
30. We find that the choice of detection scheme significantly affects
the location of the coronal hole boundary. Depending on the detection
scheme, the physical properties of the coronal hole including the area,
mean intensity, and mean magnetic field strength vary by a factor of
up to 4.5 between the maximum and minimum values. This presentation
discusses the implications of these findings for coronal hole research
from the past decade. We also outline future strategies on how to use
our results to diagnose and improve coronal magnetic field models.
Title: 3D Helioseismic Forward-Modeling and Analysis of Meridional
Circulation
Authors: Stejko, Andrey; Kosovichev, Alexander; Pipin, Valery;
Guerrero, Gustavo; Getling, Alexander; Smolarkiewicz, Piotr
Bibcode: 2021AGUFMSH55D1870S
Altcode:
The 3D Global Acoustic Linearized Euler (GALE) code is used to explore
the variance in helioseismic signatures that result from various
profiles of meridional circulation. The structure of meridional
circulation regulates the redistribution of angular momentum and
magnetic flux that governs the solar cycle. Forward-modeling is a useful
tool in exploring the structure of meridional circulation and its impact
on global parameters and can help resolve the current controversy
between single and double-cell circulation profiles. Profiles of
meridional circulation are generated using mean-field dynamo models,
which induce a reverse flow near the base of the convection zone,
characteristic of double-cell meridional circulation, with the inclusion
of turbulent pumping (-effect) resulting from a strong rotational
gradient in the region. These models provide physics-based mechanisms
for the low-end in potential differences between single- and double-cell
meridional circulation profiles. The resulting flows are used as
background velocities in the linearized acoustic GALE codesimulating the
stochastic excitation of acoustic perturbations. Techniques in local
helioseismology are then applied to measure flow signatures, showing
that within the observational time-period of the HMI instrument onboard
Solar Dynamics Observatory, it may not be possible to definitively
distinguish between single-cell and double-cell meridional circulation
structure. This analysis is extended to models of meridional circulation
generated in convectively-driven non-linear 3D global-Sun simulations
to explore the helioseismic differences generated by these models and
compare them with observations.
Title: Examining Correlations Between Helioseismic Signatures of
Active Regions and their Emergence
Authors: Stefan, John; Kosovichev, Alexander
Bibcode: 2021AGUFMSH55D1866S
Altcode:
Active regions, areas on the Sun characterized by their enhanced
magnetic field, are known for their high flare productivity. Therefore,
a comprehensive understanding of active regions and their evolution is
key to developing effective space weather forecasting tools. In order
to improve this understanding, a time-distance helioseismic method
is applied to a collection of 70 active regions observed by the HMI
instrument aboard SDO. The active regions in this analysis are chosen
based on their eventual maximum size and longitude of emergence; those
which reach a maximum size of at least 150 millionths of a hemisphere
and emerge within 45 degrees of the disk center are selected. The
corresponding Dopplergrams and Magnetograms are tracked and remapped
for two days prior to emergence and one day after emergence. The
time-distance method, building on the work of Ilonidis et al, reduces
noise in helioseismic measurements by averaging many cross-correlations
across depth - after shifting relative to reference travel times - and
across the orientation and size of correlation axes. The time-distance
method is used to obtain mean travel time perturbations and travel
time differences between depths of 40 and 70 Mm. Correlations in the
time delay between the magnitude of these travel time measurements
and the unsigned magnetic flux on the surface are examined, and the
relationship between an active regions size and magnitude of its
magnitude field with travel time measurements is estimated.
Title: Solar Gravitational Moments: What Are They and What Do They
Do? A Short Comprehensive Review
Authors: Rozelot, J. P.; Kosovichev, A. G.
Bibcode: 2021simi.conf...92R
Altcode:
Among all the fundamental solar parameters, mass, diameter, surface
gravity, temperature, luminosity..., all well inventoried since several
years in reference books, multi-gravitational moments are not yet well
documented. Several theoretical estimates have been proposed through
different approaches, mainly theory of Figure, helioseismology. We
will show their own merits. Exact values of multipolar gravitational
moments are important as they are at the crossroads of solar physics,
solar astrometry, celestial mechanics, and General Relativity. Their
temporal variations are still often neglected; they are yet an essential
aspect for constraining solar-cycle modeling or solar-evolution
theories. They induced planet-planet inclinations in multi-transiting
systems gravitating in the neighboring of a star. This paper emphasizes
some key issues to understand the role of these parameters.
Title: Probing Stellar Cores by Asteroseismic Inversions
Authors: Kosovichev, Alexander; Kitiashvili, Irina
Bibcode: 2021tsc2.confE.130K
Altcode:
Precision asteroseismology data from Kepler and TESS provide a unique
opportunity to investigate the interior structure of stars at various
stages of stellar evolution. Detection of mixed acoustic-gravity
oscillation modes has opened perspectives for probing the properties
of energy-generating cores. Most of the previous analysis was focused
on fitting standard evolutionary stellar models using mode frequency
splitting and scaling laws for oscillation properties. We present
direct asteroseismic inversions using the method of optimally localized
averages, which effectively eliminates the surface effects and attempts
to resolve the stellar core structure.
Title: Prediction of Solar Proton Events with Machine Learning:
Comparison with Operational Forecasts and "All-Clear" Perspectives
Authors: Sadykov, Viacheslav; Kosovichev, Alexander; Kitiashvili,
Irina; Oria, Vincent; Nita, Gelu M; Illarionov, Egor; O'Keefe, Patrick;
Jiang, Yucheng; Fereira, Sheldon; Ali, Aatiya
Bibcode: 2021arXiv210703911S
Altcode:
Solar Energetic Particle events (SEPs) are among the most dangerous
transient phenomena of solar activity. As hazardous radiation, SEPs may
affect the health of astronauts in outer space and adversely impact
current and future space exploration. In this paper, we consider the
problem of daily prediction of Solar Proton Events (SPEs) based on
the characteristics of the magnetic fields in solar Active Regions
(ARs), preceding soft X-ray and proton fluxes, and statistics of solar
radio bursts. The machine learning (ML) algorithm uses an artificial
neural network of custom architecture designed for whole-Sun input. The
predictions of the ML model are compared with the SWPC NOAA operational
forecasts of SPEs. Our preliminary results indicate that 1) for the
AR-based predictions, it is necessary to take into account ARs at
the western limb and on the far side of the Sun; 2) characteristics
of the preceding proton flux represent the most valuable input for
prediction; 3) daily median characteristics of ARs and the counts of
type II, III, and IV radio bursts may be excluded from the forecast
without performance loss; and 4) ML-based forecasts outperform SWPC
NOAA forecasts in situations in which missing SPE events is very
undesirable. The introduced approach indicates the possibility of
developing robust "all-clear" SPE forecasts by employing machine
learning methods.
Title: Travel Time Anisotropy Due to Subsurface Magnetic Fields
Authors: Stefan, J.; Kosovichev, A.
Bibcode: 2021AAS...23811317S
Altcode:
While there has been near-continuous observation of the Sun's surface
magnetic field over the past few decades, there remains little insight
concerning the configuration of the subsurface field. This work
presents a simple helioseismic technique to estimate the orientation
of the subsurface horizontal magnetic field as well as a proxy for
the horizontal field's magnitude. The technique is applied to several
HMI Dopplergram series of active regions and quiet-sun regions, with
notable differences between the two. Some discussion on the challenges
and limitations of the technique is provided.
Title: The Origin Of The Extended Solar Cycle
Authors: Kosovichev, A.; Pipin, V.; Getling, A.
Bibcode: 2021AAS...23830405K
Altcode:
The extended 22-year solar cycle phenomenon, discovered in observations
of the solar corona and variations of the solar differential rotation
(torsional oscillations), represents a fundamental heliophysics problem
linked to dynamo processes inside the Sun. As observed on the surface,
the extended solar cycle starts during a sunspot maximum at high
latitudes and consists of a relatively short polar branch (described
as "rush to the poles") and a long equatorward branch that continues
through the solar minimum and the next sunspot cycle. Helioseismic
observations of the internal dynamics of the Sun during the last two
solar activity cycles allow us to identify the dynamical processes
associated with the extended solar cycle throughout the depth of the
convective zone and to link them with dynamo models. Observational
data obtained from the SoHO (1996-2010) and SDO (2010-2020) spacecraft
represent measurements of the internal differential rotation, meridional
circulation, and thermodynamic parameters. The data indicate that the
development of a new extended solar cycle begins at about 60 degrees
latitude at the base of the convective zone during the maximum of
the previous cycle. Then, the process of magnetic field migration to
the Sun's surface is divided into two branches: fast (in 1-2 years)
migration to the poles in the high-latitude zone and slow migration to
the equator at middle and low latitudes for ~ 10 years. The subsurface
rotational shear layer (leptocline) plays a key role in the formation
of the magnetic butterfly diagram. Both the zonal flows (torsional
oscillations) and the meridional circulation reveal the 22-year
pattern of the extended solar cycle. A self-consistent MHD model
of the solar dynamo developed in the mean-field theory framework is
in good qualitative and quantitative agreement with the helioseismic
observations. The model shows that the extended solar-cycle phenomenon
is caused by magnetic field quenching of the convective heat flux
and modulation of the meridional circulation induced by the heat
flux variations. The model explains why the solar minimum polar field
predicts the next sunspot maximum and points to new possibilities for
predicting solar cycles from helioseismological data.
Title: Physical Properties of the Solar Atmosphere Derived from
Comparison of Spectro-Polarimetric SDO/HMI Observables with 3D
Radiative MHD Simulations
Authors: Sadykov, V.; Kitiashvili, I.; Kosovichev, A.; Wray, A.
Bibcode: 2021AAS...23832804S
Altcode:
In this study, we compare the SDO/HMI line-of-sight observables
(magnetic field, velocity, continuum intensity, and line depth) with the
related physical properties for several dynamo simulation runs performed
using the "StellarBox" 3D Radiative MHD code. The modeling of the Fe I
6173 Å Stokes profiles is performed using the SPINOR radiative transfer
code in the LTE approximation. The reproduced SDO/HMI line-of-sight
pipeline is applied to the modeled spectra, and the observables
are synthesized with high (numerical) and SDO/HMI (instrumental)
resolutions. Correlations between the observables and the physical
properties at various heights in the atmosphere are studied for a set
of view angles (0, 30, 45, 60, 70, and 80 degrees away from the solar
disk center). It is found that the SDO/HMI magnetic field and velocity
measurements are unambiguously correlated with physical parameters
at certain heights of the solar atmosphere. These heights increase
from about 100 km above the photosphere for the disk center case to
300-600 km above the photosphere for the 80-degree case. The heights
are found to be slightly lower in regions where stronger magnetic
fields are found. The comparison of the photospheric magnetic flux and
integrated continuum intensity derived from the SDO/HMI observables and
high-resolution observations and spectra is discussed. The results
of our study improve physics-based interpretations of the SDO/HMI
observables and provide a better understanding of the physical
properties of the solar atmosphere.
Title: Analysis of Time-Distance Helioseismology for Detection of
Emerging Active Regions
Authors: Stefan, John T.; Kosovichev, Alexander G.; Stejko, Andrey M.
Bibcode: 2021ApJ...913...87S
Altcode: 2020arXiv201201367S
A time-distance helioseismic technique, similar to the one used by
Ilonidis et al., is applied to two independent numerical models of
subsurface sound-speed perturbations to determine the spatial resolution
and accuracy of phase travel time shift measurements. The technique is
also used to examine pre-emergence signatures of several active regions
observed by the Michelson Doppler Imager and the Helioseismic Magnetic
Imager. In the context of similar measurements of quiet-Sun regions,
three of the five studied active regions show strong phase travel
time shifts several hours prior to emergence. These results form the
basis of a discussion of noise in the derived phase travel time maps
and possible criteria to distinguish between true and false-positive
detection of emerging flux.
Title: Understanding the Consequences Of Fields and Flows in the
Interior and Exterior of the Sun (COFFIES)
Authors: Hoeksema, J. T.; Brummell, N.; Bush, R.; Hess Webber, S.;
Kitiashvili, I.; Komm, R.; Kosovichev, A.; Mendez, B.; Scherrer, P.;
Upton, L.; Wray, A.; Zevin, D.; The Coffies Team
Bibcode: 2021AAS...23811322H
Altcode:
The solar activity cycle is the Consequence Of Fields and Flows in the
Interior and Exterior of the Sun (COFFIES). As a Phase-1 NASA DRIVE
Science Center (DSC), COFFIES ultimately aims to develop a data-driven
model of solar activity. To attain this goal COFFIES members are
learning to work together effectively to perform the investigations
needed to answer five primary science questions: 1) What drives
varying large-scale motions in the Sun? 2) How do flows interact
with the magnetic field to cause varying activity cycles? 3) Why
do active regions emerge when and where they do? 4) What do the
manifestations of activity and convection reveal about the internal
processes? 5) How does our understanding of the Sun as a star
inform us more generally about activity dynamics and structure? The
virtual COFFIES center brings together a broad spectrum of observers,
data analysts, theorists, computational scientists, and educators
who collaborate through interacting working groups of four principal
science teams. The principal objectives of the four primary science
teams are to 1) understand the generation of quasi-periodic stellar
magnetic cycles, 2) further develop 3D physical models of interior
dynamics and convection, 3) establish clear physical links between solar
flow fields and near-surface observations, and 4) develop more robust
helioseismic techniques to resolve solar interior flows. Additional
cross-team activities are facilitated by teams for numerical modeling,
center effectiveness, outreach and eduction, and diversity, equity,
inclusion and access (DEIA).
Title: The Observational Uncertainty of Coronal Hole Boundaries in
Automated Detection Schemes
Authors: Reiss, Martin A.; Muglach, Karin; Möstl, Christian; Arge,
Charles N.; Bailey, Rachel; Delouille, Véronique; Garton, Tadhg M.;
Hamada, Amr; Hofmeister, Stefan; Illarionov, Egor; Jarolim, Robert;
Kirk, Michael S. F.; Kosovichev, Alexander; Krista, Larisza; Lee,
Sangwoo; Lowder, Chris; MacNeice, Peter J.; Veronig, Astrid; Cospar
Iswat Coronal Hole Boundary Working Team
Bibcode: 2021ApJ...913...28R
Altcode: 2021arXiv210314403R
Coronal holes are the observational manifestation of the solar
magnetic field open to the heliosphere and are of pivotal importance
for our understanding of the origin and acceleration of the solar
wind. Observations from space missions such as the Solar Dynamics
Observatory now allow us to study coronal holes in unprecedented
detail. Instrumental effects and other factors, however, pose a
challenge to automatically detect coronal holes in solar imagery. The
science community addresses these challenges with different detection
schemes. Until now, little attention has been paid to assessing the
disagreement between these schemes. In this COSPAR ISWAT initiative,
we present a comparison of nine automated detection schemes widely
applied in solar and space science. We study, specifically, a prevailing
coronal hole observed by the Atmospheric Imaging Assembly instrument
on 2018 May 30. Our results indicate that the choice of detection
scheme has a significant effect on the location of the coronal hole
boundary. Physical properties in coronal holes such as the area, mean
intensity, and mean magnetic field strength vary by a factor of up
to 4.5 between the maximum and minimum values. We conclude that our
findings are relevant for coronal hole research from the past decade,
and are therefore of interest to the solar and space research community.
Title: Forward Modeling Helioseismic Signatures of One- and Two-cell
Meridional Circulation
Authors: Stejko, Andrey M.; Kosovichev, Alexander G.; Pipin, Valery V.
Bibcode: 2021ApJ...911...90S
Altcode: 2021arXiv210101220S
Using a 3D global solver of the linearized Euler equations, we
model acoustic oscillations over background velocity flow fields of
single-cell meridional circulation with deep and shallow return flows
as well as double-cell meridional circulation with strong and weak
reversals. The velocities are generated using a mean-field hydrodynamic
and dynamo model—moving through the regimes with minimal parameter
changes, counterrotation near the base of the tachocline is induced
by sign inversion of the nondiffusive action of turbulent Reynolds
stresses (Λ-effect) due to the radial inhomogeneity of the Coriolis
number. By mimicking the stochastic excitation of resonant modes in
the convective interior, we simulate realization noise present in solar
observations. Using deep-focusing to analyze differences in travel-time
signatures between the four regimes, as well as comparing to solar
observations, we show that current helioseismology techniques may offer
important insights about the location and strength of the return flow;
however, it may not currently be possible to definitively distinguish
between profiles of single-cell or double-cell meridional circulation.
Title: Characteristics of Sunquake Events Observed in Solar Cycle 24
Authors: Kosovichev, Alexander; Sharykin, Ivan
Bibcode: 2021EGUGA..23.1461K
Altcode:
Helioseismic response to solar flares ("sunquakes") occurs due to
localized force or/and momentum impacts observed during the flare
impulsive phase in the lower atmosphere. Such impacts may be caused by
precipitation of high-energy particles, downward shocks, or magnetic
Lorentz force. Understanding the mechanism of sunquakes is a key problem
of the flare energy release and transport. Our statistical analysis
of M-X class flares observed by the Solar Dynamics Observatory during
Solar Cycle 24 has shown that contrary to expectations, many relatively
weak M-class flares produced strong sunquakes, while for some powerful
X-class flares, helioseismic waves were not observed or were weak. The
analysis also revealed that there were active regions characterized by
the most efficient generation of sunquakes during the solar cycle. We
found that the sunquake power correlates with maximal values of the
X-ray flux derivative better than with the X-ray class. The sunquake
data challenge the current theories of solar flares.
Title: Helioseismic Modeling of Background Flows
Authors: Stejko, Andrey M.; Kosovichev, Alexander G.; Mansour, Nagi N.
Bibcode: 2021ApJS..253....9S
Altcode: 2020arXiv201103131S
We present a three-dimensional (3D) numerical solver of the linearized
compressible Euler equations (Global Acoustic Linearized Euler),
used to model acoustic oscillations throughout the solar interior. The
governing equations are solved in conservation form on a fully global
spherical mesh (0 ≤ ϕ ≤ 2π, 0 ≤ θ ≤ π, 0 ≤ r ≤ R
⊙) over a background state generated by the standard solar
model S. We implement an efficient pseudospectral computational method
to calculate the contribution of the compressible material derivative
dyad to internal velocity perturbations, computing oscillations over
arbitrary 3D background velocity fields. This model offers a foundation
for a “forward-modeling” approach, using helioseismology techniques
to explore various regimes of internal mass flows. We demonstrate the
efficacy of the numerical method presented in this paper by reproducing
observed solar power spectra, showing rotational splitting due to
differential rotation, and applying local helioseismology techniques
to measure travel times created by a simple model of single-cell
meridional circulation.
Title: Helioseismic Observations and Modeling of Solar Dynamo
Authors: Kosovichev, Alexander G.; Getling, Alexander V.; Pipin,
Valery V.
Bibcode: 2021csss.confE.115K
Altcode:
Helioseismological observations of the internal dynamics of the
Sun during the last two solar activity cycles make it possible
to trace the development of solar dynamo processes throughout the
depth of the convective zone and to link them with models of solar
cycles. Observational data obtained from the SoHO (1996-2010) and
SDO (2010-2020) spacecraft represent measurements of the internal
differential rotation, meridional circulation, and thermodynamic
parameters. The structure and dynamics of zonal and meridional
plasma flows reveal the processes of generation and transfer of
magnetic fields inside the Sun. The data analysis shows that active
latitudes and regions of a strong polar field on the Sun's surface
coincide with regions of deceleration of zonal currents ("torsional
oscillations"). The observed structure of zonal flows and their
latitudinal and radial migration in deep layers of the convective
zone correspond to dynamo waves predicted by dynamo theories and
numerical MHD models. The data indicate that the development of a new
solar cycle begins at about 60 degrees latitude at the base of the
convective zone during the maximum of the previous cycle. Then, the
process of magnetic field migration to the Sun's surface is divided
into two branches: fast (in 1-2 years) migration in the high-latitude
zone and slow migration at middle and low latitudes for ~ 10 years. The
subsurface rotational shear layer ("leptocline") plays a key role in
the formation of the magnetic "butterfly diagram". Both the zonal
flows ("torsional oscillations") and the meridional circulation
reveal the 22-year pattern of the "extended" solar cycle, initially
discovered from observations of Doppler velocities and the structure
of the solar corona.A self-consistent MHD model of the solar dynamo
developed in the mean-field theory framework is in good qualitative
and quantitative agreement with the helioseismic observations. The
model shows that the observed variations of the solar dynamics are
associated with a magnetic field effect on convective heat transfer
and the corresponding modulation of the meridional circulation. The
model explains why the solar minimum polar field predicts the next
sunspot maximum and points to new possibilities for predicting solar
cycles from helioseismological data.
Title: Compression of Solar Spectroscopic Observations: a Case Study
of Mg II k Spectral Line Profiles Observed by NASA's IRIS Satellite
Authors: Sadykov, Viacheslav M; Kitiashvili, Irina N; Sainz Dalda,
Alberto; Oria, Vincent; Kosovichev, Alexander G; Illarionov, Egor
Bibcode: 2021arXiv210307373S
Altcode:
In this study we extract the deep features and investigate the
compression of the Mg II k spectral line profiles observed in quiet
Sun regions by NASA's IRIS satellite. The data set of line profiles
used for the analysis was obtained on April 20th, 2020, at the
center of the solar disc, and contains almost 300,000 individual
Mg II k line profiles after data cleaning. The data are separated
into train and test subsets. The train subset was used to train the
autoencoder of the varying embedding layer size. The early stopping
criterion was implemented on the test subset to prevent the model from
overfitting. Our results indicate that it is possible to compress the
spectral line profiles more than 27 times (which corresponds to the
reduction of the data dimensionality from 110 to 4) while having a 4 DN
average reconstruction error, which is comparable to the variations in
the line continuum. The mean squared error and the reconstruction error
of even statistical moments sharply decrease when the dimensionality of
the embedding layer increases from 1 to 4 and almost stop decreasing
for higher numbers. The observed occasional improvements in training
for values higher than 4 indicate that a better compact embedding may
potentially be obtained if other training strategies and longer training
times are used. The features learned for the critical four-dimensional
case can be interpreted. In particular, three of these four features
mainly control the line width, line asymmetry, and line dip formation
respectively. The presented results are the first attempt to obtain
a compact embedding for spectroscopic line profiles and confirm the
value of this approach, in particular for feature extraction, data
compression, and denoising.
Title: Modeling Helioseismic Signatures of Meridional Circulation
Authors: Stejko, Andrey; Kosovichev, Alexander; Pipin, Valery
Bibcode: 2021csss.confE.114S
Altcode:
We present the application of a new 3D global linearized acoustic
algorithm (GALE). With the wide variance of inferred profiles of
meridional circulation currently being made from similar observational
data, we choose a "forward-modeling" approach to test the helioseismic
signatures generated by several physics-based mean-field models of
meridional velocities. We simulate the effect of realization noise
in measurements in order to place a low-end baseline for whether it
is possible to see the difference between single- and double-cell
meridional circulation. With minimal parameter changes between the
two regimes we show that it may not be possible two distinguish these
profiles within the operational time-frame of HMI. We also test several
other profiles in an attempt to use mean-field theory to help constrain
inferences made through helioseismology inversions.
Title: Connecting Atmospheric Properties and Synthetic Emission of
Shock Waves Using 3D RMHD Simulations of the Quiet Sun
Authors: Sadykov, Viacheslav M.; Kitiashvili, Irina N.; Kosovichev,
Alexander G.; Wray, Alan A.
Bibcode: 2021ApJ...909...35S
Altcode: 2020arXiv200805995S
We analyze the evolution of shock waves in high-resolution 3D radiative
MHD simulations of the quiet Sun and their synthetic emission
characteristics. The simulations model the dynamics of a 12.8 ×
12.8 × 15.2 Mm quiet-Sun region (including a 5.2 Mm layer of the
upper convection zone and a 10 Mm atmosphere from the photosphere
to corona) with an initially uniform vertical magnetic field of
10 G, naturally driven by convective flows. We synthesize the Mg
II and C II spectral lines observed by the Interface Region Imaging
Spectrograph (IRIS) satellite and extreme ultraviolet emission observed
by the Solar Dynamics Observatory (SDO)/AIA telescope. Synthetic
observations are obtained using the RH1.5D radiative transfer code and
temperature response functions at both the numerical and instrumental
resolutions. We found that the Doppler velocity jumps of the C II 1334.5
Å IRIS line and a relative enhancement of the emission in the 335 Å
SDO/AIA channel are the best proxies for the enthalpy deposited by shock
waves into the corona (with Kendall's τ correlation coefficients of
0.59 and 0.38, respectively). The synthetic emission of the lines and
the extreme ultraviolet passbands are correlated with each other during
the shock-wave propagation. All studied shocks are mostly hydrodynamic
(i.e., the magnetic energy carried by horizontal fields is ≤2.6%
of the enthalpy for all events) and have Mach numbers >1.0-1.2 in
the low corona. The study reveals the possibility of diagnosing energy
transport by shock waves into the solar corona, as well as their other
properties, by using IRIS and SDO/AIA sensing observations.
Title: Evolution of Subsurface Zonal and Meridional Flows in Solar
Cycle 24 from Helioseismological Data
Authors: Getling, Alexander V.; Kosovichev, Alexander G.; Zhao, Junwei
Bibcode: 2021ApJ...908L..50G
Altcode: 2020arXiv201215555G
The results of determinations of the azimuthal and meridional
velocities by time-distance helioseismology from Helioseismic and
Magnetic Imager on board Solar Dynamics Observatory from 2010 May to
2020 September at latitudes and Stonyhurst longitudes from - 60° to
+ 60° and depths to about 19 Mm below the photosphere are used to
analyze spatiotemporal variations of the solar differential rotation
and meridional circulation. The pattern of torsional oscillations,
or latitudinal belts of alternating "fast" and "slow" zonal flows
migrating from high latitudes toward the equator, is found to extend in
the time-latitude diagrams over the whole time interval. The oscillation
period is comparable with a doubled solar-activity-cycle and can be
described as an extended solar cycle. The zonal-velocity variations
are related to the solar-activity level, the local-velocity increases
corresponding to the sunspot-number increases and being localized at
latitudes where the strongest magnetic fields are recorded. The dramatic
growth of the zonal velocities in 2018 appears to be a precursor of
the beginning of Solar Cycle 25. The strong symmetrization of the
zonal-velocity field by 2020 can be considered another precursor. The
general pattern of poleward meridional flows is modulated by latitudinal
variations similar to the extended-solar-cycle behavior of the zonal
flows. During the activity maximum, these variations are superposed
with a higher harmonic corresponding to meridional flows converging to
the spot-formation latitudes. Our results indicate that variations of
both the zonal and meridional flows exhibit the extended-solar-cycle
behavior, which is an intrinsic feature of the solar dynamo.
Title: 3D Realistic Modeling of Main-Sequence Stars with Shallow
Outer Convection Zone
Authors: Kitiashvili, I. N.; Wray, A. A.; Kosovichev, A. G.
Bibcode: 2021AAS...23741505K
Altcode:
Our current state-of-the-art computer simulations allow us to build
3D dynamical and radiation models of F-type stars from physical first
principles. Using the stellar interior's structure from the MESA
stellar evolution code as initial conditions, we generate models of
main-sequence stars with the mass from 1.4 Msun to 2 Msun for various
metallicity composition, in the range of [Fe/H] from -0.3 to 0.2. The
radiative 3D dynamical stellar models obtained with the StellarBox
code take into account the effects of turbulence, stellar abundances,
and radiation. We investigate the turbulent dynamics from the radiative
zone to the outer convection zone and the lower atmosphere for these
stars and compare their turbulent properties. Also, we investigate
the effects of stellar rotation for a 1.47Msun star for rotational
periods of 1 and 14 days. The simulations are performed for the
different latitudinal location of the computation domain. The models
reproduce the subsurface shear layer, structural changes of convection,
and the tachocline, which is the interface between the inner radiative
zone and the outer convection zone and plays a crucial role in stellar
variability. In particular, the model results reveal the formation of
differential rotation of an anti-solar type. The simulation results shed
light on differential rotation properties, the excitation of oscillation
modes, the tachocline's dynamics and structure, and support analysis
and interpretation of observational data from Kepler and TESS missions.
Title: COFFIES - Developing a Reliable Physical Model of the Solar
Activity Cycle
Authors: Hoeksema, J. T.; Brummell, N.; Bush, R. I.; Komm, R.;
Kosovichev, A. G.; Mendez, B.; Scherrer, P. H.; Upton, L.; Wray,
A. A.; Zevin, D.
Bibcode: 2020AGUFMSH0020007H
Altcode:
The solar activity cycle is the Consequence Of Fields and Flows in
the Interior and Exterior of the Sun (COFFIES). The COFFIES Drive
Science Center ultimately aims to develop a data driven model of solar
activity. The challenging goals are 1) to understand the generation
of the quasi-periodic stellar magnetic cycles, 2) further develop 3D
physical models of interior dynamics and convection, 3) establish the
physical links between solar flow fields and near-surface observations,
and 4) develop more robust helioseismic techniques to resolve solar
interior flows. To reach these goals the COFFIES team is focusing on
what is needed to answer five primary science questions: 1) What drives
varying large-scale motions in the Sun? 2) How do flows interact with
the magnetic field to cause varying activity cycles? 3) Why do active
regions emerge when and where they do? 4) What do the manifestations
of activity and convection reveal about the internal processes? And
5) How does our understanding of the Sun as a star inform us more
generally about activity dynamics and structure? The virtual COFFIES
center is bringing together a broad spectrum of observers, analysts,
theorists, computational scientists, and educators who collaborate
through interacting teams focused on helioseismology, dynamos, solar
convection, surface links, numerical modeling, center effectiveness,
outreach, education, diversity and inclusion.
Title: Helioseismic Constraints on the Solar Interior Dynamics
and Dynamo
Authors: Kosovichev, A. G.; Brummell, N.; Dikpati, M.; Guerrero,
G.; Kitiashvili, I.; Komm, R.; Korzennik, S.; Pipin, V.; Reiter, J.;
Stejko, A.; Ulrich, R. K.; Warnecke, J.
Bibcode: 2020AGUFMSH007..04K
Altcode:
Uninterrupted helioseismic observations from the SoHO/MDI, SDO/HMI and
GONG instruments for more than two decades provide unique observational
data for studying the solar-cycle variations of the differential
rotation, large-scale and meridional flows. The data also allows
us to investigate changes in the thermodynamic structure associated
with dynamo-generated magnetic fields. The wealth of global and local
helioseismic data provides theoretical constraints on the solar dynamics
and dynamo models. The synergy of helioseismic inferences with advanced
MHD modeling sheds light on the origin of the solar activity cycles. It
helps to understand better the physical processes that control the
strength and duration of the cyclic magnetic activity and leads to
new physics-based approaches for prediction of the solar cycles. We
briefly overview the current status, discuss the solar dynamical
structure and evolution revealed by helioseismic inversions and the
forward-modeling method, and focus on the most critical points of the
problem. In particular, we discuss recent advances in measurements and
modeling of the solar-cycle variations of the meridional circulation
and migrating zonal flows (torsional oscillations) on the solar surface
and in the subsurface layers, the deep convection zone, and the solar
tachocline. The relationships between the internal dynamics and the
evolution of global magnetic fields lead to new ideas of how magnetic
fields are generated and affect the solar flows and structure.
Title: Multi-Wavelength Modeling and Analysis of the Center-to-Limb
Effects of Solar Spectroscopy and Helioseismology
Authors: Kitiashvili, I.; Zhao, J.; Sadykov, V. M.; Criscuoli, S.;
Kosovichev, A. G.; Wray, A. A.
Bibcode: 2020AGUFMSH0020003K
Altcode:
An accurate interpretation of observed solar dynamics with different
instruments requires modeling solar magnetoconvection in different
regimes, as well as taking into account center-to-limb effects,
magnetic fields, and turbulence. Realistic 3D radiative MHD modeling
of the solar magnetoconvection and atmosphere allows us to generate
synthetic observables that directly link the physical properties of
the solar plasma to spectroscopic and helioseismic observables. In
this work, we investigate the influence of the center-to-limb effects
for a wide range of wavelengths, which correspond to the operational
lines of HMI/SDO, Hinode, DKIST, and other instruments. In particular,
we discuss the wavelength-dependency of the center-to-limb helioseismic
observations of acoustic travel times that are used for diagnostics of
the deep meridional circulation, as well as 'concave' Sun effect. The
presented study will support interpretation of helioseismic inversion
results by taking into account realistic coupling of subsurface and
atmosphere, and geometry-related effects. In particular, it allows us
to improve accuracy of solar subsurface measurements from the SoHO and
SDO missions, and resolve the long-standing problem of the meridional
circulation and evolution with the solar cycle.
Title: Early Detection of Emerging Magnetic Flux using Time-Distance
Helioseismology
Authors: Stefan, J. T.; Kosovichev, A. G.
Bibcode: 2020AGUFMSH0020011S
Altcode:
We employ a time-distance measurement procedure, similar to the one
used by Ilonidis et al in their 2013 work, to detect solar subsurface
structures associated with emerging magnetic flux. We compute the
spatially-averaged cross-covariance of Dopplergram signals, and fit to
a Gabor wavelet so that the phase travel time of acoustic waves can be
extracted. Deviations from the mean phase travel time are interpreted
as fluctuations in density and gas pressure. We independently confirm
the analysis of AR10488 from Stathis et al (2013), a result that was
hotly debated after publishing. We also calibrate this time-distance
method using numerical models provided by Hartlep et al (2011) and
Stejko et al (under review). Based on this calibration, we propose an
estimate for the magnitude of local sound-speed perturbations necessary
to produce the observed signal.
Title: Challenges and Advances in Modeling of the Solar Atmosphere:
A White Paper of Findings and Recommendations
Authors: Criscuoli, Serena; Kazachenko, Maria; Kitashvili, Irina;
Kosovichev, Alexander; Martínez Pillet, Valentín; Nita, Gelu;
Sadykov, Viacheslav; Wray, Alan
Bibcode: 2021arXiv210100011C
Altcode:
The next decade will be an exciting period for solar astrophysics, as
new ground- and space-based instrumentation will provide unprecedented
observations of the solar atmosphere and heliosphere. The synergy
between modeling effort and comprehensive analysis of observations
is crucial for the understanding of the physical processes behind
the observed phenomena. However, the unprecedented wealth of data on
one hand, and the complexity of the physical phenomena on the other,
require the development of new approaches in both data analysis and
numerical modeling. In this white paper, we summarize recent numerical
achievements to reproduce structure, dynamics, and observed phenomena
from the photosphere to the low corona and outline challenges we expect
to face for the interpretation of future observations.
Title: Machine-learning Approach to Identification of Coronal Holes
in Solar Disk Images and Synoptic Maps
Authors: Illarionov, Egor; Kosovichev, Alexander; Tlatov, Andrey
Bibcode: 2020ApJ...903..115I
Altcode: 2020arXiv200608529I
Identification of solar coronal holes (CHs) provides information both
for operational space weather forecasting and long-term investigation
of solar activity. Source data for the first problem are typically
from the most recent solar disk observations, while for the second
problem it is convenient to consider solar synoptic maps. Motivated
by the idea that the concept of CHs should be similar for both cases
we investigate universal models that can learn CH segmentation in
disk images and reproduce the same segmentation in synoptic maps. We
demonstrate that convolutional neural networks trained on daily disk
images provide an accurate CH segmentation in synoptic maps and their
pole-centric projections. Using this approach we construct a catalog of
synoptic maps for the period of 2010-20 based on SDO/AIA observations
in the 193 Å wavelength. The obtained CH synoptic maps are compared
with magnetic synoptic maps in the time-latitude and time-longitude
diagrams. The initial results demonstrate that while in some cases the
CHs are associated with magnetic flux-transport events there are other
mechanisms contributing to the CH formation and evolution. To stimulate
further investigations the catalog of synoptic maps is published in
open access.
Title: Solar Multipolar Moments: an up-to-date Pedagogical
Approach. Implications for Stellar Properties. Part I: an Analytical
Overview (TUTORIAL)
Authors: Rozelot, J. P.; Kosovichev, A. G.; Kilcik, A.; Sahin, S.;
Xu, Y.; Javaraiah, J.; Georgieva, K.; Özgüҫ, A.
Bibcode: 2020simi.conf....1R
Altcode:
Solar gravitational multipolar moments have not been yet extensively
analyzed. However, they are at the crossroads of solar physics,
solar astrometry, celestial mechanics and General Relativity. Their
values reflect the physics of solar models: non-rigid rotation, solar
latitudinal rotation, solar- core properties, solar-cycle variations
and structure evolution. Their temporal variations are still often
neglected; they are yet an essential aspect for constraining
solar-cycle modeling or solar-evolution theories. They induced
planet-planet inclinations in multitransiting systems gravitating in
the neighboring of a star, leading to key issues future studies. This
paper is devoted to an analytical analysis; a second part will address
an helioseismology analysis.
Title: Torsional Oscillations in Dynamo Models with Fluctuations
and Potential for Helioseismic Predictions of the Solar Cycles
Authors: Pipin, Valery V.; Kosovichev, Alexander G.
Bibcode: 2020ApJ...900...26P
Altcode: 2020arXiv200408537P
Using a nonlinear mean-field solar dynamo model, we study relationships
between the amplitude of the "extended" mode of migrating zonal flows
("torsional oscillations") and magnetic cycles, and investigate whether
properties the torsional oscillations in subsurface layers and in the
deep convection zone can provide information about the future solar
cycles. We consider two types of dynamo models: models with regular
variations of the α-effect, and models with stochastic fluctuations,
simulating "long-memory" and "short-memory" types of magnetic activity
variations. It is found that torsional oscillation parameters, such the
zonal acceleration, show a considerable correlation with the magnitude
of the subsequent cycles with a time lag of 11-20 yr. The sign of the
correlation and the time-lag parameters can depend on the depth and
latitude of the torsional oscillations as well as on the properties of
long-term ("centennial") variations of the dynamo cycles. The strongest
correlations are found for the zonal acceleration at high latitudes
at the base of the convection zone. The model results demonstrate that
helioseismic observations of the torsional oscillations can be useful
for advanced prediction of the solar cycles, 1-2 sunspot cycles ahead.
Title: Early Detection of Emerging Magnetic Flux
Authors: Stefan, J.; Kosovichev, A.
Bibcode: 2020SPD....5120401S
Altcode:
We employ a time-distance measurement procedure, similar to the one
used by Ilonidis et al in their 2013 work, to detect solar subsurface
structures associated with emerging magnetic flux. We compute the
spatially-averaged cross-covariance of Dopplergram signals, and fit to
a Gabor wavelet so that the phase travel time of acoustic waves can be
extracted. Deviations from the mean phase travel time are interpreted as
fluctuations in density and gas pressure. We independently confirm the
analysis of AR10488 from Stathis et al (2013), a result that was hotly
debated after publishing. We also calibrate this time-distance method
using numerical models provided by Hartlep et al (2011) and Stejko
et al (2020, this meeting). Based on this calibration, we propose an
estimate for the magnitude of local sound-speed perturbations necessary
to produce the observed signal.
Title: Machine Learning in Heliophysics and Space Weather Forecasting:
A White Paper of Findings and Recommendations
Authors: Nita, Gelu; Georgoulis, Manolis; Kitiashvili, Irina; Sadykov,
Viacheslav; Camporeale, Enrico; Kosovichev, Alexander; Wang, Haimin;
Oria, Vincent; Wang, Jason; Angryk, Rafal; Aydin, Berkay; Ahmadzadeh,
Azim; Bai, Xiaoli; Bastian, Timothy; Filali Boubrahimi, Soukaina; Chen,
Bin; Davey, Alisdair; Fereira, Sheldon; Fleishman, Gregory; Gary, Dale;
Gerrard, Andrew; Hellbourg, Gregory; Herbert, Katherine; Ireland,
Jack; Illarionov, Egor; Kuroda, Natsuha; Li, Qin; Liu, Chang; Liu,
Yuexin; Kim, Hyomin; Kempton, Dustin; Ma, Ruizhe; Martens, Petrus;
McGranaghan, Ryan; Semones, Edward; Stefan, John; Stejko, Andrey;
Collado-Vega, Yaireska; Wang, Meiqi; Xu, Yan; Yu, Sijie
Bibcode: 2020arXiv200612224N
Altcode:
The authors of this white paper met on 16-17 January 2020 at the New
Jersey Institute of Technology, Newark, NJ, for a 2-day workshop that
brought together a group of heliophysicists, data providers, expert
modelers, and computer/data scientists. Their objective was to discuss
critical developments and prospects of the application of machine and/or
deep learning techniques for data analysis, modeling and forecasting
in Heliophysics, and to shape a strategy for further developments in
the field. The workshop combined a set of plenary sessions featuring
invited introductory talks interleaved with a set of open discussion
sessions. The outcome of the discussion is encapsulated in this white
paper that also features a top-level list of recommendations agreed
by participants.
Title: A Method for the Estimation of f- and p-mode Parameters and
Rotational Splitting Coefficients from Un-averaged Solar Oscillation
Power Spectra
Authors: Reiter, J.; Rhodes, E. J., Jr.; Kosovichev, A. G.; Scherrer,
P. H.; Larson, T. P.; , S. F. Pinkerton, II
Bibcode: 2020ApJ...894...80R
Altcode:
We present a new methodology for the fitting of the peaks in
solar oscillation power spectra that is equally well-suited for the
estimation of low-, medium, and high-degree f- and p-mode parameters and
frequency-splitting coefficients. The method can provide accurate input
data over a wide portion of the dispersion plane for both structural and
rotational inversions. This method, which we call the Multiple-Peak,
Tesseral-Spectrum (MPTS) method, operates directly upon of all of the
modes in a multiplet (n, l) of radial order n and degree l, and employs
a fitting profile that consists of the sum of numerous individual
overlapping profiles whose relative amplitudes are determined by the
leakage matrix appropriate to the targeted mode. Hence, 2l + 1 sets
of modal parameters are obtained simultaneously for each multiplet
(n, l). By fitting an appropriate polynomial to the run of the
fitted frequencies versus the azimuthal order, frequency-splitting
coefficients are also obtained for the same multiplet. Using power
spectra obtained from the 66 day long 2010 MDI Dynamics Run, we
present sample structural and rotational inversions that employed
frequencies and frequency-splitting coefficients from modes in the
degree range of 0-1000 and the frequency range of 965-4600 μHz. The
structural inversion confirms evidence for a pronounced departure of
the sound speed in the outer solar convection zone from the radial
sound-speed profile contained in Model S of Christensen-Dalsgaard
and his collaborators that we obtained previously using a different
fitting method.
Title: Sunquakes of Solar Cycle 24
Authors: Sharykin, Ivan N.; Kosovichev, Alexander G.
Bibcode: 2020ApJ...895...76S
Altcode: 2019arXiv191104197S
The paper presents results of a search for helioseismic events
(sunquakes) produced by M-X class solar flares during Solar Cycle
24. The search is performed by analyzing photospheric Dopplergrams
from the Helioseismic Magnetic Imager. Among the total number of 500
M-X class flares, 94 helioseismic events were detected. Our analysis
has shown that many strong sunquakes were produced by solar flares of
low M class (M1-M5), while in some powerful X-class flares helioseismic
waves were not observed or were weak. Our study has also revealed that
only several active regions were characterized by the most efficient
generation of helioseismic waves during flares. We found that the
sunquake power correlates with the maximum value of the soft X-ray flux
time derivative better than with the X-ray class, indicating that the
sunquake mechanism is associated with high-energy particles. We also
show that the seismically active flares are more impulsive than the
flares without helioseismic perturbations. We present a new catalog of
helioseismic solar flares, which opens opportunities for performing
statistical studies to better understand the physics of sunquakes as
well as the flare-energy release and transport.
Title: Estimation of Key Sunquake Parameters through Hydrodynamic
Modeling and Cross-correlation Analysis
Authors: Stefan, John T.; Kosovichev, Alexander G.
Bibcode: 2020ApJ...895...65S
Altcode: 2019arXiv191106839S
Sunquakes are one of the more distinct secondary phenomena related to
solar flares, where energy deposition in the lower layers of the Sun's
atmosphere excites acoustic waves easily visible in photospheric
dopplergrams. We explore two possible excitation mechanisms
of sunquakes in the context of the electron beam hypothesis: an
instantaneous transfer of momentum and a gradual applied force due
to flare eruption. We model the sunquake excitation and compare with
five observed sunquake events using a cross-correlation analysis. We
find that at least half the events studied are consistent with the
electron beam hypothesis and estimate the energy required to excite
the sunquakes to be within the range determined by previous studies.
Title: Determination of the solar rotation parameters via orthogonal
polynomials
Authors: Mdzinarishvili, T. G.; Shergelashvili, B. M.; Japaridze,
D. R.; Chargeishvili, B. B.; Kosovichev, A. G.; Poedts, S.
Bibcode: 2020AdSpR..65.1843M
Altcode:
Accurate measurements of the solar differential rotation parameters
are necessary for understanding the solar dynamo mechanism. We use the
orthogonalization process to estimate these parameters. The advantage
of the orthogonalization of the data in the tracer motion statistical
analysis is outlined. The differential rotation is represented in
terms of various types of polynomials. We compare the quality of a
set of models of the solar differential rotation using the Akaike
information criterion and choose the best one. Applying the proposed
method, we studied the solar differential rotation and its North-South
asymmetry using observations of coronal holes. A statistical analysis
of observations from the Atmospheric Imaging Assembly (AIA) on Solar
Dynamics Observatory (SDO) reveals the differential rotation pattern
of coronal holes and its North-South asymmetry.
Title: Response of SDO/HMI Observables to Heating of the Solar
Atmosphere by Precipitating High-energy Electrons
Authors: Sadykov, Viacheslav M.; Kosovichev, Alexander G.; Kitiashvili,
Irina N.; Kerr, Graham S.
Bibcode: 2020ApJ...893...24S
Altcode: 2019arXiv190610788S
We perform an analysis of the line-of-sight (LOS) observables of the
Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics
Observatory (SDO) for models of the solar atmosphere heated by
precipitating high-energy electrons during solar flares. The radiative
hydrodynamic (RADYN) flare models are obtained from the F-CHROMA
database. The Stokes profiles for the Fe 6173 Å line observed by
SDO/HMI are calculated using the radiative transfer code RH1.5D,
assuming statistical equilibrium for atomic level populations,
and imposing uniform background vertical magnetic fields of various
strengths. The SDO/HMI observing sequence and LOS data processing
pipeline algorithm are applied to derive the observables (continuum
intensity, line depth, Doppler velocity, LOS magnetic field). Our
results reveal that the strongest deviations of the observables from
the actual spectroscopic line parameters are found for the model with
a total energy deposited of Etotal = 1.0 × 1012
erg cm-2, injected with a power-law spectral index of δ =
3 above a low-energy cutoff of Ec = 25 keV. The magnitudes
of the velocity and magnetic field deviations depend on the imposed
magnetic field, and can reach 0.35 km s-1 for LOS velocities,
90 G for LOS magnetic field, and 3% for continuum enhancement for the
1000 G imposed LOS magnetic field setup. For Etotal ≥
3.0 × 1011 erg cm-2 models, the velocity and
magnetic field deviations are most strongly correlated with the energy
flux carried by ∼50 keV electrons, and the continuum enhancement
is correlated with the synthesized ∼55-60 keV hard X-ray photon
flux. The relatively low magnitudes of perturbations of the observables
and absence of magnetic field sign reversals suggest that the considered
RADYN beam heating models augmented with the uniform vertical magnetic
field setups cannot explain the strong transient changes found in the
SDO/HMI observations.
Title: Resolving Power of Asteroseismic Inversion of the Kepler
Legacy Sample
Authors: Kosovichev, Alexander G.; Kitiashvili, Irina N.
Bibcode: 2020IAUS..354..107K
Altcode: 2020arXiv200209839K
The Kepler Asteroseismic Legacy Project provided frequencies, separation
ratios, error estimates, and covariance matrices for 66 Kepler main
sequence targets. Most of the previous analysis of these data was
focused on fitting standard stellar models. We present results of
direct asteroseismic inversions using the method of optimally localized
averages (OLA), which effectively eliminates the surface effects and
attempts to resolve the stellar core structure. The inversions are
presented for various structure properties, including the density
stratification and sound speed. The results show that the mixed modes
observed in post-main sequence F-type stars allow us to resolve the
stellar core structure and reveal significant deviations from the
evolutionary models obtained by the grid-fitting procedure to match
the observed oscillation frequencies.
Title: Realistic 3D MHD modeling of self-organized magnetic
structuring of the solar corona
Authors: Kitiashvili, Irina N.; Wray, Alan A.; Sadykov, Viacheslav;
Kosovichev, Alexander G.; Mansour, Nagi N.
Bibcode: 2020IAUS..354..346K
Altcode:
The dynamics of solar magnetoconvection spans a wide range of spatial
and temporal scales and extends from the interior to the corona. Using
3D radiative MHD simulations, we investigate the complex interactions
that drive various phenomena observed on the solar surface, in the
low atmosphere, and in the corona. We present results of our recent
simulations of coronal dynamics driven by underlying magnetoconvection
and atmospheric processes, using the 3D radiative MHD code StellarBox
(Wray et al. 2018). In particular, we focus on the evolution of
thermodynamic properties and energy exchange across the different
layers from the solar interior to the corona.
Title: Solar oblateness & asphericities temporal variations:
Outstanding some unsolved issues
Authors: Rozelot, Jean P.; Kosovichev, Alexander G.; Kilcik, Ali
Bibcode: 2020IAUS..354..232R
Altcode:
Solar oblateness has been the subject of several studies dating back
to the nineteenth century. Despite difficulties, both theoretical
and observational, tangible results have been achieved. However,
variability of the solar oblateness with time is still poorly
known. How the solar shape evolves with the solar cycle has been a
challenging problem. Analysis of the helioseismic data, which are the
most accurate measure of the solar structure up to now, leads to the
determination of asphericity coefficients which have been found to
change with time. We show here that by inverting even coefficients
of f-mode oscillation frequency splitting to obtain the oblateness
magnitude and its temporal dependence can be inferred. It is found
that the oblateness variations lag the solar activity cycles by about
3 years. A major change occurred between solar cycles 23 and 24 is that
the oblateness was greater in cycle 24 despite the lower solar activity
level. Such results may help to better understand the near-subsurface
layers as they strongly impacts the internal dynamics of the Sun and
may induce instabilities driving the transport of angular momentum.
Title: 3D MHD Modeling of the Impact of Subsurface Stratification
on the Solar Dynamo
Authors: Stejko, Andrey M.; Guerrero, Gustavo; Kosovichev, Alexander
G.; Smolarkiewicz, Piotr K.
Bibcode: 2020ApJ...888...16S
Altcode:
Various models of solar subsurface stratification are tested in the
global EULAG-MHD solver to simulate diverse regimes of near-surface
convective transport. Sub- and superadiabacity are altered at the
surface of the model (r > 0.95R ⊙) to either suppress or
enhance convective flow speeds in an effort to investigate the impact
of the near-surface layer on global dynamics. A major consequence
of increasing surface convection rates appears to be a significant
alteration of the distribution of angular momentum, especially below
the tachocline where the rotational frequency predominantly increases
at higher latitudes. These hydrodynamic changes correspond to large
shifts in the development of the current helicity in this stable layer
(r < 0.72R ⊙), significantly altering its impact on
the generation of poloidal and toroidal fields at the tachocline and
below, acting as a major contributor toward transitions in the dynamo
cycle. The enhanced near-surface flow speed manifests in a global shift
of the toroidal field (B ϕ ) in the butterfly diagram, from
a north-south symmetric pattern to a staggered antisymmetric emergence.
Title: Solar and Stellar Magnetic Fields: Origins and Manifestations
Authors: Kosovichev, A.; Strassmeier, S.; Jardine, M.
Bibcode: 2020IAUS..354.....K
Altcode:
No abstract at ADS
Title: A Generalized Spectral Kurtosis Solar Image Variability
Descriptor
Authors: Nita, G. M.; Sadykov, V. M.; Kosovichev, A. G.; Oria, V.
Bibcode: 2019AGUFMSH31E3340N
Altcode:
The growing need for accurate Space Weather forecasts motivates
researchers to implement comprehensive prediction algorithms for solar
transient events that rely on large amounts of observational data, which
often require sophisticated processing in order to extract relevant
information that is used to guide or constrain the forecasts. From
the perspective of such ongoing efforts, continuing to search for
alternative physical or mathematical descriptors and investigating their
ability to encode the dynamics of solar variability may provide means
for improved forecast performance. Here we introduce a novel solar
image variability descriptor built from time series of SDO/AIA images,
which can automatically identify and discriminate quite-Sun areas,
active regions, and developing eruptive events. Our approach is based
on the empirical observation that the statistical distribution of the
squared pixel intensities corresponding to quiet-Sun areas imaged by
SDO/AIA in certain observing channels may be well approximated by Gamma
distributions, while active and eruptive solar disk areas obey different
statistics, also distinguishable from each other. This finding justifies
the direct use of the well-developed Generalized Spectral Kurtosis
Estimator theory, which offers reliable means to discriminate non-quiet
image areas in terms of analytically defined detection thresholds having
known probabilities of false alarm. We present the results of our
analysis in the case of several active regions, which were obtained
using an automated data processing pipeline that we have developed under
the framework provided by two ongoing projects, namely "EarthCube Data
Infrastructure: Intelligent Databases and Analysis Tools for Geospace
Data and Models," and "EarthCube Research Coordination Network:
Towards Integration of Heliophysics Data, Modeling, and Analysis Tools."
Title: Cluster Analysis of Spectroscopic Line Profiles in IRIS
Observations and RMHD Simulations of the Solar Atmosphere
Authors: Sadykov, V. M.; Kitiashvili, I.; Kosovichev, A. G.
Bibcode: 2019AGUFMSH31E3345S
Altcode:
Spatially-resolved spectroscopic observations from IRIS satellite,
especially when coupled with realistic 3D RMHD simulations, are a
powerful tool for analysis of processes in the solar chromosphere
and transition region. However, the complexity of spectroscopic data
makes comparison of observations and modeling results difficult. In
this work, we apply unsupervised clustering algorithms for analysis
of observational and synthetic line profiles to find a compact
representation of spectroscopic data and classification in terms
of physical characteristics of the radiating solar plasma. In
particular, we utilize the quiet-Sun observations from IRIS, and
for their interpretation compute synthetic line profiles of the
chromospheric Mg II h&k 2796 Å & 2803 Å and transition
region C II 1334 Å & 1335 Å lines using the realistic 3D RMHD
simulations of the quiescent solar atmosphere (using StellarBox and
RH1.5 codes). K-Means clustering algorithm is applied separately
to the observed or synthetic spectroscopic line profiles, as well
as to their statistical moments (intensity maxima, Doppler shifts,
line widths etc). The average silhouette width maximization technique
for the K-Means algorithm is utilized to obtain optimal numbers of
clusters. We discuss applications of the line profile clustering method
to 1) visualizations of computational and observational spectroscopic
imaging data; 2) understanding of evolutionary trends and behavior
patterns; 3) recognition of heating events and shock waves.
Title: Solar activity modeling: from subgranular scales to the
solar cycles
Authors: Kitiashvili, I.; Wray, A. A.; Sadykov, V. M.; Kosovichev,
A. G.; Mansour, N. N.
Bibcode: 2019AGUFMSH31E3350K
Altcode:
Dynamical effects of solar magnetoconvection span a wide range
spatial and temporal scales that extends from the interior to the
corona and from fast turbulent motions to the global-Sun magnetic
activity. To study the solar activity on short temporal scales (from
minutes to hours), we use 3D radiative MHD simulations that allow
us to investigate complex turbulent interactions that drive various
phenomena, such as plasma eruptions, spontaneous formation of magnetic
structures, funnel-like structures and magnetic loops in the corona,
and others. In particular, we focus on multi-scale processes of energy
exchange across the different layers, which contribute to the corona
heating and eruptive dynamics, as well as interlinks between different
layers of the solar interior and atmosphere. For modeling the
global-scale activity we use the data assimilation approach that has
demonstrated great potential for building reliable long-term forecasts
of solar activity. In particular, it has been shown that the Ensemble
Kalman Filter (EnKF) method applied to the Parker-Kleeorin-Ruzmakin
dynamo model is capable of predicting solar activity up to one
sunspot cycle ahead in time, as well as estimating the properties of
the next cycle a few years before it begins. In this presentation,
using the available magnetogram data, we discuss development of
the methodology and forecast quality criteria (including forecast
uncertainties and sources of errors). We demonstrate the influence
of observational limitation on the prediction accuracy. We present
the EnKF predictions of the upcoming Solar Cycle 25 based on both
the sunspot number series and observed magnetic fields, and discuss
the uncertainties and potential of the data assimilation approach for
modeling and forecasting the solar activity.
Title: On the Origin of Solar Torsional Oscillations and Extended
Solar Cycle
Authors: Pipin, V. V.; Kosovichev, A. G.
Bibcode: 2019ApJ...887..215P
Altcode: 2019arXiv190804525P
We present a nonlinear mean-field model of the solar interior dynamics
and dynamo, which reproduces the observed cyclic variations of the
global magnetic field of the Sun, as well as the differential rotation
and meridional circulation. Using this model, we explain, for the first
time, the extended 22 yr pattern of the solar torsional oscillations,
observed as propagation of zonal variations of the angular velocity
from high latitudes to the equator during the time equal to the full
dynamo cycle. In the literature, this effect is usually attributed to
the so-called “extended solar cycle.” In agreement with the commonly
accepted idea, our model shows that the torsional oscillations can be
driven by a combination of magnetic field effects acting on turbulent
angular momentum transport and the large-scale Lorentz force. We find
that the 22 yr pattern of the torsional oscillations can result from a
combined effect of an overlap of subsequent magnetic cycles and magnetic
quenching of the convective heat transport. The latter effect results in
cyclic variations of the meridional circulation in the sunspot formation
zone, in agreement with helioseismology results. The variations of the
meridional circulation, together with other drivers of the torsional
oscillations, maintain their migration to the equator during the 22
yr magnetic cycle, resulting in the observed extended pattern of the
torsional oscillations.
Title: Characterization of Subsurface Flow Dynamics for Forecasting
of Solar Activity
Authors: Kosovichev, A. G.; Sadykov, V. M.
Bibcode: 2019AGUFMSH31E3347K
Altcode:
Evolution of large-scale magnetic field structures in the solar
photosphere and corona is controlled by motions beneath the visible
surface of the Sun. Subsurface plasma flows play a critical role
in formation and evolution of active regions and their activity. We
analyze subsurface flow maps provided by the local helioseismology
pipeline from the Helioseismic and Magnetic Imager (HMI) data on
board the Solar Dynamics Observatory, and investigate links between
flow characteristics and magnetic activity. The primary goal is
to determine flow descriptors, which can improve solar activity
forecasts. In particular, by employing machine learning classifiers,
we test how the flow helicity and velocity shear descriptors can
improve the prediction of initiation of flares and CME eruptions.
Title: 3D MHD Modeling of the Impact of Subsurface Stratification
on the Solar Dynamo
Authors: Stejko, Andrey M.; Guerrero, Gustavo; Kosovichev, Alexander
G.; Smolarkiewicz, Piotr K.
Bibcode: 2019arXiv191109658S
Altcode:
Various models of solar subsurface stratification are tested in the
global EULAG-MHD solver to simulate diverse regimes of near-surface
convective transport. Sub- and superadiabacity are altered at the
surface of the model ($ r > 0.95~R_{\odot}$) to either suppress or
enhance convective flow speeds in an effort to investigate the impact
of the near-surface layer on global dynamics. A major consequence
of increasing surface convection rates appears to be a significant
alteration of the distribution of angular momentum, especially below
the tachocline where the rotational frequency predominantly increases
at higher latitudes. These hydrodynamic changes correspond to large
shifts in the development of the current helicity in this stable
layer ($r<0.72R_{\odot}$), significantly altering its impact on
the generation of poloidal and toroidal fields at the tachocline and
below, acting as a major contributor towards transitions in the dynamo
cycle. The enhanced near-surface flow speed manifests in a global shift
of the toroidal field ($B_{\phi}$) in the butterfly diagram - from a
North-South symmetric pattern to a staggered anti-symmetric emergence.
Title: What Sets the Magnetic Field Strength and Cycle Period in
Solar-type Stars?
Authors: Guerrero, G.; Zaire, B.; Smolarkiewicz, P. K.; de Gouveia
Dal Pino, E. M.; Kosovichev, A. G.; Mansour, N. N.
Bibcode: 2019ApJ...880....6G
Altcode: 2018arXiv181007978G
Two fundamental properties of stellar magnetic fields have been
determined by observations for solar-like stars with different Rossby
numbers ({{Ro}}), namely, the magnetic field strength and the magnetic
cycle period. The field strength exhibits two regimes: (1) for fast
rotation, it is independent of {{Ro}}, and (2) for slow rotation,
it decays with {{Ro}} following a power law. For the magnetic cycle
period, two regimes of activity, the active and inactive branches,
have also been identified. For both of them, the longer the rotation
period, the longer the activity cycle. Using global dynamo simulations
of solar-like stars with Rossby numbers between ∼0.4 and ∼2, this
paper explores the relevance of rotational shear layers in determining
these observational properties. Our results, consistent with nonlinear
{α }2{{Ω }} dynamos, show that the total magnetic field
strength is independent of the rotation period. Yet at surface levels,
the origin of the magnetic field is determined by {{Ro}}. While for
{{Ro}}≲ 1, it is generated in the convection zone, for {{Ro}}≳ 1,
strong toroidal fields are generated at the tachocline and rapidly
emerge toward the surface. In agreement with the observations, the
magnetic cycle period increases with the rotational period. However,
a bifurcation is observed for {{Ro}}∼ 1, separating a regime where
oscillatory dynamos operate mainly in the convection zone from the
regime where the tachocline has a predominant role. In the latter,
the cycles are believed to result from the periodic energy exchange
between the dynamo and the magneto-shear instabilities developing in
the tachocline and the radiative interior.
Title: Modeling of Subsurface Shear with EULAG-MHD
Authors: Stejko, Andrey Maksimovich; Guerrero, Gustavo; Kosovichev,
Alexander G.
Bibcode: 2019AAS...23430208S
Altcode:
The 3D Global solar convection algorithm EULAG-MHD has recently been
used to recreate solar hydrodynamic profiles of differential rotation
and meridional circulation along with simulation of long-term evolution
of the global magnetic field. In previous studies we saw some of the
large effects that are created by including strong shear layers in the
tachocline and on the surface; changing the period of the solar cycle
from an order of years to that of decades. We attempt to explore the
global consequences of these regions by simulating various levels of
sub/super-adiabaticity in the background potential temperature profiles
- modeled by a polytropic ideal gas with manually controlled polytropic
indeces, simulating the creation or suppression of a near-surface shear
layer (NSSL). We also attempt to understand how the MPDATA algorithm
that is implemented in the model will function under various resolutions
(512 φ, 256 θ, 256 R; 256 φ, 128 θ, 128 R) where an implicit viscous
dissipation is implemented using the implicit Large-Eddy simulation
(ILES) approach for turbulent advective motions - a method that can be
unpredictable as large changes in resolution begin to change scales of
dissipation in low viscosity environments. The simulation of our NSSL
is done at a lower resolution (128 φ, 64 θ, 64 R) that has repeatedly
been shown to adequately simulate solar-like hydrodynamic effects such
as differential rotation, as well as generate well defined cyclical
magnetic dynamo patterns. These models show the NSSL as playing an
important role in the proper distribution of angular momentum and
resulting in more realistic solar-like magnetic and hydrodynamic
patterns.
Title: Long-Term Prediction of Solar Activity Using Magnetogram Data
and Ensemble Kalman Filter
Authors: Kitiashvili, Irina; Kosovichev, Alexander G.
Bibcode: 2019AAS...23440101K
Altcode:
Solar activity predictions using the data assimilation approach have
demonstrated great potential to build reliable long-term forecasts of
solar activity. In particular, it has been shown that the Ensemble
Kalman Filter (EnKF) method applied to a non-linear dynamo model is
capable of predicting solar activity up to one sunspot cycle ahead
in time, as well as estimating the properties of the next cycle a
few years before it begins. These developments assume an empirical
relationship between the mean toroidal magnetic field flux and the
sunspot number. Estimated from the sunspot number series, variations
of the toroidal field have been used to assimilate the data into the
Parker-Kleeorin-Ruzmakin (PKR) dynamo model by applying the EnKF
method. The dynamo model describes the evolution of the toroidal
and poloidal components of the magnetic field and the magnetic
helicity. Full-disk magnetograms provide more accurate and complete
input data by constraining both the toroidal and poloidal global
field components, but these data are available only for the last four
solar cycles. In this presentation, using the available magnetogram
data, we discuss development of the methodology and forecast quality
criteria (including forecast uncertainties and sources of errors). We
demonstrate the influence of limited time series observations on the
accuracy of solar activity predictions. We present EnKF predictions of
the upcoming Solar Cycle 25 based on both the sunspot number series and
observed magnetic fields and discuss the uncertainties and potential
of the data assimilation approach. The research is funded by the NSF
SHINE program AGS-1622341.
Title: Evolution of Magnetic Helicity in Solar Cycle 24
Authors: Pipin, Valery V.; Pevtsov, Alexei A.; Liu, Yang; Kosovichev,
Alexander G.
Bibcode: 2019ApJ...877L..36P
Altcode: 2019arXiv190500772P
We propose a novel approach to reconstruct the surface magnetic
helicity density on the Sun or Sun-like stars. The magnetic vector
potential is determined via decomposition of vector magnetic-field
measurements into toroidal and poloidal components. The method is
verified using data from a non-axisymmetric dynamo model. We apply the
method to vector field synoptic maps from the Helioseismic and Magnetic
Imager on board the Solar Dynamics Observatory to study the evolution
of the magnetic helicity density during solar cycle 24. It is found
that the mean helicity density of the non-axisymmetric magnetic field
of the Sun evolves in a way similar to that reported for the current
helicity density of the solar active regions. It predominantly has a
negative sign in the northern hemisphere, while it is mainly positive
in the southern hemisphere. Also, the hemispheric helicity rule for
the non-axisymmetric magnetic field showed the sign inversion at the
end of cycle 24. The evolution of the magnetic helicity density of
a large-scale axisymmetric magnetic field is different from what is
predicted by dynamo theory. On one hand, the mean large- and small-scale
components of magnetic helicity density display the hemispheric helicity
rule of opposite signs at the beginning of cycle 24. However, later
in the cycle, the two helicities exhibit the same sign, in contrast
with theoretical expectations.
Title: 3D Realistic Modeling of Chromospheric and Coronal Heating
and Self-Organization
Authors: Kitiashvili, Irina; Wray, Alan A.; Kosovichev, Alexander G.;
Sadykov, Viacheslav M.; Mansour, Nagi N.
Bibcode: 2019AAS...23410615K
Altcode:
Turbulent magnetoconvection is a primary driver of the dynamics and
structure of the solar atmosphere and corona. Realistic high-resolution
radiative MHD simulations reveal a complex multiscale structuring
and dynamics above the photosphere. We present a detailed study of
dynamical links between small-scale magnetic fields generated by
local dynamo action and properties of the chromosphere and corona,
as well as effects of coherent self-organized magnetic structures. In
particular, we discuss formation of coherent structures, eruptive
dynamics, and contributions of multi-scale structuring and highly
non-linear dynamics to heating of the chromosphere and corona.
Title: Detection of Dynamo Waves in the Solar Convection Zone by
Helioseismology
Authors: Kosovichev, Alexander G.; Pipin, Valery
Bibcode: 2019AAS...23430703K
Altcode:
Analysis of helioseismology data obtained in 1996-2019 for two solar
cycles from two space missions, Solar and Heliospheric Observatory
(SoHo) and Solar Dynamics Observatory (SDO), reveals that latitudinal
variations of solar rotation ('torsional oscillations') are associated
with hydromagnetic dynamo waves initiated in the solar tachocline
and travelling in radius and latitude towards the surface during the
solar cycles. On the surface, the waves form two branches of zonal
deceleration migrating towards the poles and equator, and coinciding
with the large-scale magnetic field patterns observed in synoptic
magnetograms. The results explain the phenomenon of 'extended solar
cycle', and provide first observational evidence for magnetic dynamo
waves predicted by the Parker's theory of solar activity cycles. We
compare the observational results with dynamic models of the solar
dynamo, and discuss driving mechanisms of the torsional oscillations.
Title: Time-Dependent Hydrodynamic Modeling of Solar Acoustic Waves
Authors: Stefan, John; Kosovichev, Alexander G.
Bibcode: 2019AAS...23430209S
Altcode:
We consider linear perturbations to appropriate hydrodynamic equations,
such as mass continuity and the adiabatic condition. Using the Solar
Model S (Christensen-Dalsgaard 1996- 2014) as mesh, we discretize
the governing equations as well as decompose the various modes
using spherical harmonics. We take advantage of parallel computing
resources by running simulations up to very high modes of order
l=3000. This allows us to model so-called "sunquakes" with fairly high
precision; since the governing equations allow for different types
of perturbations, we aim to determine the location and mechanism of
these impulsive events.
Title: Cluster analysis of spectroscopic line profiles in RMHD
simulations and observations of the solar atmosphere
Authors: Sadykov, Viacheslav M.; Kitiashvili, Irina N.; Kosovichev,
Alexander G.
Bibcode: 2019shin.confE..11S
Altcode:
Spatially-resolved spectroscopic observations from the IRIS space
mission and ground-based telescopes, coupled with realistic 3D RMHD
simulations, are a powerful tool for analysis of processes in the
solar atmosphere. To better understand the dynamical and thermodynamic
properties in the simulation data and their connection to observations,
it is essential to determine similarities in the behaviors of the
synthesized and observed spectral line profiles. In this work, we
utilize realistic 3D RMHD simulations of the solar atmosphere (using
the StellarBox and Bifrost codes) and compute synthetic line profiles of
photospheric (Fe I 6173A), chromospheric (H-alpha 6563A, Ca II h&k
3969A&3934A, Mg II h&k 2796A&2803A), and transition region
(C II 1334A&1335A) lines. Several clustering algorithms (k-Means
clustering, Density-based spatial clustering of applications with
noise) are applied separately to the profiles of each calculated
spectroscopic line, as well as to the multi-line synthetic data. We
discuss application of line profile clustering to visualizations of
the computational volume, understanding its evolutionary trends and
behavior patterns, and inversion (reconstruction) of physical properties
of the solar atmosphere from multi-line spectroscopic data.
Title: Helioseismic Effects of Solar Flares in Cycle 24
Authors: Kosovichev, Alexander; Sharykin, Ivan
Bibcode: 2019shin.confE.204K
Altcode:
Helioseismic flare waves (""sunquakes"") occur due to localized force
or/and momentum impacts observed during the flare impulsive phase in
the lower atmosphere. Such impacts may be caused by precipitation of
high-energy particles, downward traveling shock wave, or by magnetic
Lorentz force associated with plasma eruption. Understanding the
mechanism of sunquakes is a key problem of the flare energy release and
transport. We present results of a statistical study of the helioseismic
response of M-X class solar flares observed during Solar Cycle 24. For
detection of sunquakes, we apply the helioseismic holography technique
to analysis of Dopplergram data from Helioseismic Magnetic Imager
(HMI). Among the total number ( 500) of the M - X class flares, we
found 82 helioseismic events. This result is quite unexpected because
it was previously thought that sunquakes are rare and observed mainly
during strong flares. However, our analysis has shown that there
are many M-class flares, which produced strong sunquakes, while in
some powerful X flares helioseismic waves were not observed or were
weak. The analysis also revealed that during the solar cycle, there
were active regions that are characterized by most efficient generation
of sunquakes. We found that the sunquake power correlates with maximal
values of the GOES X-ray lightcurve time derivative better than with
the GOES X-ray class. The new catalog of helioseismic solar flares
allows us to make new steps for deeper understanding of the mechanism
of sunquakes and the flare physics, in general.
Title: Hydrodynamic Modeling of Impulsive Helioseismic Events
Authors: Stefan, John T.; Kosovichev, Alexander G.
Bibcode: 2019shin.confE..14S
Altcode:
We consider impulsive helioseismic events (sunquakes) as a hydrodynamic
phenomenon, modeling them through linear perturbations to a modified
form of Euler's equations. Realistic wave damping is simulated from
measurements of characteristic decay times, and the resulting waveforms
are compared with observational data to constrain possible excitation
mechanisms.
Title: Statistical Properties of Soft X-Ray Emission of Solar Flares
Authors: Sadykov, Viacheslav M.; Kosovichev, Alexander G.; Kitiashvili,
Irina N.; Frolov, Alexander
Bibcode: 2019ApJ...874...19S
Altcode: 2018arXiv181005610S
We present a statistical analysis of properties of Soft X-Ray (SXR)
emission, plasma temperature (T), and emission measure (EM), derived
from Geostationary Operational Environmental Satellite observations
of flares in 2002-2017. The temperature and EMs are obtained using
the Temperature and EM-based Background Subtraction algorithm, which
delivers reliable results together with uncertainties even for weak
B-class flare events. More than 96% of flares demonstrate a sequential
appearance of T, SXR, and EM maxima, in agreement with the expected
behavior of the chromospheric evaporation process. The relative number
of such flares increases with increasing the SXR flux maximum. The
SXR maximum is closer in time to the T maximum for B-class flares than
for ≥C-class flares, while it is very close to the EM maximum for M-
and X-class flares. We define flares as “T-controlled” if the time
interval between the SXR and T maxima is at least two times shorter than
the interval between the EM and SXR maxima, and as “EM-controlled”
if the time interval between the EM and SXR maxima is at least two
times shorter than the interval between the SXR and T maxima. For
any considered flare class range, the T-controlled events compared
to EM-controlled events have: (a) higher EM but lower T; (b) longer
durations and shorter relative growth times; and (c) longer FWHM and
characteristic decay times. Interpretation of these statistical results
based on analysis of a single loop dynamics suggests that for flares
of the same class range, the T-controlled events can be developed in
longer loops than the EM-controlled events.
Title: The Origin of Deep Acoustic Sources Associated with Solar
Magnetic Structures
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Mansour, N. N.; Wray,
A. A.; Sandstrom, T. A.
Bibcode: 2019ApJ...872...34K
Altcode: 2018arXiv181006133K
It is generally accepted that solar acoustic (p) modes are excited
by near-surface turbulent motions, in particular by downdrafts and
interacting vortices in intergranular lanes. Recent analysis of Solar
Dynamics Observatory data by Zhao et al. (2015) revealed fast-moving
waves around sunspots, which are consistent with magnetoacoustic waves
excited approximately 5 Mm beneath the sunspot. We analyzed 3D radiative
MHD simulations of solar magnetoconvection with a self-organized
pore-like magnetic structure, and identified more than 600 individual
acoustic events both inside and outside this structure. By performing
a case-by-case study, we found that acoustic sources surrounding
the magnetic structure are associated with downdrafts. Their depth
correlates with downdraft speed and magnetic field strength. The
sources often can be transported into deeper layers by downdrafts. The
wave front shape, in the case of a strong or inclined downdraft,
can be stretched along the downdraft. Inside the magnetic structure,
excitation of acoustic waves is driven by converging flows. Frequently,
strong converging plasma streams hit the structure boundaries, causing
compressions in its interior that excite acoustic waves. Analysis
of the depth distribution of acoustic events shows the strongest
concentration at 0.2-1 Mm beneath the surface for the outside sources
and mostly below 1 Mm inside the magnetic region, that is, deeper than
their counterparts outside the magnetic region.
Title: Dynamo Wave Patterns inside of the Sun Revealed by Torsional
Oscillations
Authors: Kosovichev, Alexander G.; Pipin, Valery V.
Bibcode: 2019ApJ...871L..20K
Altcode:
Torsional oscillations represent bands of fast and slow zonal flows
around the Sun, which extend deep into the convection zone and
migrate during solar cycles toward the equator following the sunspot
“butterfly” diagram. Analysis of helioseismology data obtained in
1996-2018 for almost two solar cycles reveals zones of deceleration of
the torsional oscillations inside of the Sun due to dynamo-generated
magnetic field. The zonal deceleration originates near the bottom of
the convection zone at high latitudes, and migrates to the surface
revealing patterns of magnetic dynamo waves predicted by Parker’s
dynamo theory. The analysis reveals that the primary seat of the
solar dynamo is located in a high-latitude zone of the tachocline. It
suggests a dynamo scenario that can explain “extended solar cycles”
previously observed in the evolving shape of the solar corona. The
results show a substantial decrease of the zonal acceleration in the
current solar cycle and indicate a further decline of activity in
the next solar cycle. Although the relationship between the magnitude
of zonal deceleration and the amount of emerged toroidal field that
leads to formation of sunspots is not yet established, the results
reveal a new perspective for solar cycle modeling and prediction using
helioseismology data.
Title: Statistical Study of Chromospheric Evaporation in Impulsive
Phase of Solar Flares
Authors: Sadykov, Viacheslav M.; Kosovichev, Alexander G.; Sharykin,
Ivan N.; Kerr, Graham S.
Bibcode: 2019ApJ...871....2S
Altcode: 2018arXiv180510729S
We present a statistical study of chromospheric evaporation in solar
flares using simultaneous observations by the RHESSI X-ray telescope
and the Interface Region Imaging Spectrograph UV spectrograph. The
results are compared with radiation hydrodynamic flare models from
the F-CHROMA RADYN database. For each event, we study spatially
resolved Doppler shifts of spectral lines formed in the transition
region (C II 1334.5 Å) and hot coronal plasma (Fe XXI 1354.1 Å)
to investigate the dynamics of the solar atmosphere during the
flare impulsive phase. We estimate the energy fluxes deposited by
high-energy electrons using X-ray imaging spectroscopy and assuming
the standard thick-target model. Using the RADYN flare models, the RH
1.5D radiative transfer code, and the Chianti atomic line database,
we calculate C II and Fe XXI line profiles and compare with the
observations. While the RADYN models predict a correlation between
the Doppler shifts and deposited energy flux for both lines, this was
only observed in the C II data. Several quantitative discrepancies are
found between the observations and models: the Fe XXI Doppler shifts
are substantially stronger in the models than in the data, and the
C II mean blueshifts are absent in the observations but predicted
by the models. The transition energies between “gentle” and
“explosive” evaporation regimes estimated from the observations
((2{--}8)× {10}9 erg cm-2 s-1)
and derived from the models ((2.2{--}10.1)× {10}9 erg
cm-2 s-1) are comparable with each other. The
results illustrate relationships among the processes of chromospheric
evaporation, the response of the colder layers, and the flare energy
flux deposited by high-energy electrons, although demonstrating
discrepancy between analyzed observations and RADYN models.
Title: Does Nonaxisymmetric Dynamo Operate in the Sun?
Authors: Pipin, V. V.; Kosovichev, A. G.
Bibcode: 2018ApJ...867..145P
Altcode: 2018arXiv180805332P
We explore effects of random nonaxisymmetric perturbations of kinetic
helicity (the α effect) and diffusive decay of bipolar magnetic
regions on generation and evolution of large-scale nonaxisymmetric
magnetic fields on the Sun. Using a reduced 2D nonlinear mean-field
dynamo model and assuming that bipolar regions emerge due to magnetic
buoyancy in situ of the large-scale dynamo action, we show that
fluctuations of the α effect can maintain the nonaxisymmetric magnetic
fields through a solar-type α 2Ω dynamo process. It is
found that diffusive decay of bipolar active regions is likely to be
the primary source of nonaxisymmetric magnetic fields observed on the
Sun. Our results show that nonaxisymmetric dynamo models with stochastic
perturbations of the α effect can explain periods of extremely high
activity (“super-cycle” events) as well as periods of deep decline
of magnetic activity. We compare the models with synoptic observations
of solar magnetic fields for the last four activity cycles and discuss
implications of our results for interpretation of observations of
stellar magnetic activity.
Title: Roadmap for Reliable Ensemble Forecasting of the Sun-Earth
System
Authors: Nita, Gelu; Angryk, Rafal; Aydin, Berkay; Banda, Juan;
Bastian, Tim; Berger, Tom; Bindi, Veronica; Boucheron, Laura; Cao,
Wenda; Christian, Eric; de Nolfo, Georgia; DeLuca, Edward; DeRosa,
Marc; Downs, Cooper; Fleishman, Gregory; Fuentes, Olac; Gary, Dale;
Hill, Frank; Hoeksema, Todd; Hu, Qiang; Ilie, Raluca; Ireland,
Jack; Kamalabadi, Farzad; Korreck, Kelly; Kosovichev, Alexander;
Lin, Jessica; Lugaz, Noe; Mannucci, Anthony; Mansour, Nagi; Martens,
Petrus; Mays, Leila; McAteer, James; McIntosh, Scott W.; Oria, Vincent;
Pan, David; Panesi, Marco; Pesnell, W. Dean; Pevtsov, Alexei; Pillet,
Valentin; Rachmeler, Laurel; Ridley, Aaron; Scherliess, Ludger; Toth,
Gabor; Velli, Marco; White, Stephen; Zhang, Jie; Zou, Shasha
Bibcode: 2018arXiv181008728N
Altcode:
The authors of this report met on 28-30 March 2018 at the New Jersey
Institute of Technology, Newark, New Jersey, for a 3-day workshop
that brought together a group of data providers, expert modelers, and
computer and data scientists, in the solar discipline. Their objective
was to identify challenges in the path towards building an effective
framework to achieve transformative advances in the understanding
and forecasting of the Sun-Earth system from the upper convection
zone of the Sun to the Earth's magnetosphere. The workshop aimed to
develop a research roadmap that targets the scientific challenge
of coupling observations and modeling with emerging data-science
research to extract knowledge from the large volumes of data (observed
and simulated) while stimulating computer science with new research
applications. The desire among the attendees was to promote future
trans-disciplinary collaborations and identify areas of convergence
across disciplines. The workshop combined a set of plenary sessions
featuring invited introductory talks and workshop progress reports,
interleaved with a set of breakout sessions focused on specific topics
of interest. Each breakout group generated short documents, listing
the challenges identified during their discussions in addition to
possible ways of attacking them collectively. These documents were
combined into this report-wherein a list of prioritized activities
have been collated, shared and endorsed.
Title: Dynamo Wave Patterns Inside the Sun Revealed by Torsional
Oscillations
Authors: Kosovichev, Alexander G.; Pipin, Valery V.
Bibcode: 2018arXiv180910776K
Altcode:
Torsional oscillations represent bands of fast and slow zonal flows
around the Sun, which extend deep into the convection zone and migrate
during solar cycles towards the equator following the sunspot butterfly
diagram. Analysis of helioseismology data obtained in 1996-2018 for
almost two solar cycles reveals zones of deceleration of the torsional
oscillations inside the Sun due to dynamo-generated magnetic field. The
zonal deceleration originates near the bottom of the convection zone
at high latitudes, and migrates to the surface revealing patterns
of magnetic dynamo waves predicted by the Parker's dynamo theory. The
analysis reveals that the primary seat of the solar dynamo is located in
a high-latitude zone of the tachocline. It suggests a dynamo scenario
that can explain 'extended solar cycles' previously observed in the
evolving shape of the solar corona. The results show a substantial
decrease of the zonal acceleration in the current solar cycle and
indicate further decline of activity in the next solar cycle. Although
the relationship between the magnitude of zonal deceleration and the
amount of emerged toroidal field that leads to formation of sunspots
is not yet established, the results open a new perspective for solar
cycle modeling and prediction using helioseismology data.
Title: Onset of Photospheric Impacts and Helioseismic Waves in X9.3
Solar Flare of 2017 September 6
Authors: Sharykin, Ivan N.; Kosovichev, Alexander G.
Bibcode: 2018ApJ...864...86S
Altcode: 2018arXiv180406565S
The X9.3 flare of 2017 September 6, was the most powerful flare of
Solar Cycle 24. It generated strong white-light emission and multiple
helioseismic waves (sunquakes). By using data from the Helioseismic and
Magnetic Imager (HMI) on board the Solar Dynamics Observatory as well
as hard X-ray (HXR) data from the KONUS instrument on board the WIND
spacecraft, and Anti-Coincidence System on board the INTERGRAL space
observatory, we investigate spatio-temporal dynamics of photospheric
emission sources, identify sources of helioseismic waves, and compare
the flare photospheric dynamics with the HXR temporal profiles. The
results show that the photospheric flare impacts started to develop in
compact regions in close vicinity of the magnetic polarity inversion
line (PIL) in the preimpulsive phase before detection of the HXR
emission. The initial photospheric disturbances were localized in the
region of strong horizontal magnetic field of the PIL, and, thus,
are likely associated with a compact sheared magnetic structure
elongated along the PIL. The acoustic egression power maps revealed
two primary sources of generation of sunquakes, which were associated
with places of the strongest photospheric impacts in the preimpulsive
phase and the early impulsive phase. This can explain the two types of
helioseismic waves observed in this flare. Analysis of the high-cadence
HMI filtergrams suggests that the flare energy release developed in
the form of sequential involvement of compact low-lying magnetic loops
that were sheared along the PIL.
Title: Solar cycle variations of rotation and asphericity in the
near-surface shear layer
Authors: Kosovichev, A. G.; Rozelot, J. P.
Bibcode: 2018JASTP.176...21K
Altcode: 2018arXiv180405081K
The precise shape of the Sun is sensitive to the influence of gravity,
differential rotation, local turbulence and magnetic fields. So its
precise measurement is a long-standing astrometric objective. It
has been previously shown by different methods that the solar shape
exhibits asphericity that evolves with the solar cycle. Thanks to the
Michelson Doppler Imager (MDI) on Solar and Heliospheric Observatory
(SoHO) and the Helioseismic and Magnetic Imager (HMI) aboard NASA's
Solar Dynamics Observatory (SDO), and their capability to observe with
an unprecedented accuracy the surface gravity oscillation (f) modes,
it is possible to extract information concerning the coefficients
of rotational frequency splitting, a1 , a3 and
a5 , that measure the latitudinal differential rotation,
together with the a2 , a4 and a6
asphericity coefficients. Analysis of these helioseismology data with
time for almost two solar cycles, from 1996 to 2017, reveals a close
correlation of the a1 and a5 coefficients with
the solar activity, whilst a3 exhibits a long-term trend
and a weak correlation with the solar activity in the current solar
cycle indicating a substantial change of the global solar rotation,
potentially associated with a long-term evolution of the solar
cycles. Looking in more details, the asphericity coefficients,
a2 , a4 and a6 are more strongly
associated with the solar cycle when applying a time lag of respectively
0.1, 1.6 and -1.6 years. The magnitude of a6 -coefficient
varies in phase with the sunspot number (SN), but its amplitude is
ahead of the SN variation. The latest measurements made in mid 2017
indicate that the magnitude of the a6 -coefficient has
probably reached its minimum; therefore, the next solar minimum can be
expected by the end of 2018 or in the beginning of 2019. The so-called
"seismic radius" in the range of f-mode angular degree: ℓ = 137 - 299
exhibits a temporal variability in anti-phase with the solar activity;
its relative value decreased by ∼ 2.3 ×10-5 in Solar
Cycle 23 and ∼ 1.7 ×10-5 in Cycle 24. Such results will
be useful for better understanding the physical mechanisms which act
inside the Sun, and so, better constrain dynamo models for forecasting
the solar cycles.
Title: Modeling Impulsive Helioseismic Events
Authors: Stefan, John Thomas; Kosovichev, Alexander
Bibcode: 2018shin.confE.246S
Altcode:
We consider linear perturbations to appropriate hydrodynamic equations,
such as mass continuity and the adiabatic condition. Using the Solar
Model S (Christensen-Dalsgaard 1996-2014) as mesh, we discretize
the governing equations as well as decompose the various modes using
spherical harmonics. We take advantage of parallel computing resources
by running simulations up to very high modes of order 10E3 . This allows
us to model so-called "sunquakes" with fairly high precision; since
the governing equations allow for different types of perturbations, we
aim to determine the location and mechanism of these impulsive events.
Title: Using Machine-Learning Methods and Expert Prediction
Probabilities to Forecast Solar Flares
Authors: Sadykov, Viacheslav M.; Kosovichev, Alexander G.
Bibcode: 2018shin.confE.144S
Altcode:
It has long been known that studying connection between solar
flares and properties of magnetic field in active regions is very
important for understanding the flare physics and developing space
weather forecasts. The Helioseismic and Magnetic Imager onboard the
Solar Dynamics Observatory (SDO/HMI) obtains tremendous amounts of
the magnetic field data products. However, the operational NOAA
Space Weather Prediction Center (SWPC) forecasts of solar flares
still represent prediction probabilities issued by the experts. In
this research we investigate the possibilities to enhance the daily
operational flare forecasts performed at the SWPC/NOAA by developing
a synergy of the expert predictions and physics-based criteria,
and by employing machine-learning methods. Among the physics-based
criteria, we consider the descriptors of the Polarity Inversion Line
(PIL, previously tested by Sadykov and Kosovichev 2017, ApJ, 849, 148)
and Space weather HMI Active Region Patches (SHARP, Bobra et al. 2014,
SoPh, 289, 3549) and derive from them the daily-cadence characteristics
the entire Sun. We also consider the daily descriptors of the GOES
Soft X-ray 1-8Å flux such as previous day flare history and averaged
X-ray flux. We estimate the effectiveness in separation of flaring and
non-flaring cases for each characteristic, as well as for the expert
prediction probabilities, and find that some PIL and SHARP descriptors
are as effective as the expert prediction probabilities and should be
considered to issue the flare forecast. Finally, we train and test three
Machine-Learning classification algorithms (Support Vector Classifier,
k-Nearest Neighbor Classifier, and Random Forest Classifier) using the
most effective descriptors and expert prediction probabilities, and
compare the received predictions with the current SWPC/NOAA forecasts.
Title: Cyclic Changes of the Sun’s Seismic Radius
Authors: Kosovichev, Alexander; Rozelot, Jean-Pierre
Bibcode: 2018ApJ...861...90K
Altcode: 2018arXiv180509385K
The questions asking whether the Sun shrinks with the solar activity
and what causes this have been a subject of debate. Helioseismology
provides a means to measure with high precision the radial displacement
of subsurface layers, the so-called “seismic radius,” through the
analysis of oscillation frequencies of surface gravity (f) modes. Here,
we present results of a new analysis of 21 years of helioseismology
data from two space missions, the Solar and Heliospheric Observatory
and the Solar Dynamics Observatory, which allow us to resolve previous
uncertainties and compare variations of the seismic radius in two solar
cycles. After removing the f-mode frequency changes associated with
the surface activity, we find that the mean seismic radius is reduced
by 1-2 km during the solar maxima and that most significant variations
of the solar radius occur beneath the visible surface of the Sun at a
depth of about 5 ± 2 Mm, where the radius is reduced by 5-8 km. These
variations can be interpreted as changes in the solar subsurface
structure caused by the predominately vertical ∼10 kG magnetic field.
Title: Helioseismic Observations of Torsional Oscillations Inside
the Sun and Their Potential for Predicting Solar Cycles
Authors: Kosovichev, Alexander G.
Bibcode: 2018shin.confE.152K
Altcode:
The helioseismic analysis of torsional oscillations of the Sun from
the SOHO and SDO, obtained in 1996-2018, reveals the spatio-temporal
dynamics of the solar convection zone, associated with the dynamo
process. The data reveal new relationships between the migrating
magnetic field patterns observed in synoptic magnetograms and
the dynamics of torsional oscillations near the surface and in
the interior. In particular, it is found that the evolution of
torsional oscillations in the deep convection zone is ahead of
the surface magnetic evolution by several years, and that it is
related to the extended solar cycle phenomenon previously observed
in the solar corona. The data show substantial differences in the
torsional oscillation properties between Cycles 23 and 24 indicating
on fundamental changes in the dynamo regime. The helioseismology
observations of the torsional oscillations open new perspectives for
understanding the global dynamo processes inside the Sun, and for
predicting the next solar cycle.
Title: Dynamics of Self-Formed Funnel Structure in 3D Realistic
Simulations of a Quiet-Sun Region
Authors: Kitiashvili, Irina; Wray, Alan A.; Kosovichev, Alexander G.;
Mansour, Nagi Nicolas
Bibcode: 2018tess.conf10629K
Altcode:
Dynamical interaction of the solar convection zone dynamics,
chromosphere and corona is challenging in both observational and
modeling aspects. Because of complex multi-scale interactions of
turbulent MHD flows and structures realistic 3D radiative MHD numerical
simulations are needed to shed light on self-organization processes
of the turbulent magnetic fields, and investigate physical properties
of the solar plasma and dynamical coupling across the layers from the
subphotosphere to the corona. We present 3D MHD realistic simulations
of the quiet-Sun dynamics, which covers upper layers of the convection
zone to 10Mm above the photosphere. The simulations reveal a spontaneous
formation of a self-organized funnel-like structure that extends through
the chromosphere and corona. We will present thermodynamical properties
of the structure, its influence on the dynamics of surrounding areas
of the chromosphere and corona, discuss the formation mechanism, and
compare the simulation results with IRIS, Hinode and SDO observations.
Title: Subsurface Flow Dynamics and Flaring Activity of AR 12673
Authors: Kosovichev, Alexander G.; Sadykov, Viacheslav M.; Sharykin,
Ivan; Zhao, Junwei
Bibcode: 2018tess.conf30605K
Altcode:
The flare-rich AR 12673 is characterized by rapid magnetic flux
emergence, strong shearing and twisting flows, and complex magnetic
topology. We use time-distance helioseismology data from SDO/HMI to
investigate relationships among the subsurface dynamics, emergence,
evolution and flaring activity of this active region. The subsurface
flow maps are obtained with 1-hour cadence in the depth range of
0-20 Mm during August 29 - September 9, 2017, covering the whole AR
passage on the disk, from its emergence to the period of high flaring
activity. Using the maps, we calculate the flow characteristics:
divergence, vorticity, shear rate and helicity, and their correlations
with the GOES X-ray flux, as well as with global and local magnetic
field properties, including the vector magnetogram descriptors from
the SHARP database, characteristics of the Polarity Inversion Line
(PIL) and reconstructed topological properties (flux of high-twist
magnetic field lines and free-energy excess). Among the subsurface
flow characteristics, the highest correlation with the X-ray flux (with
correlation coefficient 0.82) is found for the total unsigned kinetic
helicity in the depth range 1.2-7.5 Mm beneath the PIL region. We
discuss relationship of the flow and magnetic field characteristics to
the onset of major flares of AR 12673, and compare the subsurface flow
patterns with those observed for AR 10486 that produced the largest
flares in the declining phase of the previous solar cycle.
Title: Effects of Distributed Magnetic Fields and Compact Magnetic
Structures on Properties of Acoustic Waves Excitation on the Sun
Authors: Kitiashvili, Irina; Kosovichev, Alexander G.; Wray, Alan A.;
Mansour, Nagi Nicolas
Bibcode: 2018tess.conf11504K
Altcode:
Recent helioseismology interferences have shown possibility of acoustic
waves excitation in the subsurface layers much deeper than 200 km. Using
3D radiative MHD numerical simulations, we investigate acoustic wave
excitation in the case of distributed magnetic field and spontaneously
formed highly magnetized pore-like structures, and show that in the
presence of strong magnetic field structures acoustic waves can be
excited much deeper than in the quiet-Sun regions. The distribution
of acoustic events with depth depends on the magnetic field scale. In
particular, in the case of small-scale magnetic patches the acoustic
events are located in a relative shallow, 1.5 Mm deep layer, while
the acoustic sources located inside of a self-organized pore-like
magnetic structure can be found up to 3-4Mm below the surface. We
discuss the excitation mechanism, and present an explanation of recent
helioseismology observations of deep acoustic sources in sunspots.
Title: Intelligent Database of Solar Events and Active Regions
(IDSEAR)
Authors: Sadykov, Viacheslav M.; Kosovichev, Alexander G.; Nita,
Gelu M.; Oria, Vincent; Wang, Wei
Bibcode: 2018tess.conf30713S
Altcode:
The growing needs for the accurate Space Weather Forecasts motivate
the researchers to implement comprehensive prediction algorithms
for solar transient events (flares, coronal mass ejections,
filament eruptions). However, each prediction attempt will face
the data preparation and processing phase, which usually takes
a significant part of the research time. Besides some attempts
(e.g. SDO SHARP archive), currently there are no databases
containing processed descriptors of the Active Regions (AR) and
related observations of flares and eruptive events. We develop an
Intelligent Database of Solar Events and Active Regions (IDSEAR)
which is the continuation of the Interactive Multi-Instrument Database
of Solar Flares (IMIDSF, <a href="https://solarflare.njit.edu/"
https://solarflare.njit.edu/</a>) previously developed by
our team. The IDSEAR will have full IMIDSF functionality (search of
solar flares based on their physical descriptors) extended to the
integration of Solar Events, ARs and related observations, which will
allow users to make queries using descriptors of ARs and solar events
linked to the ARs. In addition to the commonly-used AR descriptors,
we plan to add new physical data products (NLFFF extrapolations and
local helioseismology subsurface flow maps) and their descriptors, as
well as operational data from SWPC NOAA (to include expert estimates in
machine-learning procedures). The structure of the IDSEAR is scalable
and allows addition of any descriptors and data products via the user
contribution system (UCS). We envision that the developed database
will allow the researchers to significantly speed up data processing
and preparation for statistical analysis and physics-based prediction
of solar events, as well as get access to and share high-quality
scientific data products.
Title: Testing the Standard Model of Solar Flares through a
Statistical Study of Soft X-ray Emission Properties from GOES
observations
Authors: Sadykov, Viacheslav M.; Kosovichev, Alexander G.
Bibcode: 2018tess.conf40804S
Altcode:
We present a statistical analysis of properties of Soft X-Ray
(SXR) emission, plasma Temperatures (T) and Emission Measures (EM),
derived from GOES observations of flares for 2002-2017 time period,
and compare with qualitative predictions of the standard thick-target
flare model. Temperatures and emission measures are obtained using the
TEBBS algorithm (Ryan et al. 2012), which delivers reliable results
even for weak B-class flare events. For each event, we compute
the maxima of SXR, SXR derivative, T and EM, as well as their time
delays with respect to each other. Our study of distributions of
these parameters for the flares of different GOES classes leads to
the following findings. First, more than 96% of flares demonstrate
the sequential appearance of T, SXR, and EM maxima, which agrees with
expected behavior of the chromospheric evaporation process predicted
by radiative hydrodynamics simulations. The relative number of such
flares increases with the increase of the GOES class. Second, the
SXR maxima occur closer to the T maxima for the B-class flares, but
closer to the EM maxima for the M-class flares, demonstrating that the
different physical parameters (T and EM correspondingly) contribute
differently to the SXR emission of strong and weak flares. Third,
we selected two subgroups of C-class flares (one with dominating T
contribution to SXR, the other with dominating EM contribution) and
found no difference in distribution of their GOES classes, T and EM
maxima. However, the events with dominating EM contribution are 30%
longer and grow 72% slower in average. We discuss such statistical
relationships in terms of the standard flare model, and, in particular,
in terms of the influence of electron beam heating parameters on the
chromospheric evaporation rates.
Title: Machine-Learning Approach for Solar Flare Forecasts Based
on Automatic Identification and Characterization of Magnetic Field
Polarity Inversion Line
Authors: Sadykov, Viacheslav M.; Kosovichev, Alexander G.
Bibcode: 2018tess.conf32202S
Altcode:
It has long been known that major flares and eruptions are associated
with polarity inversion line (PIL) in regions of strong magnetic field
(such as in delta-type sunspots). However, what physical characteristics
of PIL are the primary factors, which define the flare triggering
mechanism and energetics, is still an open question. We analyze the
relationship between the flare X-ray peak flux, and characteristics
of the PIL and active regions (ARs), derived from line-of-sight (LOS)
magnetograms. The automatic PIL detection procedure is based on a
magnetogram segmentation algorithm using a variational principle. The
procedure is applied to SDO/HMI magnetograms to calculate a set of the
PIL physical characteristics for each AR with 1 hr cadence. The PIL
characteristics are associated with the AR flare history and divided
into flaring and nonflaring cases. Effectiveness of the derived
characteristics for flare forecasting is determined by the number of
nonflaring cases separated from flaring cases by a certain threshold,
and by their Fisher ranking score. The Support Vector Machine (SVM)
classifier trained only on the PIL characteristics is used for the
flare prediction. We have obtained the following results: (1) the PIL
characteristics are more effective than global characteristics of ARs,
(2) the highest True Skill Statistics (TSS) values of 0.76 ± 0.03
for ≥M1.0 flares and 0.84 ± 0.07 for ≥X1.0 flares are obtained
using the "Sigmoid" SVM kernel, (3) the TSS scores obtained using only
the LOS magnetograms are slightly lower than the scores obtained using
vector magnetograms, but significantly better than current expert-based
predictions, (4) for prediction of ≥M1.0 class flares 74.4% of all
cases, and 91.2% for ≥X1.0 class, can be pre-classified as negative
with no significant effect on the results, (5) the inclusion of global
AR characteristics does not improve the forecast. The study confirms the
unique role of the PIL region characteristics in the flare initiation
process and demonstrates possibilities for operational flare forecasting
using only the LOS magnetograms. We also discuss the extension of the
proposed algorithm for prediction of the flare onset times and GOES
classes (and other physical parameters).
Title: How big is the Sun: Solar diameter changes over time
Authors: Rozelot, J. P.; Kosovichev, A. G.; Kilcik, A.
Bibcode: 2018SunGe..13...63R
Altcode: 2018arXiv180406930R
The measurement of the Sun's diameter has been first tackled by the
Greek astronomers from a geometric point of view. Their estimation of
? 1800?, although incorrect, was not truly called into question for
several centuries. The first pioneer works for measuring the Sun's
diameter with an astrometric precision were made around the year 1660
by Gabriel Mouton, then by Picard and La Hire. A canonical value of
the solar radius of 959?.63 was adopted by Auwers in 1891. In spite
of considerable efforts during the second half of the XXth century,
involving dedicated space instruments, no consensus was reached on
this issue. However, with the advent of high sensitivity instruments
on board satellites, such as the Michelson Doppler Imager (MDI) on
Solar and Heliospheric Observatory (SoHO) and the Helioseismic and
Magnetic Imager (HMI) aboard NASA's Solar Dynamics Observatory (SDO),
it was possible to extract with an unprecedented accuracy the surface
gravity oscillation ? modes, over nearly two solar cycles, from 1996 to
2017. Their analysis in the range of angular degree l = 140 - 300 shows
that the so-called "seismic radius" exhibits a temporal variability in
anti-phase with the solar activity. Even if the link between the two
radii (photospheric and seismic) can be made only through modeling,
such measurements provide an interesting alternative which led to a
revision of the standard solar radius by the International Astronomical
Union in 2015. This new look on such modern measurements of the Sun's
global changes from 1996 to 2017 gives a new way for peering into
the solar interior, mainly to better understand the subsurface fields
which play an important role in the implementation of the solar cycles.
Title: A brief history of the solar diameter measurements: a critical
quality assessment of the existing data
Authors: Rozelot, Jean Pierre; Kosovichev, Alexander G.; Kilcik, Ali
Bibcode: 2018vsss.book...89R
Altcode: 2016arXiv160902710R
The size of the diameter of the Sun has been debated for a very
long time. First tackled by the Greek astronomers from a geometric
point of view, an estimate, although incorrect, has been determined,
not truly called into question for several centuries. The French
school of astronomy, under the impetus of Mouton and Picard in the
XVIIth century can be considered as a pioneer in this issue. It was
followed by the German school at the end of the XIXth century whose
works led to a canonical value established at 959".63 (second of
arc). A number of ground-based observations has been made in the
second half of the XIXth century leading to controversial results
mainly due to the difficulty to disentangle between the solar and
atmospheric effects. Dedicated space measurements yield to a very
faint dependence of the solar diameter with time. New studies over
the entire radiation spectrum lead to a clear relationship between
the solar diameter and the wavelength, reflecting the height at which
the lines are formed. Thus the absolute value of the solar diameter,
which is a reference for many astrophysical applications, must be
stated according to the wavelength. Furthermore, notable features
of the Near Sub-Surface Layer (NSSL), called the leptocline, can be
established in relation to the solar limb variations, mainly through
the shape asphericities coefficients. The exact relationship has not
been established yet, but recent studies encourage further in-depth
investigations of the solar subsurface dynamics, both observationally
and by numerical MHD simulations.
Title: Realistic Simulations of Stellar Radiative MHD
Authors: Wary, Alan A.; Bensassiy, Khalil; Kitiashvili, Irina N.;
Mansour, Nagi N.; Kosovichev, Alexander G.
Bibcode: 2018vsss.book...39W
Altcode:
No abstract at ADS
Title: Local Helioseismology of Emerging Active Regions: A Case Study
Authors: Kosovichev, Alexander G.; Zhao, Junwei; Ilonidis, Stathis
Bibcode: 2018vsss.book...15K
Altcode: 2016arXiv160704987K
Local helioseismology provides a unique opportunity to investigate the
subsurface structure and dynamics of active regions and their effect
on the large-scale flows and global circulation of the Sun. We use
measurements of plasma flows in the upper convection zone, provided
by the Time-Distance Helioseismology Pipeline developed for analysis
of solar oscillation data obtained by Helioseismic and Magnetic
Imager (HMI) on Solar Dynamics Observatory (SDO), to investigate the
subsurface dynamics of emerging active region NOAA 11726. The active
region emergence was detected in deep layers of the convection zone
about 12 hours before the first bipolar magnetic structure appeared on
the surface, and 2 days before the emergence of most of the magnetic
flux. The speed of emergence determined by tracking the flow divergence
with depth is about 1.4 km/s, very close to the emergence speed in
the deep layers. As the emerging magnetic flux becomes concentrated
in sunspots local converging flows are observed beneath the forming
sunspots. These flows are most prominent in the depth range 1-3 Mm,
and remain converging after the formation process is completed. On the
larger scale converging flows around active region appear as a diversion
of the zonal shearing flows towards the active region, accompanied
by formation of a large-scale vortex structure. This process occurs
when a substantial amount of the magnetic flux emerged on the surface,
and the converging flow pattern remains stable during the following
evolution of the active region. The Carrington synoptic flow maps
show that the large-scale subsurface inflows are typical for active
regions. In the deeper layers (10-13 Mm) the flows become diverging,
and surprisingly strong beneath some active regions. In addition,
the synoptic maps reveal a complex evolving pattern of large-scale
flows on the scale much larger than supergranulation
Title: Influence of stellar radiation pressure on flow structure in
the envelope of hot-Jupiter HD 209458b
Authors: Cherenkov, A. A.; Bisikalo, D. V.; Kosovichev, A. G.
Bibcode: 2018MNRAS.475..605C
Altcode:
Close-in exoplanets are subjected to extreme radiation of their
host stars. Photometric observations of the hot-Jupiter HD 209458b
transit by HST/STIS detected strong absorption in the Ly α line, thus
indicating the existence of a hydrogen envelope extending beyond the
Roche lobe. The gasdynamic modelling (Bisikalo et al.) showed that the
stable structure of this envelope is maintained by the balance between
the Roche lobe overfilling and stellar wind pressure. Obviously, the
dynamics and stability of the envelope can be affected by stellar
radiation pressure. Using 3D gasdynamic simulations, we study the
impact of radiation pressure in the Ly α line on the envelope of
hot-Jupiter HD 209458b, and show that the effect is not strong enough
to significantly affect the gasdynamics in the system. For a detectable
radiation pressure effect the intensity of the Ly α line has to be
by two orders of magnitude greater.
Title: On the Origin of the Double-cell Meridional Circulation in
the Solar Convection Zone
Authors: Pipin, V. V.; Kosovichev, A. G.
Bibcode: 2018ApJ...854...67P
Altcode: 2017arXiv170803073P
Recent advances in helioseismology, numerical simulations and
mean-field theory of solar differential rotation have shown that the
meridional circulation pattern may consist of two or more cells in
each hemisphere of the convection zone. According to the mean-field
theory the double-cell circulation pattern can result from the sign
inversion of a nondiffusive part of the radial angular momentum
transport (the so-called Λ-effect) in the lower part of the solar
convection zone. Here, we show that this phenomenon can result from
the radial inhomogeneity of the Coriolis number, which depends on
the convective turnover time. We demonstrate that if this effect is
taken into account then the solar-like differential rotation and
the double-cell meridional circulation are both reproduced by the
mean-field model. The model is consistent with the distribution of
turbulent velocity correlations determined from observations by tracing
motions of sunspots and large-scale magnetic fields, indicating that
these tracers are rooted just below the shear layer.
Title: 3D Realistic Radiative Hydrodynamic Modeling of a Moderate-Mass
Star: Effects of Rotation
Authors: Kitiashvili, Irina; Kosovichev, Alexander G.; Mansour,
Nagi N.; Wray, Alan A.
Bibcode: 2018AAS...23133404K
Altcode:
Recent progress in stellar observations opens new perspectives in
understanding stellar evolution and structure. However, complex
interactions in the turbulent radiating plasma together with effects
of magnetic fields and rotation make inferences of stellar properties
uncertain. The standard 1D mixing-length-based evolutionary models
are not able to capture many physical processes of stellar interior
dynamics, but they provide an initial approximation of the stellar
structure that can be used to initialize 3D time-dependent radiative
hydrodynamics simulations, based on first physical principles, that
take into account the effects of turbulence, radiation, and others. In
this presentation we will show simulation results from a 3D realistic
modeling of an F-type main-sequence star with mass 1.47 Msun, in which
the computational domain includes the upper layers of the radiation
zone, the entire convection zone, and the photosphere. The simulation
results provide new insight into the formation and properties of the
convective overshoot region, the dynamics of the near-surface, highly
turbulent layer, the structure and dynamics of granulation, and the
excitation of acoustic and gravity oscillations. We will discuss the
thermodynamic structure, oscillations, and effects of rotation on the
dynamics of the star across these layers.
Title: Helioseismic Observations of Two Solar Cycles and Constraints
on Dynamo Theory
Authors: Kosovichev, Alexander
Bibcode: 2018AAS...23131505K
Altcode:
Helioseismology data from the SOHO and SDO, obtained in 1996-2017 for
almost two solar cycles, provide a unique opportunity to investigate
variations of the solar interior structure and dynamics, and link these
variations to the current dynamo models and simulations. The solar
oscillation frequencies and frequency splitting of medium-degree p- and
f-modes, as well as helioseismic inversions have been used to analyze
variations of the differential rotation (“torsional oscillations”)
and the global asphericity. By comparing the helioseismology results
with the synoptic surface magnetic fields we identify characteristic
changes associated the initiation and evolution of the solar cycles,
23 and 24. The observational results are compared with the current
mean-field dynamo models and 3D MHD dynamo simulations. It is shown
that the helioseismology inferences provide important constraints on
the dynamics of the tachocline and near-surface shear layer, and also
may explain the fundamental difference between the two solar cycles
and detect the onset of the next cycle.
Title: Helioseismology Observations of Solar Cycles and Dynamo
Modeling
Authors: Kosovichev, A. G.; Guerrero, G.; Pipin, V.
Bibcode: 2017AGUFMSH12A..04K
Altcode:
Helioseismology observations from the SOHO and SDO, obtained in
1996-2017, provide unique insight into the dynamics of the Sun's
deep interior for two solar cycles. The data allow us to investigate
variations of the solar interior structure and dynamics, and compare
these variations with dynamo models and simulations. We use results
of the local and global helioseismology data processing pipelines
at the SDO Joint Science Operations Center (Stanford University) to
study solar-cycle variations of the differential rotation, meridional
circulation, large-scale flows and global asphericity. By comparing
the helioseismology results with the evolution of surface magnetic
fields we identify characteristic changes associated the initiation and
development of Solar Cycles 23 and 24. For the physical interpretation
of observed variations, the results are compared with the current
mean-field dynamo models and 3D MHD dynamo simulations. It is shown
that the helioseismology inferences provide important constraints on
the solar dynamo mechanism, may explain the fundamental difference
between the two solar cycles, and also give information about the next
solar cycle.
Title: Solar activity across the scales: from small-scale quiet-Sun
dynamics to magnetic activity cycles
Authors: Kitiashvili, I.; Collins, N.; Kosovichev, A. G.; Mansour,
N. N.; Wray, A. A.
Bibcode: 2017AGUFMSH13A2466K
Altcode:
Observations as well as numerical and theoretical models show that
solar dynamics is characterized by complicated interactions and
energy exchanges among different temporal and spatial scales. It
reveals magnetic self-organization processes from the smallest scale
magnetized vortex tubes to the global activity variation known as
the solar cycle. To understand these multiscale processes and their
relationships, we use a two-fold approach: 1) realistic 3D radiative MHD
simulations of local dynamics together with high-resolution observations
by IRIS, Hinode, and SDO; and 2) modeling of solar activity cycles by
using simplified MHD dynamo models and mathematical data assimilation
techniques. We present recent results of this approach, including
the interpretation of observational results from NASA heliophysics
missions and predictive capabilities. In particular, we discuss the
links between small-scale dynamo processes in the convection zone and
atmospheric dynamics, as well as an early prediction of Solar Cycle 25.
Title: Relationship between Hard X-Ray Footpoint Sources and
Photospheric Electric Currents in Solar Flares: a Statistical Study
Authors: Zimovets, I. V.; Sharykin, I. N.; Wang, R.; Liu, Y. D.;
Kosovichev, A. G.
Bibcode: 2017AGUFMSH41A2751Z
Altcode:
It is believed that solar flares are a result of release of free
magnetic energy contained in electric currents (ECs) flowing in active
regions (ARs). However, there are still debates whether the primary
energy release and acceleration of electrons take place in coronal
current sheets or in chromospheric footpoints of current-carrying
magnetic flux tubes (loops). We present results of an observational
statistical study of spatial relationship between hard X-ray (HXR;
EHXR≥50keV) footpoint sources detected by RHESSI and vertical
photospheric ECs calculated using vector magnetograms obtained from the
SDO/HMI data. We found that for a sample of 47 flares (from C3.0 to X3.1
class) observed on the solar disk by both instruments in 2010-2016,
at least one HXR source was in a region of strong (within 20% of the
maximum EC density in the corresponding ARs) vertical ECs having the
form of a ribbon (79%) or an island (21%). The total vertical ECs in
such HXR sources are in the range of 1010-1013 A. The EC density is in
the range of 0.01-1.0 A/m2. We found no correlation between intensity
of the HXR sources and the EC density. By comparing pre-flare and
post-flare EC maps we did not find evidences of significant dissipation
of vertical ECs in the regions corresponding to the HXR sources. In
some cases, we found amplification of ECs during flares. We discuss
effects of sensitivity and angular resolution of RHESSI and SDO/HMI. In
general, the results indicate that there is a link between the flare HXR
footpoint sources and enhanced vertical ECs in the photosphere. However,
the results do not support a concept of electron acceleration by the
electric field excited in footpoints of current-carrying loops due
to some (e.g. Rayleigh-Taylor) instabilities (Zaitsev et al., 2016),
since strong correlation between the HXR intensity and the EC density
is expected in such concept.
Title: Reduction of mass loss by the hot Jupiter WASP-12b due to
its magnetic field
Authors: Arakcheev, A. S.; Zhilkin, A. G.; Kaigorodov, P. V.; Bisikalo,
D. V.; Kosovichev, A. G.
Bibcode: 2017ARep...61..932A
Altcode:
The influence of the dipolar magnetic field of a "hot Jupiter" with
the parameters of the object WASP-12b on the mass-loss rate from
its atmosphere is investigated. The results of three-dimensional
gas-dynamical and magnetohydrodynamical computations show that the
presence of a magnetic moment with a strength of 0.1 the magnetic
moment of Jupiter leads to appreciable variations of the matter
flow structure. For example, in the case of the exoplanet WASP-12b
with its specified set of atmospheric parameters, the stream from
the vicinity of the Lagrange point L1 is not stopped
by the dynamical pressure of the stellar wind, and the envelope
remains open. Including the effect of the magnetic field leads to
a variation in this picture—the atmosphere becomes quasi-closed,
with a characteristic size of order 14 planetary radii, which, in turn,
substantially decreases the mass-loss rate by the exoplanet atmosphere
(by 70%). This reduction of the mass-loss rate due to the influence of
the magnetic fieldmakes it possible for exoplanets to form closed and
quasi-closed envelopes in the presence of more strongly overflowing
Roche lobes than is possible without a magnetic field.
Title: Relationships between Characteristics of the Line-of-sight
Magnetic Field and Solar Flare Forecasts
Authors: Sadykov, Viacheslav M.; Kosovichev, Alexander G.
Bibcode: 2017ApJ...849..148S
Altcode: 2017arXiv170403423S
We analyze the relationship between the flare X-ray peak flux,
and characteristics of the polarity inversion line (PIL) and active
regions (ARs), derived from line-of-sight (LOS) magnetograms. The PIL
detection algorithm based on a magnetogram segmentation procedure is
applied for each AR with 1 hr cadence. The PIL and AR characteristics
are associated with the AR flare history and divided into flaring
and nonflaring cases. Effectiveness of the derived characteristics
for flare forecasting is determined by the number of nonflaring cases
separated from flaring cases by a certain threshold, and by their Fisher
ranking score. The Support Vector Machine (SVM) classifier trained only
on the PIL characteristics is used for the flare prediction. We have
obtained the following results: (1) the PIL characteristics are more
effective than global characteristics of ARs, (2) the highest True
Skill Statistics (TSS) values of 0.76 ± 0.03 for ≥M1.0 flares and
0.84 ± 0.07 for ≥X1.0 flares are obtained using the “Sigmoid”
SVM kernel, (3) the TSS scores obtained using only the LOS magnetograms
are slightly lower than the scores obtained using vector magnetograms,
but significantly better than current expert-based predictions, (4)
for prediction of ≥M1.0 class flares 74.4% of all cases, and 91.2%
for ≥X1.0 class, can be pre-classified as negative with no significant
effect on the results, (5) the inclusion of global AR characteristics
does not improve the forecast. The study confirms the unique role
of the PIL region characteristics in the flare initiation process,
and demonstrates possibilities of flare forecasting using only the
LOS magnetograms.
Title: Are tachoclines important for solar and stellar dynamos? What
can we learn from global simulations
Authors: Guerrero, G.; Smolarkiewicz, P. K.; de Gouveia Dal Pino,
E. M.; Kosovichev, A. G.; Zaire, B.; Mansour, N. N.
Bibcode: 2017IAUS..328...61G
Altcode:
The role of tachoclines, the thin shear layers that separate solid
body from differential rotation in the interior of late-type stars, in
stellar dynamos is still controversial. In this work we discuss their
relevance in view of recent results from global dynamo simulations
performed with the EULAG-MHD code. The models have solar-like
stratification and different rotation rates (i.e., different Rossby
number). Three arguments supporting the key role of tachoclines are
presented: the solar dynamo cycle period, the origin of torsional
oscillations and the scaling law of stellar magnetic fields as function
of the Rossby number. This scaling shows a regime where the field
strength increases with the rotation and a saturated regime for fast
rotating stars. These properties are better reproduced by models that
consider the convection zone and a fraction of the radiative core,
naturally developing a tachocline, than by those that consider only
the convection zone.
Title: Fine Structure and Dynamics of the Solar Atmosphere
Authors: Vargas Domínguez, S.; Kosovichev, A. G.; Antolin, P.;
Harra, L.
Bibcode: 2017IAUS..327.....V
Altcode:
No abstract at ADS
Title: Magnetic field generation in PMS stars with and without
radiative core
Authors: Zaire, B.; Guerrero, G.; Kosovichev, A. G.; Smolarkiewicz,
P. K.; Landin, N. R.
Bibcode: 2017IAUS..328...30Z
Altcode: 2017arXiv171102057Z
Recent observations of the magnetic field in pre-main sequence stars
suggest that the magnetic field topology changes as a function of
age. The presence of a tachocline could be an important factor in the
development of magnetic field with higher multipolar modes. In this
work we performed MHD simulations using the EULAG-MHD code to study the
magnetic field generation and evolution in models that mimic stars at
two evolutionary stages. The stratification for both stellar phases was
computed by fitting stellar structure profiles obtained with the ATON
stellar evolution code. The first stage is at 1.1Myr, when the star
is completely convective. The second stage is at 14Myrs, when the star
is partly convective, with a radiative core developed up to 30% of the
stellar radius. In this proceedings we present a preliminary analysis
of the resulting mean-flows and magnetic field. The mean-flow analysis
shown that the star rotate almost rigidly on the fully convective
phase, whereas at the partially convective phase there is differential
rotation with conical contours of iso-rotation. As for the mean magnetic
field both simulations show similarities with respect to the field
evolution. However, the topology of the magnetic field is different.
Title: Exploring shallow sunspot formation by using Implicit
Large-eddy simulations
Authors: Camacho, F. J.; Guerrero, G.; Smolarkiewicz, P. K.;
Kosovichev, A. G.; Mansour, N. N.
Bibcode: 2017IAUS..328..117C
Altcode:
The mechanism by which sunspots are generated at the surface of
the sun remains unclear. In the current literature two types of
explanations can be found. The first one is related to the buoyant
emergence of toroidal magnetic fields generated at the tachocline. The
second one states that active regions are formed, from initially
diffused magnetic flux, by MHD instabilities that develop in the
near-surface layers of the Sun. Using the anelastic MHD code EULAG
we address the problem of sunspot formation by performing implicit
large-eddy simulations of stratified magneto-convection in a domain
that resembles the near-surface layers of the Sun. The development
of magnetic structures is explored as well as their effect on the
convection dynamics. By applying a homogeneous magnetic field over an
initially stationary hydrodynamic convective state, we investigate
the formation of self-organized magnetic structures in the range of
the initial magnetic field strength, 0.01 < B 0/B
eq < 0.5, where B eq is the characteristic
equipartition field strength.
Title: Initiation and chromospheric effects of a M1.0 class solar
flare from high-resolution multi-wavelength observations
Authors: Sadykov, V. M.; Kosovichev, A. G.; Sharykin, I. N.; Zimovets,
I. V.; Vargas Dominguez, S.
Bibcode: 2017IAUS..327..103S
Altcode:
Initiation and development of a M 1.0 class flare of June 12, 2014, was
observed by space and ground-based telescopes, including EUV and X-ray
imaging spectroscopy by IRIS and RHESSI, and high-resolution optical
imaging by 1.6 m New Solar Telescope (NST). Analyzing the NST data,
we found small-scale loop-like structures in the region of the magnetic
field Polarity Inversion Line (PIL), the emergence and interaction of
which caused photospheric brightenings temporarily coinciding with hard
X-ray impulses. Detailed studies of the PIL region reveal signatures of
photospheric plasma downflows and dissipation of electric currents. The
reconstructed magnetic field topology shows a bundle of lines connecting
the PIL region with the flare ribbons which were places of chromospheric
evaporation observed by IRIS. The observations suggest a scenario with
the primary energy release processes located in the low atmospheric
layers of the PIL, energizing the overlying large-scale magnetic
structure and causing ``gentle'' chromospheric evaporation.
Title: A Look on the Solar Diameter Data Analysis over the Centuries
Authors: Rozelot, J. P.; Kosovichev, A. G.; Kilcik, A.
Bibcode: 2017simi.conf...70R
Altcode:
The measurement of the Sun's diameter has been first tackled by the
Greek astronomers from a geometric point of view. Their estimation of
≈ 1800″, although incorrect, was not truly called into question
for several centuries. The first pioneer works for measuring the Sun's
diameter with an astrometric precision were made around the year 1660
by Gabriel Mouton, then by Picard and La Hire. A canonical value of
the solar radius of 959″.63 was adopted by Auwers in 1891. In spite
of considerable efforts during the second half of the XXth century,
involving dedicated space instruments, no consensus was reached on
this issue. However, with the advent of high sensitivity instruments
on board satellites, such as the Michelson Doppler Imager (MDI)
on Solar and Heliospheric Observatory (SoHO) and the Helioseismic
and Magnetic Imager (HMI) aboard NASA's Solar Dynamics Observatory
(SDO), it was possible to extract with an unprecedented accuracy the
surface gravity oscillation f modes, over nearly two solar cycles,
from 1996 to 2017. Their analysis in the range of angular degree =
140 300 shows that the so-called "seismic radius" exhibits a temporal
variability in antiphase with the solar activity; This new look on
such modern measurements of the Sun's global changes from 1996 to 2017
gives a new way for peering into the solar interior.
Title: Characteristics of the Polarity Inversion Line and Solar
Flare Forecasts
Authors: Sadykov, Viacheslav M.; Kosovichev, Alexander G.
Bibcode: 2017SPD....4811601S
Altcode:
Studying connection between solar flares and properties of magnetic
field in active regions is very important for understanding the flare
physics and developing space weather forecasts. In this work, we analyze
relationship between the flare X-ray peak flux from the GOES satellite,
and characteristics of the line-of-sight (LOS) magnetograms obtained
by the SDO/HMI instrument during the period of April, 2010 - June,
2016. We try to answer two questions: 1) What characteristics of
the LOS magnetic field are most important for the flare initiation
and magnitude? 2) Is it possible to construct a reliable forecast
of ≥ M1.0 and ≥ X1.0 class flares based only on the LOS magnetic
field characteristics? To answer these questions, we apply a Polarity
Inversion Line (PIL) detection algorithm, and derive various properties
of the PIL and the corresponding Active Regions (AR). The importance
of these properties for flare forecasting is determined by their
ability to separate flaring cases from non-flaring, and their Fisher
ranking score. It is found that the PIL characteristics are of special
importance for the forecasts of both ≥ M1.0 and ≥ X1.0 flares,
while the global AR characteristics become comparably discriminative
only for ≥ X1.0 flares. We use the Support Vector Machine (SVM)
classifier and train it on the six characteristics of the most
importance for each case. The obtained True Skill Statistics (TSS)
values of 0.70 for ≥ M1.0 flares and 0.64 for ≥ X1.0 flares are
better than the currently-known expert-based predictions. Therefore,
the results confirm the importance of the LOS magnetic field data and,
in particular, the PIL region characteristics for flare forecasts.
Title: Analysis of Chromospheric Evaporation in Solar Flares
Authors: Sadykov, Viacheslav M.; Kosovichev, Alexander G.
Bibcode: 2017SPD....4810817S
Altcode:
Chromospheric evaporation is one of the key processes of solar
flares. Properties of chromospheric evaporation are thought to be
closely connected to the energy release rates and energy transport
mechanisms. Previous investigations revealed that in addition to
electron-beam heating the chromospheric evaporation can be driven
by heat fluxes and, probably, by other mechanisms. In this work,
we present a study of flare events simultaneously observed by IRIS,
SDO and RHESSI, focusing on spatio-temporal characteristics of the
flare dynamics and its relation to the magnetic field topology. Event
selection is performed using the Interactive Multi-Instrument
Database of Solar Flares (IMIDSF) recently developed by the Center
for Computational Heliophysics (CCH) at NJIT. The selection of
IRIS observations was restricted to the fast-scanning regimes
(coarse-raster or sparse-raster modes with ≥ 4 slit positions, ≥
6`` spatial coverage, and ≤ 60 sec loop time). We have chosen 14
events, and estimated the spatially-resolved intensities and Doppler
shifts of the chromospheric (Mg II), transition region (C II) and hot
coronal (Fe XXI) lines reflecting the dynamics of the chromospheric
evaporation. The correlations among the derived line profile properties,
flare morphology, magnetic topology and hard X-ray characteristics
will be presented, and compared with the RADYN flare models and other
scenarios of chromospheric evaporations.
Title: Interactive Multi-Instrument Database of Solar Flares (IMIDSF)
Authors: Sadykov, Viacheslav M.; Nita, Gelu M.; Oria, Vincent;
Kosovichev, Alexander G.
Bibcode: 2017SPD....48.0101S
Altcode:
Solar flares represent a complicated physical phenomenon observed
in a broad range of the electromagnetic spectrum, from radiowaves to
gamma-rays. For a complete understanding of the flares it is necessary
to perform a combined multi-wavelength analysis using observations
from many satellites and ground-based observatories. For efficient
data search, integration of different flare lists and representation of
observational data, we have developed the Interactive Multi-Instrument
Database of Solar Flares (https://solarflare.njit.edu/). The
web database is fully functional and allows the user to search
for uniquely-identified flare events based on their physical
descriptors and availability of observations of a particular set of
instruments. Currently, data from three primary flare lists (GOES,
RHESSI and HEK) and a variety of other event catalogs (Hinode,
Fermi GBM, Konus-Wind, OVSA flare catalogs, CACTus CME catalog,
Filament eruption catalog) and observing logs (IRIS and Nobeyama
coverage), are integrated. An additional set of physical descriptors
(temperature and emission measure) along with observing summary, data
links and multi-wavelength light curves is provided for each flare
event since January 2002. Results of an initial statistical analysis
will be presented.
Title: Realistic Modeling of Interaction of Quiet-Sun Magnetic Fields
with the Chromosphere
Authors: Kitiashvili, Irina; Kosovichev, Alexander G.; Mansour,
Nagi N.; Wray, Alan A.
Bibcode: 2017SPD....4810502K
Altcode:
High-resolution observations and 3D MHD simulations reveal intense
interaction between the convection zone dynamics and the solar
atmosphere on subarcsecond scales. To investigate processes of
the dynamical coupling and energy exchange between the subsurface
layers and the chromosphere we perform 3D radiative MHD modeling
for a computational domain that includes the upper convection zone
and the chromosphere, and investigate the structure and dynamics for
different intensity of the photospheric magnetic flux. For comparison
with observations, the simulation models have been used to calculate
synthetic Stokes profiles of various spectral lines. The results show
intense energy exchange through small-scale magnetized vortex tubes
rooted below the photosphere, which provide extra heating of the
chromosphere, initiate shock waves, and small-scale eruptions.
Title: Solar-Cycle Variations Observed by Helioseismology and
Constraints on Solar Dynamo
Authors: Kosovichev, Alexander G.; Larson, Timothy P.; Guerrero,
Gustavo; Pipin, Valery
Bibcode: 2017SPD....4840303K
Altcode:
Helioseismology data from the SOHO and SDO, obtained in 1996-2017 for
almost two solar cycles, provide a unique opportunity to investigate
variations of the solar interior structure and dynamics, and link
these variations to the current dynamo models and simulations. The
solar oscillation frequencies and frequency splitting of medium-degree
p- and f-modes, as well as helioseismic inversions have been used to
analyze the differential rotation and global asphericity. By comparing
the helioseismology results with the synoptic surface magnetic fields
we identify characteristic changes associated the initiation and
evolution of the solar cycles, 23 and 24. The observational results are
compared with the current mean-field dynamo models and 3D MHD dynamo
simulations. It is shown that the helioseismology inferences provide
important constraints on the dynamics of the tachocline and near-surface
shear layer, and also may explain the fundamental difference between
the two solar cycles and detect the onset of the next cycle.
Title: Relationship Between High-Energy X-ray Sources and Helioseismic
Impact of X-Class Flare
Authors: Kosovichev, Alexander G.; Sharykin, Ivan N.; Sadykov,
Viacheslav M.; Zimovets, Ivan V.; Myshyakov, Ivan I.
Bibcode: 2017SPD....48.0203K
Altcode:
The X-class solar flare of October 23, 2012, generated the strongest
sunquake event of the current solar cycle. We study properties of
the energy release with high temporal and spatial resolutions, using
photospheric data from the Helioseismic Magnetic Imager (HMI) onboard
Solar Dynamics Observatory (SDO), and hard X-ray observations made
by the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). To
investigate the photospheric impact with high temporal resolution
we developed a special procedure for analysis of level-1 HMI data
(filtergrams), obtained by scanning the Fe I line (6731 A) with
the time cadence of 3.6 s and spatial resolution of 0.5 arcsec per
pixel. The helioseismic holography technique was used to reconstruct
the helioseismic impact. It is found that the photospheric disturbances
caused by the flare spatially coincide with the region of hard X-ray
emission, but are delayed by 4 seconds. This delay is consistent
with predictions of the flare hydrodynamics RADYN models. However,
the models fail to explain the magnitude of variations observed by the
HMI. The data indicate that the photospheric impact and helioseismic
wave might be caused by the electron energy flux substantially higher
than that in the current flare radiative hydrodynamic models.
Title: An Interactive Multi-instrument Database of Solar Flares
Authors: Sadykov, Viacheslav M.; Kosovichev, Alexander G.; Oria,
Vincent; Nita, Gelu M.
Bibcode: 2017ApJS..231....6S
Altcode: 2017arXiv170202991S
Solar flares are complicated physical phenomena that are observable
in a broad range of the electromagnetic spectrum, from radio waves
to γ-rays. For a more comprehensive understanding of flares, it
is necessary to perform a combined multi-wavelength analysis using
observations from many satellites and ground-based observatories. For
an efficient data search, integration of different flare lists,
and representation of observational data, we have developed the
Interactive Multi-Instrument Database of Solar Flares (IMIDSF, https://solarflare.njit.edu/).
The web-accessible database is fully functional and allows the user to
search for uniquely identified flare events based on their physical
descriptors and the availability of observations by a particular set
of instruments. Currently, the data from three primary flare lists
(Geostationary Operational Environmental Satellites, RHESSI, and HEK)
and a variety of other event catalogs (Hinode, Fermi GBM, Konus-WIND,
the OVSA flare catalogs, the CACTus CME catalog, the Filament eruption
catalog) and observing logs (IRIS and Nobeyama coverage) are integrated,
and an additional set of physical descriptors (temperature and emission
measure) is provided along with an observing summary, data links,
and multi-wavelength light curves for each flare event since 2002
January. We envision that this new tool will allow researchers to
significantly speed up the search of events of interest for statistical
and case studies.
Title: Investigation of Relationship between High-energy X-Ray
Sources and Photospheric and Helioseismic Impacts of X1.8 Solar
Flare of 2012 October 23
Authors: Sharykin, I. N.; Kosovichev, A. G.; Sadykov, V. M.; Zimovets,
I. V.; Myshyakov, I. I.
Bibcode: 2017ApJ...843...67S
Altcode: 2017arXiv170303767S
The X-class solar flare of 2012 October 23 generated continuum
photospheric emission and a strong helioseismic wave (“sunquake”)
that points to an intensive energy release in the dense part of the
solar atmosphere. We study properties of the energy release with high
temporal and spatial resolutions, using photospheric data from the
Helioseismic Magnetic Imager (HMI) on board Solar Dynamics Observatory,
and hard X-ray observations made by RHESSI. For this analysis we use
level-1 HMI data (filtergrams), obtained by scanning the Fe I line
(6731 Å) with the time cadence of ∼3.6 s and spatial resolution of
∼0.″5 per pixel. It is found that the photospheric disturbances
caused by the flare spatially coincide with the region of hard X-ray
emission but are delayed by ≲4 s. This delay is consistent with
predictions of the flare hydrodynamics RADYN models. However, the models
fail to explain the magnitude of variations observed by the HMI. The
data indicate that the photospheric impact and helioseismic wave might
be caused by the electron energy flux, which is substantially higher
than that in the current flare radiative hydrodynamic models.
Title: Statistical properties of coronal hole rotation rates: Are
they linked to the solar interior?
Authors: Bagashvili, S. R.; Shergelashvili, B. M.; Japaridze,
D. R.; Chargeishvili, B. B.; Kosovichev, A. G.; Kukhianidze, V.;
Ramishvili, G.; Zaqarashvili, T. V.; Poedts, S.; Khodachenko, M. L.;
De Causmaecker, P.
Bibcode: 2017A&A...603A.134B
Altcode: 2017arXiv170604464B
Context. The present paper discusses results of a statistical study
of the characteristics of coronal hole (CH) rotation in order to
find connections to the internal rotation of the Sun.
Aims:
The goal is to measure CH rotation rates and study their distribution
over latitude and their area sizes. In addition, the CH rotation rates
are compared with the solar photospheric and inner layer rotational
profiles.
Methods: We study CHs observed within ± 60° latitude
and longitude from the solar disc centre during the time span from
the 1 January 2013 to 20 April 2015, which includes the extended peak
of solar cycle 24. We used data created by the spatial possibilistic
clustering algorithm (SPoCA), which provides the exact location and
characterisation of solar coronal holes using SDO/AIA193 Å channel
images. The CH rotation rates are measured with four-hour cadence
data to track variable positions of the CH geometric centre.
Results: North-south asymmetry was found in the distribution of coronal
holes: about 60 percent were observed in the northern hemisphere and
40 percent were observed in the southern hemisphere. The smallest and
largest CHs were present only at high latitudes. The average sidereal
rotation rate for 540 examined CHs is 13.86( ± 0.05)°/d.
Conclusions: The latitudinal characteristics of CH rotation do
not match any known photospheric rotation profile. The CH angular
velocities exceed the photospheric angular velocities at latitudes
higher than 35-40 degrees. According to our results, the CH rotation
profile perfectly coincides with tachocline and the lower layers of
convection zone at around 0.71 R⊙; this indicates that
CHs may be linked to the solar global magnetic field, which originates
in the tachocline region.
Title: Helioseismic Observations and Constraints on Solar and Stellar
Dynamo Models
Authors: Kosovichev, Alexander G.; Guerrero, Gustavo; Pipin, Valery V.
Bibcode: 2017shin.confE..56K
Altcode:
Helioseismology data from the SOHO and SDO, obtained in 1996-2017 for
almost two solar cycles, provide a unique opportunity to investigate
variations of the solar interior structure and dynamics, and link
these variations to the current dynamo models and simulations. The
solar oscillation frequencies and frequency splitting of medium-degree
p- and f-modes, as well as helioseismic inversions have been used to
analyze the differential rotation and global asphericity. By comparing
the helioseismology results with the synoptic surface magnetic fields
we identify characteristic changes associated the initiation and
evolution of the solar cycles, 23 and 24. The observational results are
compared with the current mean-field dynamo models and 3D MHD dynamo
simulations. It is shown that the helioseismology inferences provide
important constraints on the dynamics of the tachocline and near-surface
shear layer, and also may explain the fundamental difference between
the two solar cycles and detect the onset of the next cycle.
Title: Realistic 3D radiative modeling of turbulent structure of
moderate-mass stars and Sun
Authors: Kitiashvili, Irina N.; Kosovichev, Alexander G.; Wray,
Alan A.; Mansour, Nagi N.
Bibcode: 2017shin.confE..60K
Altcode:
Understanding the turbulent dynamics of the Sun and stars is a
critical element for interpreting observed processes and phenomena on
different scales and for predicting extreme events such as flares and
superflares. High-resolution observations of the Sun and high-fidelity
radiative MHD numerical simulations have substantially advanced our
understanding of solar and stellar local dynamics and magnetism from
the upper convection zone to the atmosphere and corona. However,
global modeling of the Sun with such a high degree of realism is
currently not affordable due to the extremely high computational cost
of resolving the scales in the convection zone. The physics of the
deep solar dynamics can be effectively addressed through modeling
more massive solar-type stars where the convection zone is shallower
and the convective overturning time is much shorter than those on the
Sun. We present recent 3D realistic simulation results of moderate-mass
stars and discuss links between solar and stellar dynamics, such
as the multiscale structure of granulation, convective overshoot,
and others. In particular, these simulations have provided better
understanding of the dynamics of the tachocline (the overshoot layer
at the bottom of the convection zone) and have explained long-standing
results from helioseismology.
Title: Flare Energy Release in the Lower Solar Atmosphere near the
Magnetic Field Polarity Inversion Line
Authors: Sharykin, I. N.; Sadykov, V. M.; Kosovichev, A. G.; Vargas
Dominguez, S.; Zimovets, I. V.
Bibcode: 2017ApJ...840...84S
Altcode: 2018arXiv180104921S
We study flare processes in the solar atmosphere using observational
data for an M1-class flare of 2014 June 12, obtained by the New Solar
Telescope (NST/BBSO) and Helioseismic Magnetic Imager (HMI/SDO). The
main goal is to understand triggers and manifestations of the flare
energy release in the photosphere and chromosphere using high-resolution
optical observations and magnetic field measurements. We analyze
optical images, HMI Dopplergrams, and vector magnetograms, and use
nonlinear force-free field (NLFFF) extrapolations for reconstruction
of the magnetic topology and electric currents. The NLFFF modeling
reveals the interaction of two magnetic flux ropes with oppositely
directed magnetic fields in the polarity inversion line (PIL). These
flux ropes are observed as a compact sheared arcade along the PIL in the
high-resolution broadband continuum images from NST. In the vicinity of
the PIL, the NST {{H}}α observations reveal the formation of a thin
three-ribbon structure corresponding to a small-scale photospheric
magnetic arcade. The observational results are evidence in favor of
the primary energy release site located in the chromospheric plasma
with strong electric currents concentrated near the PIL. In this case,
magnetic reconnection is triggered by the interacting magnetic flux
ropes forming a current sheet elongated along the PIL.
Title: Magnetoacoustic Waves Excitation in Self-Organized Solar
Magnetic Structures
Authors: Kitiashvili, I.; Kosovichev, A. G.; Mansour, N. N.; Sandstrom,
T. A.; Wray, A. A.
Bibcode: 2016AGUFMSH21E2570K
Altcode:
Interaction of the turbulent plasma and magnetic fields is of great
interest as a key to understanding self-organization processes and
dynamics of the solar magnetism. We develop 3D time-dependent radiative
MHD simulations that are based on first principles and provide an
important tool for uncovering the basic physical mechanisms. Our
simulations are able to reproduce many observed phenomena, and, in
particular, allow us to investigate spontaneous formation of coherent
highly magnetized flux-robe structures that are observed as "pores"
in the photosphere. The dynamical evolution of these structures is
accompanied by numerous magnetoacoustic waves that are excited in
subphotospheric layers and propagate into the solar atmosphere. We
present analysis of the pore dynamics, and properties and excitation
mechanism of the magnetoacoustic waves, as well as a comparison with
observations.
Title: Three-Fluid collisional and reactive magnetic reconnection
with radiative effects in chromospheric conditions
Authors: Alvarez Laguna, A.; Ozak, N. O.; Maneva, Y. G.; Lani, A.;
Kosovichev, A. G.; Mansour, N. N.; Poedts, S.
Bibcode: 2016AGUFMSH21E2566A
Altcode:
The partially ionized chomosphere hosts the interplay of complex
physical phenomena, i.e., collisional processes, non-chemical
equilibrium conditions and non-LTE radiation effects, etc. We study the
magnetic reconnection in different ionization levels under chromospheric
conditions for a multi-fluid, compressible, collisional and reactive
model. We will extend previous work that considers two-fluid models
(plasma + neutrals), to a three-fluid model accounting for electron
dynamics. The model includes chemical reactions of ionization,
recombination and charge exchange collisions. The transport fluxes
consider the anisotropy introduced by the magnetic field in the charged
species. The radiative losses, that are known to play an important role
in the chromosphere, are modeled with an effectively thin radiation
loss function, fitting a three-level Hydrogen atom. In a set of 2-D
computational simulations, we study different ionization levels from
0.5% to 50%, with fixed Lundquist number, analyzing the radiation
effects on the tearing mode instability.
Title: Triggers and Manifestations of Flare Energy Release in the
Low Atmosphere
Authors: Kosovichev, A. G.; Sharykin, I. N.; Sadykov, V. M.; Vargas,
S.; Zimovets, I. V.
Bibcode: 2016AGUFMSH14B..05K
Altcode:
The main goal is to understand triggers and manifestations of the
flare energy release in the lower layers of the solar atmosphere (the
photosphere and chromosphere) using high-resolution optical observations
and magnetic field measurements. As a case study we present results
for an M-class flare. We analyze optical images, HMI Dopplergrams
and vector magnetograms, and use Non-Linear Force-Free Field (NLFFF)
extrapolation for reconstruction of the magnetic topology. The NLFFF
modelling reveals interaction of oppositely directed magnetic flux-tubes
in the Polarity Inversion Line (PIL). These two interacting magnetic
flux tubes are observed as a compact sheared arcade along the PIL in the
high-resolution broad-band continuum images from New Solar Telescope
(NST). In the vicinity of the PIL, the NST H-alpha observations
reveal formation of a thin three-ribbon structure corresponding to
the small-scale photospheric magnetic arcade. Magnetic reconnection is
triggered by two interacting magnetic flux tubes with forming current
sheet extended along the PIL. Presented observational results evidence
in favor of location of the primary energy release site in the dense
chromosphere where plasma is partially ionized in the region of strong
electric currents concentrated near the polarity inversion line.
Title: Angular momentum fluxes caused by Λ -effect and meridional
circulation structure of the Sun
Authors: Pipin, Valery V.; Kosovichev, Alexander G.
Bibcode: 2016AdSpR..58.1490P
Altcode: 2016arXiv160105178P
Using mean-field hydrodynamic models of the solar angular momentum
balance we show that the non-monotonic latitudinal dependence of
the radial angular momentum fluxes caused by Λ -effect can affect
the number of the meridional circulation cells stacking in the radial
direction in the solar convection zone. In particular, our results show
the possibility of a complicated triple-cell meridional circulation
structure. This pattern consists of two large counterclockwise
circulation cells (the N-hemisphere) and a smaller clockwise cell
located at low latitudes at the bottom of the convection zone. Similar
triple-cell circulation patterns were previously earlier found in a
number of 3D global simulations models. The effect is demonstrated for
the first time using the mean-field parametrization of the Λ -effect.
Title: Understanding Solar Torsional Oscillations from Global
Dynamo Models
Authors: Guerrero, G.; Smolarkiewicz, P. K.; de Gouveia Dal Pino,
E. M.; Kosovichev, A. G.; Mansour, N. N.
Bibcode: 2016ApJ...828L...3G
Altcode: 2016arXiv160802278G
The phenomenon of solar “torsional oscillations” (TO) represents
migratory zonal flows associated with the solar cycle. These flows
are observed on the solar surface and, according to helioseismology,
extend through the convection zone. We study the origin of the TO
using results from a global MHD simulation of the solar interior that
reproduces several of the observed characteristics of the mean-flows and
magnetic fields. Our results indicate that the magnetic tension (MT)
in the tachocline region is a key factor for the periodic changes in
the angular momentum transport that causes the TO. The torque induced
by the MT at the base of the convection zone is positive at the poles
and negative at the equator. A rising MT torque at higher latitudes
causes the poles to speed up, whereas a declining negative MT torque
at the lower latitudes causes the equator to slow-down. These changes
in the zonal flows propagate through the convection zone up to the
surface. Additionally, our results suggest that it is the magnetic
field at the tachocline that modulates the amplitude of the surface
meridional flow rather than the opposite as assumed by flux-transport
dynamo models of the solar cycle.
Title: Relationship Between Chromospheric Evaporation and Magnetic
Field Topology in an M-Class Solar Flare
Authors: Sadykov, Viacheslav M.; Kosovichev, Alexander G.; Sharykin,
Ivan N.; Zimovets, Ivan V.; Vargas Dominguez, Santiago
Bibcode: 2016ApJ...828....4S
Altcode: 2016arXiv160405346S
Chromospheric evaporation is observed as Doppler blueshift during
solar flares. It plays a key role in the dynamics and energetics of
solar flares; however, its mechanism is still unknown. In this paper,
we present a detailed analysis of spatially resolved multi-wavelength
observations of chromospheric evaporation during an M 1.0-class
solar flare (SOL2014-06-12T21:12) using data from NASA’s Interface
Region Imaging Spectrograph and HMI/SDO (the Helioseismic and Magnetic
Imager on board the Solar Dynamics Observatory), and high-resolution
observations from VIS/NST (the Visible Imaging Spectrometer at the New
Solar Telescope). The results show that the averaged over the flare
region Fe xxi blueshift of the hot (107 K) evaporating
plasma is delayed relative to the C II redshift of the relatively
cold (104 K) chromospheric plasma by about one minute. The
spatial distribution of the delays is not uniform across the region
and can be as long as two minutes in several zones. Using vector
magnetograms from HMI, we reconstruct the magnetic field topology and
the quasi-separatrix layer, and find that the blueshift delay regions
as well as the Hα flare ribbons are connected to the region of the
magnetic polarity inversion line (PIL) and an expanding flux rope
via a system of low-lying loop arcades with a height of ≲4.5 Mm. As
a result, the chromospheric evaporation may be driven by the energy
release in the vicinity of PIL, and has the observed properties due
to a local magnetic field topology.
Title: 3D Realistic MHD Modeling of Solar Activity in Quiet-Sun
Regions
Authors: Kitiashvili, Irina N.; Kosovichev, Alexander G.; Mansour,
Nagi N.; Wray, Alan A.
Bibcode: 2016shin.confE.149K
Altcode:
Despite on the absence of strong highly-energetic events in quiet-Sun
regions the amount of energy flux into the chromosphere and corona,
generated by small-scale events, makes these areas of great interest
to address problems of coronal heating as well as solar wind
acceleration. The quiet-Sun regions represent an evolving background
state in which the turbulent MHD environment affects properties of
sunspots formed from magnetic fluxes emerging from the deep convection,
formation of filaments, coronal loops etc. We present recent 3D
radiative MHD numerical studies of magnetic self-organization processes
driven by the turbulent magnetoconvection in quiet-Sun regions,
such as small-scale dynamo and plasma eruptions, and discuss their
physical nature, links to high-resolution observations by modeling
synthetic spectro-polarimetric data, and also potential impacts on
the energetics and dynamics of the corona and inner heliosphere.
Title: Links Between Global Solar Magnetic Fields and Subsurface Flows
Authors: Kosovichev, Alexander; Zhao, Junwei
Bibcode: 2016shin.confE.145K
Altcode:
The distribution of magnetic fields on the solar surface and magnetic
flux transport are controlled by surface and subsurface plasma
flows. Thus, understanding the links between the flow dynamics and
global magnetic fields is potentially important for predicting the
flux evolution and estimating the structure of the corona. We use local
helioseismology and magnetic field measurements from the HMI instrument
on SDO to investigate detailed dynamics of the upper convection zone
and its relation to the magnetic field evolution during the first five
years of the current solar cycle. The helioseismology data represent 3D
flow maps in the depth range of 0-20 Mm, obtained uninterruptedly every
8 hours for almost the whole solar disk with the spatial sampling of
two arcsec. We calculate the flow characteristics (such as divergence,
vorticity and kinetic helicity) on different spatio-temporal scales
from supergranulation to global-scale zonal and meridional flows. The
subsurface flow maps reveal a complex multi-scale organization linked
to the solar magnetic activity. This study is focused on comparison
of the flow maps with the corresponding HMI magnetic field and AIA
coronal maps. In particular, we look for a relationship between the
near-surface flows, coronal holes and other large-scale structures. We
present the flow maps for the SHINE initial challenge event of July
8, 2010, and analyze the synoptic evolution of the Solar Subsurface
Weather associated with this event.
Title: Initiation and Energy Release Mechanisms of Non-eruptive
Flares: Two Case Studies
Authors: Kosovichev, Alexander G.; Sharykin, Ivan N.; Sadykov,
Viacheslav M.; Zimovets, Ivan V.
Bibcode: 2016shin.confE.160K
Altcode:
We present a detailed analysis of two non-eruptive solar flares. The
solar flare on July 30, 2011 was of a modest X-ray class (M9.3),
but it made a strong photospheric impact and produced a sunquake,
observed with the Helioseismic and Magnetic Imager (HMI) on Solar
Dynamics Observatory (SDO). In addition to the helioseismic waves
(also observed with the SDO/AIA instrument), the flare caused a
large expanding area of white-light emission and was accompanied by
substantial restructuring of magnetic fields, leading to the rapid
formation of a sunspot structure in the flare region. The flare produced
no significant hard X-ray emission and no coronal mass ejection. This
indicates that the flare energy release was mostly confined to the lower
atmosphere. Another non-eruptive M1-class flare of June 12, 2014, is
studied by using high-resolution images obtained by New Solar Telescope
(NST/BBSO) and HMI vector magnetograms. The NLFFF modelling reveals
interaction of oppositely directed magnetic flux-tubes in the polarity
inversion line (PIL). These two interacting magnetic flux tubes are
observed as a compact sheared arcade along the PIL. In the vicinity
of the PIL, the NST H-alpha observations reveal formation of a thin
three-ribbon structure corresponding to the small-scale photospheric
magnetic arcade. The observational results evidence in favor of location
of the primary energy release site in the dense chromosphere where
plasma is partially ionized in the region of strong electric currents
and shearing plasma flows concentrated near the polarity inversion
line. Magnetic reconnection may be triggered by interacting magnetic
flux tubes with forming a current sheet elongated along the PIL.
Title: Dependence of Stellar Magnetic Activity Cycles on Rotational
Period in a Nonlinear Solar-type Dynamo
Authors: Pipin, V. V.; Kosovichev, A. G.
Bibcode: 2016ApJ...823..133P
Altcode: 2016arXiv160207815P
We study the turbulent generation of large-scale magnetic fields using
nonlinear dynamo models for solar-type stars in the range of rotational
periods from 14 to 30 days. Our models take into account nonlinear
effects of dynamical quenching of magnetic helicity, and escape of
magnetic field from the dynamo region due to magnetic buoyancy. The
results show that the observed correlation between the period of
rotation and the duration of activity cycles can be explained in the
framework of a distributed dynamo model with a dynamical magnetic
feedback acting on the turbulent generation from either magnetic
buoyancy or magnetic helicity. We discuss implications of our findings
for the understanding of dynamo processes operating in solar-like stars.
Title: Solar-Cycle Evolution of Subsurface Flows and Magnetic Field
Authors: Kosovichev, Alexander G.; Zhao, Junwei
Bibcode: 2016SPD....47.0709K
Altcode:
Local helioseismology and magnetic field measurements from the HMI
instrument on SDO provide unique high-resolution data that allow us
to investigate detailed dynamics of the upper convection zone and its
relation to the magnetic field evolution during the first five years
of the current solar cycle. This study is focused on the understanding
the role of the near-surface shear layer (NSSL) in the dynamo process,
generation, emergence and transport of the solar magnetic flux. The
helioseismology data represent 3D flow maps in the depth range of 0-20
Mm, obtained uninterruptedly every 8 hours for almost the whole solar
disk with the spatial sampling of two arcsec. We calculate the flow
characteristics (such as divergence, vorticity and kinetic helicity) on
different spatio-temporal scales from supergranulation to global-scale
zonal and meridional flows. We investigate the multi-scale organization
of the subsurface flows, including the inflows into active regions,
the hemispheric `flip-flop’ asymmetry of variations of the meridional
flows, the structure and dynamics of torsional oscillations, and compare
the flow behavior with the evolution of the observed magnetic activity
of the current cycle.
Title: Reconstruction of Solar Subsurfaces by Local Helioseismology
Authors: Kosovichev, Alexander G.; Zhao, Junwei
Bibcode: 2016LNP...914...25K
Altcode: 2016arXiv160705681K
Local helioseismology has opened new frontiers in our quest for
understanding of the internal dynamics and dynamo on the Sun. Local
helioseismology reconstructs subsurface structures and flows by
extracting coherent signals of acoustic waves traveling through the
interior and carrying information about subsurface perturbations
and flows, from stochastic oscillations observed on the surface. The
initial analysis of the subsurface flow maps reconstructed from the
5 years of SDO/HMI data by time-distance helioseismology reveals
the great potential for studying and understanding of the dynamics
of the quiet Sun and active regions, and the evolution with the solar
cycle. In particular, our results show that the emergence and evolution
of active regions are accompanied by multi-scale flow patterns, and
that the meridional flows display the North-South asymmetry closely
correlating with the magnetic activity. The latitudinal variations
of the meridional circulation speed, which are probably related to
the large-scale converging flows, are mostly confined in shallow
subsurface layers. Therefore, these variations do not necessarily
affect the magnetic flux transport. The North-South asymmetry is also
pronounced in the variations of the differential rotation (`torsional
oscillations'). The calculations of a proxy of the subsurface kinetic
helicity density show that the helicity does not vary during the
solar cycle, and that supergranulation is a likely source of the
near-surface helicity.
Title: Observational Investigation of Energy Release in the Lower
Solar Atmosphere of a Solar Flare
Authors: Sharykin, I. N.; Sadykov, V. M.; Kosovichev, A. G.; Vargas
Dominguez, S.; Zimovets, I. V.
Bibcode: 2016arXiv160405380S
Altcode:
We study flare processes in the lower solar atmosphere using
observational data for a M1-class flare of June 12, 2014, obtained
by New Solar Telescope (NST/BBSO) and Helioseismic Magnetic Imager
(HMI/SDO). The main goal is to understand triggers and manifestations of
the flare energy release in the lower layers of the solar atmosphere
(the photosphere and chromosphere) using high-resolution optical
observations and magnetic field measurements. We analyze optical
images, HMI Dopplergrams and vector magnetograms, and use Non-Linear
Force-Free Field (NLFFF) extrapolations for reconstruction of the
magnetic topology. The NLFFF modelling reveals interaction of oppositely
directed magnetic flux-tubes in the PIL. These two interacting magnetic
flux tubes are observed as a compact sheared arcade along the PIL in the
high-resolution broad-band continuum images from NST. In the vicinity
of the PIL, the NST H alpha observations reveal formation of a thin
three-ribbon structure corresponding to the small-scale photospheric
magnetic arcade. Presented observational results evidence in favor of
location of the primary energy release site in the dense chromosphere
where plasma is partially ionized in the region of strong electric
currents concentrated near the polarity inversion line. Magnetic
reconnection may be triggered by two interacting magnetic flux tubes
with forming current sheet elongated along the PIL.
Title: Dynamics of Turbulent Convection and Convective Overshoot in
a Moderate-mass Star
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Mansour, N. N.; Wray,
A. A.
Bibcode: 2016ApJ...821L..17K
Altcode: 2015arXiv151207298K
We present results of realistic three-dimensional (3D) radiative
hydrodynamic simulations of the outer layers of a moderate-mass
star (1.47 M ⊙), including the full convection zone,
the overshoot region, and the top layers of the radiative zone. The
simulation results show that the surface granulation has a broad range
of scales, from 2 to 12 Mm, and that large granules are organized in
well-defined clusters, consisting of several granules. Comparison of
the mean structure profiles from 3D simulations with the corresponding
one-dimensional (1D) standard stellar model shows an increase of the
stellar radius by ∼800 km, as well as significant changes in the
thermodynamic structure and turbulent properties of the ionization
zones. Convective downdrafts in the intergranular lanes between
granulation clusters reach speeds of more than 20 km s-1,
penetrate through the whole convection zone, hit the radiative zone,
and form an 8 Mm thick overshoot layer. Contrary to semi-empirical
overshooting models, our results show that the 3D dynamic overshoot
region consists of two layers: a nearly adiabatic extension of
the convection zone and a deeper layer of enhanced subadiabatic
stratification. This layer is formed because of heating caused by
the braking of the overshooting convective plumes. This effect has to
be taken into account in stellar modeling and the interpretation of
asteroseismology data. In particular, we demonstrate that the deviations
of the mean structure of the 3D model from the 1D standard model of the
same mass and composition are qualitatively similar to the deviations
for the Sun found by helioseismology.
Title: Commission 12: Solar Radiation and Structure
Authors: Cauzzi, Gianna; Shchukina, Nataliya; Kosovichev, Alexander;
Bianda, Michele; Brandenburg, Axel; Chou, Dean-Yi; Dasso, Sergio; Ding,
Ming-De; Jefferies, Stuart; Krivova, Natalie; Kuznetsov, Vladimir D.;
Moreno-Insertis, Fernando
Bibcode: 2016IAUTA..29..278C
Altcode:
Commission 12 of the International Astronomical Union encompasses
investigations of the internal structure and dynamics of the Sun,
the quiet solar atmosphere, solar radiation and its variability, and
the nature of relatively stable magnetic structures like sunspots,
faculae and the magnetic network. The Commission sees participation
of over 300 scientists worldwide.
Title: On the Role of Tachoclines in Solar and Stellar Dynamos
Authors: Guerrero, G.; Smolarkiewicz, P. K.; de Gouveia Dal Pino,
E. M.; Kosovichev, A. G.; Mansour, N. N.
Bibcode: 2016ApJ...819..104G
Altcode: 2015arXiv150704434G
Rotational shear layers at the boundary between radiative and convective
zones, tachoclines, play a key role in the process of magnetic
field generation in solar-like stars. We present two sets of global
simulations of rotating turbulent convection and dynamo. The first set
considers a stellar convective envelope only; the second one, aiming
at the formation of a tachocline, also considers the upper part of the
radiative zone. Our results indicate that the resulting properties of
the mean flows and dynamo, such as the growth rate, saturation energy,
and mode, depend on the Rossby number (Ro). For the first set of models
either oscillatory (with ∼2 yr period) or steady dynamo solutions are
obtained. The models in the second set naturally develop a tachocline,
which in turn leads to the generation of a strong mean magnetic
field. Since the field is also deposited in the stable deeper layer,
its evolutionary timescale is much longer than in the models without
a tachocline. Surprisingly, the magnetic field in the upper turbulent
convection zone evolves on the same timescale as the deep field. These
models result in either an oscillatory dynamo with a ∼30 yr period
or a steady dynamo depending on Ro. In terms of the mean-field dynamo
coefficients computed using the first-order smoothing approximation,
the field evolution in the oscillatory models without a tachocline
seems to be consistent with dynamo waves propagating according to
the Parker-Yoshimura sign rule. In the models with tachoclines the
dynamics is more complex and involves other transport mechanisms as
well as tachocline instabilities.
Title: Spectroscopic UV observations of M1.0 class solar flare from
IRIS satellite
Authors: Sadykov, Viacheslav M.; Kosovichev, Alexander G.; Sharykin,
Ivan N.; Vargas Dominguez, Santiago
Bibcode: 2016IAUS..320...64S
Altcode:
This work presents an analysis of UV spectroscopic observations from the
IRIS satellite of an M1.0 class flare occurred on 12 June 2014 in active
region NOAA 12087. Our analysis of the IRIS spectra and Slit-Jaw images
revealed presence of a strongly redshifted chromospheric jet before
the flare. We also found strong emission of the chromospheric lines,
and studied the C II 1334.5 Å line emission distribution in details. A
blueshift of the Fe XXI line across the flaring region corresponds to
evaporation flows of the hot chromospheric plasma with a speed of 50
km/s. Although the enhancement of the C II line integrated redshift
correlates with the flare X-ray emission, we classify the evaporation
as of a ``gentle'' type because of its long time scale and subsonic
velocities. Analysis of X-ray data from the RHESSI satellite showed that
both, an injection of accelerated particles and a heat flux from the
energy release site can explain the energetics of the observed event.
Title: 2-D and 3-D models of convective turbulence and oscillations
in intermediate-mass main-sequence stars
Authors: Guzik, Joyce A.; Morgan, T. H.; Nelson, N. J.; Lovekin, C.;
Kosak, K.; Kitiashvili, I. N.; Mansour, N. N.; Kosovichev, A.
Bibcode: 2016IAUFM..29B.540G
Altcode: 2016arXiv160504455G
We present multidimensional modeling of convection and oscillations in
main-sequence stars somewhat more massive than the Sun, using three
separate approaches: 1) Using the 3-D planar StellarBox radiation
hydrodynamics code to model the envelope convection zone and part
of the radiative zone. Our goals are to examine the interaction
of stellar pulsations with turbulent convection in the envelope,
excitation of acoustic modes, and the role of convective overshooting;
2) Applying the spherical 3-D MHD ASH (Anelastic Spherical Harmonics)
code to simulate the core convection and radiative zone. Our goal is
to determine whether core convection can excite low-frequency gravity
modes, and thereby explain the presence of low frequencies for some
hybrid γ Dor/δ Sct variables for which the envelope convection zone
is too shallow for the convective blocking mechanism to drive gravity
modes; 3) Applying the ROTORC 2-D stellar evolution and dynamics
code to calculate evolution with a variety of initial rotation rates
and extents of core convective overshooting. The nonradial adiabatic
pulsation frequencies of these nonspherical models are calculated using
the 2-D pulsation code NRO. We present new insights into pulsations
of 1-2 M⊙ stars gained by multidimensional modeling.
Title: Solar radius variations: new look on the wavelength dependence
Authors: Rozelot, Jean-Pierre; Kosovichev, Alexander; Kilcik, Ali
Bibcode: 2016IAUS..320..342R
Altcode:
The possibility that the Sun's radius is changing, even at a faint
level, has been discussed over a long time. As the solar radius is
certainly one of the most important basic pieces of astrophysical
information, it is crucial to determine the physical mechanisms
that may cause shrinking or expansion of the solar envelope. The
wavelength dependence has been poorly inspected up to now. Here we
examine recent solar radius determinations from space observations,
mainly from Mercury and Venus transits, made by different teams
in 2006, 2012 and 2014. Seemingly, the results are not consistent:
authors interpreted the discrepancies because of the different methods
of analysis used in their work. However, looking at the wavelength
dependence, adding other available observations, from X-EUV up to
radio, a typical relationship between the radius and the wavelength
can be found, reflecting the different heights at which the lines
are formed. Possible explanations are discussed. Such results can be
interesting for studying solar-stellar connections.
Title: Solar and Stellar Flares and their Effects on Planets
Authors: Kosovichev, A. G.; Hawley, S. L.; Heinzel, P.
Bibcode: 2016IAUS..320.....K
Altcode:
No abstract at ADS
Title: Solar Dynamo on Small and Global Scales
Authors: Kitiashvili, I.; Kosovichev, A. G.; Mansour, N. N.; Wray,
A. A.
Bibcode: 2015AGUFMSH23A2432K
Altcode:
Phenomenon of the solar variability is primarily driven by the evolution
of magnetic fields on both small and global scales. Because connection
between the dynamo processes on different scales remains unclear,
we consider them separately. In particular, we analyze 1) a global
dynamo model, which is reduced to a dynamical system in the context
of the solar cycle variations, and 2) realistic-type 3D numerical
simulations of the small-scale dynamo, and discuss possible interlinks
between these dynamo processes.
Title: Relationship Between Subsurface Flows and Magnetic Field
Evolution in Solar Cycle 24
Authors: Kosovichev, A. G.; Zhao, J.
Bibcode: 2015AGUFMSH33D..05K
Altcode:
Local time-distance helioseismology and magnetic field measurements from
the HMI instrument on SDO provide unique high-resolution data that allow
us to investigate detailed dynamics of the upper convection zone and
its relation to the magnetic field evolution during the first five years
of the current solar cycle. This study is focused on the understanding
the role of the near-surface shear layer (NSSL) in the dynamo process,
generation, emergence and transport of the solar magnetic flux. The
helioseismology data represent 3D flow maps in the depth range of 0-20
Mm, obtained uninterruptedly every 8 hours for almost the whole solar
disk with the spatial sampling of two arcsec. We calculate the flow
characteristics (such as divergence, vorticity and kinetic helicity) on
different spatio-temporal scales from supergranulation to global-scale
zonal and meridional flows. We investigate the multi-scale organization
of the subsurface flows, including the inflows into active regions,
the hemispheric `flip-flop' asymmetry of variations of the meridional
flows, the structure and dynamics of torsional oscillations, and compare
the flow behavior with the evolution of the observed magnetic activity.
Title: Partially Ionized Plasma Three-Fluid Modeling of Magnetic
Reconnection in the Sun Chromosphere
Authors: Alvarez Laguna, A.; Lani, A.; Mansour, N. N.; Kosovichev,
A. G.; Poedts, D. S.
Bibcode: 2015AGUFMSH43A2424A
Altcode:
Magnetic reconnection is present in most of the unsteady and eruptive
phenomena in the Sun atmosphere, including Coronal Mass Ejections (CMEs)
and solar flares. Also, it occurs in the chromosphere, bringing about
chromospheric jets and spicules and being considered a likely mechanism
to play an important role in heating up the corona. In this work,
we present a computational model that simulates magnetic reconnection
in the Sun chromosphere using a three-fluid model (electrons + ions +
neutrals). The model treats separately ions, electrons and neutrals,
considering mass, momentum and energy conservation for each fluid. The
fluids interact among each other by means of collisions and chemical
reactions. The charged particles heat fluxes are anisotropic with
the magnetic field, following Braginskii's description. This model
also considers non-equilibrium partial ionization effects including
electron impact ionization, radiative recombination reactions and charge
exchange. The electromagnetic field evolution is represented by the full
Maxwell's equations, allowing for high frequency waves disregarded by
the MHD approximation. Previous two-fluid simulations showed that the
dynamics of ions and neutrals are decoupled during the reconnection
process when the width of the current sheet becomes comparable to the
ion scales. Also, the effect of the chemical non-equilibrium in the
reconnection region plays a crucial role, yielding faster reconnection
rates. We extended these simulations with a three-fluid model that
considers separately the dynamics of electrons. This new model provides
a better description of the complex dynamics taking place during the
reconnection, both in Sweet-Parker reconnections and during the tearing
instability. The results are compared with the two-fluid simulations.
Title: Effects of Large-scale Non-axisymmetric Perturbations in the
Mean-field Solar Dynamo.
Authors: Pipin, V. V.; Kosovichev, A. G.
Bibcode: 2015ApJ...813..134P
Altcode: 2015arXiv150500219P
We explore the response of a nonlinear non-axisymmetric mean-field
solar dynamo model to shallow non-axisymmetric perturbations. After a
relaxation period, the amplitude of the non-axisymmetric field depends
on the initial condition, helicity conservation, and the depth of
perturbation. It is found that a perturbation that is anchored at 0.9 R
⊙ has a profound effect on the dynamo process, producing
a transient magnetic cycle of the axisymmetric magnetic field, if it
is initiated at the growing phase of the cycle. The non-symmetric,
with respect to the equator, perturbation results in a hemispheric
asymmetry of the magnetic activity. The evolution of the axisymmetric
and non-axisymmetric fields depends on the turbulent magnetic
Reynolds number R m . In the range of R m =
104-106 the evolution returns to the normal course
in the next cycle, in which the non-axisymmetric field is generated
due to a nonlinear α-effect and magnetic buoyancy. In the stationary
state, the large-scale magnetic field demonstrates a phenomenon of
“active longitudes” with cyclic 180° “flip-flop” changes of
the large-scale magnetic field orientation. The flip-flop effect is
known from observations of solar and stellar magnetic cycles. However,
this effect disappears in the model, which includes the meridional
circulation pattern determined by helioseismology. The rotation rate
of the non-axisymmetric field components varies during the relaxation
period and carries important information about the dynamo process.
Title: Solar Radius Variations: An Inquisitive Wavelength Dependence
Authors: Rozelot, Jean Pierre; Kosovichev, Alexander; Kilcik, Ali
Bibcode: 2015ApJ...812...91R
Altcode:
Recent solar radius determinations from space observations of Mercury
and Venus transits have been made by different teams in 2003, 2006,
2012, and 2014. Seemingly the results are not consistent: the authors
interpreted the discrepancies as caused by the different methods of
analysis. However, looking at the wavelength dependence and adding other
available observations from X-EUV up to radio, a typical wavelength
dependence can be found, reflecting the different heights at which
the lines are formed. Measurements obtained during different periods
of time would, in principle, allow us to detect a signature of radius
temporal dependence. However, the available data are not sufficiently
numerous to detect a significant dependence, at least at the level
of the uncertainty at which the observations were made. Lastly, no
unique theoretical model is available today to reproduce the strong
wavelength dependence of the solar radius, which shows an unexpected
minimum at around (6.6 ± 1.9) μm, after a parabolic fit.
Title: Dynamo Effects Near the Transition from Solar to Anti-Solar
Differential Rotation
Authors: Simitev, Radostin D.; Kosovichev, Alexander G.; Busse,
Friedrich H.
Bibcode: 2015ApJ...810...80S
Altcode: 2015arXiv150407835S
Numerical MHD simulations play an increasingly important role
for understanding the mechanisms of stellar magnetism. We present
simulations of convection and dynamos in density-stratified rotating
spherical fluid shells. We employ a new 3D simulation code for obtaining
the solution of a physically consistent anelastic model of the process
with a minimum number of parameters. The reported dynamo simulations
extend into a “buoyancy-dominated” regime where the buoyancy
forcing is dominant while the Coriolis force is no longer balanced
by pressure gradients, and strong anti-solar differential rotation
develops as a result. We find that the self-generated magnetic fields,
despite being relatively weak, are able to reverse the direction of
differential rotation from anti-solar to solar-like. We also find
that convection flows in this regime are significantly stronger in the
polar regions than in the equatorial region, leading to non-oscillatory
dipole-dominated dynamo solutions, and to a concentration of magnetic
field in the polar regions. We observe that convection has a different
morphology in the inner and the outer part of the convection zone
simultaneously such that organized geostrophic convection columns
are hidden below a near-surface layer of well-mixed highly chaotic
convection. While we focus our attention on the buoyancy-dominated
regime, we also demonstrate that conical differential rotation profiles
and persistent regular dynamo oscillations can be obtained in the
parameter space of the rotation-dominated regime even within this
minimal model.
Title: Effects of Tachocline in Solar-Stellar Dynamo Simulations
Authors: Kosovichev, Alexander; Guerrero, Gustavo; Smolarkiewicz,
Petr; de Gouveia Dal Pino, Elisabete; Mansour, Nagi N.
Bibcode: 2015IAUGA..2258507K
Altcode:
Using 3D implicit large-eddy simulations and comparing these with
observational constrains we investigate variations of the solar rotation
and dynamo-generated magnetic fields. Two sets of simulations models are
explored, the first one considers the solar convective envelope only;
the second one, aiming at the formation of a tachocline, considers also
a fraction of the radiative core. In both kind of models the rotation
profile and the dynamo solution depend on the Rossby number, however,
the flux of angular momentum is also affected by the Lorentz force. The
slow rotating models without tachocline reproduce remarkably well the
solar differential rotation in the convection zone (CZ) including
the tilted iso-rotation contours and the near-surface shear layer
(NSSL). Two of these models exhibit periodic reversals of the magnetic
field with a cycle of ∼ 2 yr. With this short dynamic timescale, the
NSSL apparently does not affect the migration of the surface magnetic
field which results to be poleward. The radial shear at the tachocline
gives rise to a strong toroidal magnetic field at this location. Since
it is also deposited in the stable layer below, it diffuses in longer
timescales than the field at the CZ. These models result either in
an oscillatory dynamo with a ∼ 30 yr cycle period or in steady
fields. In all the models with magnetic cycles the evolution of the
fields seems to be consistent with dynamo waves propagating according
to the Parker-Yoshimura sign rule. We compare the modeling results
with recent helioseismology inferences, and observations of solar and
stellar differential rotation and cycles, including intermittency of
the Maunder-minimum type.
Title: Detection of Fast-moving Waves Propagating Outward along
Sunspots’ Radial Direction in the Photosphere
Authors: Zhao, Junwei; Chen, Ruizhu; Hartlep, Thomas; Kosovichev,
Alexander G.
Bibcode: 2015ApJ...809L..15Z
Altcode: 2015arXiv150704795Z
Helioseismic and magnetohydrodynamic waves are abundant in and
above sunspots. Through cross-correlating oscillation signals in the
photosphere observed by the Solar Dynamics Observatory/Helioseismic
and Magnetic Imager, we reconstruct how waves propagate away from
virtual wave sources located inside a sunspot. In addition to the
usual helioseismic wave, a fast-moving wave is detected traveling along
the sunspot’s radial direction from the umbra to about 15 Mm beyond
the sunspot boundary. The wave has a frequency range of 2.5-4.0 mHz
with a phase velocity of 45.3 km s-1, substantially faster
than the typical speeds of Alfvén and magnetoacoustic waves in the
photosphere. The observed phenomenon is consistent with a scenario of
that a magnetoacoustic wave is excited at approximately 5 Mm beneath the
sunspot. Its wavefront travels to and sweeps across the photosphere with
a speed higher than the local magnetoacoustic speed. The fast-moving
wave, if truly excited beneath the sunspot’s surface, will help open a
new window for studying the internal structure and dynamics of sunspots.
Title: Sunquakes and Two Types of Solar Flares
Authors: Kosovichev, Alexander
Bibcode: 2015IAUGA..2258219K
Altcode:
Uninterrupted observations from Solar Dynamics Observatory provide
unique opportunity for investigation of "sunquakes", helioseismic
waves caused by strong localized impacts in the low atmosphere during
impulsive phase of solar flares. The SDO observations show that these
events are more frequent than previously thought. They are observed in
solar flares from C- to X-classes. However, not all X-class flares
produce sunquakes, and it is puzzling why some relatively weak
flares produce sunquakes, while significantly more powerful flares
do not. Using data from the Helioseismic and Magnetic Imager (HMI)
and Atmospheric Imaging Assembly (AIA) I investigate properties of
sunquakes by detecting and analyzing the seismic wave fronts and the
sources of the flare impact, and discuss physical mechanisms of the
impact. By comparing energetic and morphological characteristics of
the flares with and without sunquakes, I present arguments that this
phenomenon reflects a division between two classes of solar flares:
confined and eruptive, which may be fundamentally different in terms
of the energy release mechanism.
Title: Solar Radius Variations: a New Look on the Wavelength
Dependence
Authors: Rozelot, Jean-Pierre; Kosovichev, Alexander; Kilcik, Ali
Bibcode: 2015IAUGA..2297672R
Altcode:
The possibility that the the Sun's radius is changing, even at a faint
level, has been talk over a long time. As solar radius is certainly one
of the most important basic pieces of astrophysical information, it is
crucial to determine the physical mechanisms that may cause shrinking
or expansion of the solar envelope. The wavelength dependence has been
poorly examine up to now. In this talk, we examine recent solar radius
determinations from space observation of Mercury and Venus transits that
have been made by different teams, in 2006, 2012 and 2014. Seemingly
results are not consistent: authors interpreted the discrepancies by the
different methods of analysis used. However, looking at the wavelength
dependence, adding other available observations, from X-EUV up to radio,
a typical wavelength dependence can be found, reflecting the different
heights at which the lines are formed. A possible explanation is
proposed. Such results can be interesting for solar-stellar connections.
Title: Division II: Commission 12: Solar Radiation and Structure
Authors: Kosovichev, Alexander; Cauzzi, Gianna; Martinez Pillet,
Valentin; Asplund, Martin; Brandenburg, Axel; Chou, Dean-Yi;
Christensen-Dalsgaard, Jorgen; Gan, Weiqun; Kuznetsov, Vladimir D.;
Rovira, Marta G.; Shchukina, Nataliya; Venkatakrishnan, P.
Bibcode: 2015IAUTB..28..109K
Altcode:
The President of C12, Alexander Kosovichev, presented the status of
the Commission and its working Group(s). Primary activities included
organization of international meetings (IAU Symposia, Special Sessions
and Joint Discussion); review and support of proposals for IAU sponsored
meetings; organization of working groups on the Commission topics
to promote the international cooperation; preparation of triennial
report on the organizational and science activities of Commission
members. Commission 12 broadly encompasses topics of solar research
which include studies of the Sun's internal structure, composition,
dynamics and magnetism (through helioseismology and other techniques),
studies of the quiet photosphere, chromosphere and corona, and also
research of the mechanisms of solar radiation, and its variability on
various time scales. Some overlap with topics covered by Commission
10 Solar Activity is unavoidable, and many activities are sponsored
jointly by these two commissions. The Commission website can be found
at http://sun.stanford.edu/IAU-Com12/, with information about related
IAU Symposiums and activities, and links to appropriate web sites.
Title: Dynamics of Electric Currents, Magnetic Field Topology and
Helioseismic Response of a Solar Flare
Authors: Sharykin, Ivan; Kosovichev, Alexander
Bibcode: 2015IAUGA..2286139S
Altcode:
The solar flare on July 30, 2011 was of a modest X-ray class (M9.3),
but it made a strong photospheric impact and produced a "sunquake,'"
observed with the Helioseismic and Magnetic Imager (HMI) on NASA's
Solar Dynamics Observatory (SDO). In addition to the helioseismic
waves (also observed with the SDO/AIA instrument), the flare caused a
large expanding area of white-light emission and was accompanied by
the rapid formation of a sunspot structure in the flare region. The
flare produced hard X-ray emission less than 300 keV and no coronal
mass ejection. The absence of significant coronal mass ejection rules
out magnetic rope eruption as a mechanism of helioseismic waves. We
discuss the connectivity of the flare energy release with the electric
currents dynamics and show the potential importance of high-speed plasma
flows in the lower solar atmosphere during the flare energy release.
Title: 2D and 3D Models of Convective Turbulence and Oscillations
in Intermediate-Mass Main-Sequence Stars
Authors: Guzik, Joyce Ann; Morgan, Taylor H.; Nelson, Nicholas
J.; Lovekin, Catherine; Kitiashvili, Irina N.; Mansour, Nagi N.;
Kosovichev, Alexander
Bibcode: 2015IAUGA..2255601G
Altcode:
We present multidimensional modeling of convection and oscillations
in main-sequence stars somewhat more massive than the sun, using
three separate approaches: 1) Applying the spherical 3D MHD ASH
(Anelastic Spherical Harmonics) code to simulate the core convection
and radiative zone. Our goal is to determine whether core convection
can excite low-frequency gravity modes, and thereby explain the
presence of low frequencies for some hybrid gamma Dor/delta Sct
variables for which the envelope convection zone is too shallow for
the convective blocking mechanism to drive g modes; 2) Using the 3D
planar ‘StellarBox’ radiation hydrodynamics code to model the
envelope convection zone and part of the radiative zone. Our goals
are to examine the interaction of stellar pulsations with turbulent
convection in the envelope, excitation of acoustic modes, and the
role of convective overshooting; 3) Applying the ROTORC 2D stellar
evolution and dynamics code to calculate evolution with a variety of
initial rotation rates and extents of core convective overshooting. The
nonradial adiabatic pulsation frequencies of these nonspherical models
will be calculated using the 2D pulsation code NRO of Clement. We will
present new insights into gamma Dor and delta Sct pulsations gained
by multidimensional modeling compared to 1D model expectations.
Title: Waldmeier's Rules in the Solar and Stellar Dynamos
Authors: Pipin, Valery; Kosovichev, Alexander
Bibcode: 2015IAUGA..2255933P
Altcode:
The Waldmeier's rules [1] establish important empirical relations
between the general parameters of magnetic cycles (such as the
amplitude, period, growth rate and time profile) on the Sun and
solar-type stars [2]. Variations of the magnetic cycle parameters
depend on properties of the global dynamo processes operating in the
stellar convection zones. We employ nonlinear mean-field axisymmetric
dynamo models [3] and calculate of the magnetic cycle parameters,
such as the dynamo cycle period, total magnetic and Poynting fluxes
for the Sun and solar-type stars with rotational periods from 15 to
30 days. We consider two types of the dynamo models: 1) distributed
(D-type) models employing the standard α - effect distributed in
the whole convection zone, and 2) Babcock-Leighton (BL-type) models
with a non-local α - effect. The dynamo models take into account the
principal mechanisms of the nonlinear dynamo generation and saturation,
including the magnetic helicity conservation, magnetic buoyancy effects,
and the feedback on the angular momentum balance inside the convection
zones. Both types of models show that the dynamo generated magnetic flux
increases with the increase of the rotation rate. This corresponds to
stronger brightness variations. The distributed dynamo model reproduces
the observed dependence of the cycle period on the rotation rate for
the Sun analogs better than the BL-type model. For the solar-type
stars rotating more rapidly than the Sun we find dynamo regimes with
multiple periods. Such stars with multiple cycles form a separate
branch in the variability-rotation diagram.1. Waldmeier, M., Prognose
für das nächste Sonnenfleckenmaximum, 1936, Astron. Nachrichten,
259,262. Soon,W.H., Baliunas,S.L., Zhang,Q.,An interpretation
of cycle periods of stellar chromospheric activity, 1993, ApJ,
414,333. Pipin,V.V., Dependence of magnetic cycle parameters on period
of rotation in nonlinear solar-type dynamos, 2015, astro-ph: 14125284
Title: Properties of Turbulent Dynamics and Oscillations of
Main-Sequence Stars Deduced From Numerical Simulations
Authors: Kitiashvili, Irina N.; Mansour, Nagi N.; Kosovichev,
Alexander; Wray, Alan A.
Bibcode: 2015IAUGA..2258520K
Altcode:
Unique observational data from the Kepler mission open new perspectives
for detail investigation of dynamical and internal properties of
numerous stars. However, the new observational results require
better understand links between the stellar turbulent convection
and oscillations. We perform 3D numerical radiative hydrodynamics
simulations of convective and oscillation properties of main-sequence
stars from the solar-type stars to more massive F- and A-type stars. As
the stellar mass increases the convection zone shrinks making it
possible to include the whole convection zone in the computational
domain. Also in more massive stars the scale and intensity of the
turbulent motions dramatically increases, providing more energy for
excitation of acoustic and gravity modes. In this talk I will discuss
properties of the turbulent dynamics of the stars, interaction between
the radiative and convection zones, and excitation of acoustic and
gravity modes.
Title: Generation and Properties of Large-Scale Non-axisymmetric
Magnetic Fields by Solar Dynamo
Authors: Pipin, Valery; Kosovichev, Alexander
Bibcode: 2015IAUGA..2258424P
Altcode:
Large-scale non-axisymmetric magnetic fields generated by the solar
dynamo, and presumably responsible for the phenomenon of "active
longitudes", play an important role in the distribution of solar
activity and flares. By calculating 3D mean-field dynamo models, we
show that nonlinear coupling between axisymmetric and non-axisymmetric
modes, e.g. due to the magnetic feedback on the alpha-effect (see,
e.g., [1]), can maintain a large-scale non-axisymmetric dynamo
process. Non-axisymmetric random fluctuations of dynamo parameters
can be another source for the non-axisymmetric magnetic fields on the
Sun. Such fluctuations can provide a mechanism of the magnetic energy
transfer from the global field to the non-axisymmetric modes. It
is shown that the rotational periods of the non-axisymmetric field
correspond to the dynamo process operating in the subsurface shear
layer which is located in the range of depths 0.85-0.95R. We find
that the magnetic helicity conservation quenches generation of the
non-axisymmetric dynamo modes as well as it does for the axisymmetric
dynamo. It is concluded that the 3D mean-field non-axisymmetric dynamo
models can potentially explain the observed distribution of the solar
magnetic activity.1. Moss, D.,Non-axisymmetric solar magnetic fields,
1999, MNRAS, 306, 300On 3/18/2015 2:29 PM, Valery Pipin wrote:
Title: NST and IRIS multi-wavelength observations of an M1.0 class
solar flare
Authors: Vargas Domínguez, Santiago; Sadykov, Viacheslav; Kosovichev,
Alexander; Sharykin, Ivan; Struminsky, Alexei; Zimovets, Ivan
Bibcode: 2015IAUGA..2257574V
Altcode:
Although solar flares are the most energetic events in the Solar System
and have direct impact in the interplanetary space and ultimately
in our planet, there are still many unresolved issues concerning
their generation, the underlying processes of particle acceleration
involved, the effect at different layer in the solar atmosphere,
among others. This work presents new coordinated observations from the
New Solar Telescope (NST) and the space telescope IRIS that acquired
simultaneous observations of an M1.0 class flare occurred on 12 June,
2014 in active region NOAA 12087. NST filtergrams using the TiO filter,
together with chromospheric data from the Halpha line allow us to study
the evolution of the event from the first signs of the intensification
of the intensity in the region. We focused on a small portion where
the intensity enhancement in Halpha (blue and red wings) seems to be
triggered, and discovered a rapid expansion of a flux-rope structure
near the magnetic neutral line, in the sequence of high-resolution
photospheric images. IRIS observations evidenced strong emission of the
chromospheric and transition region lines during the flare. Jet-like
structures are detected before the initiation of the flare in
chromospheric lines and strong non-thermal emission in the transition
region at the beginning of the impulsive phase. Evaporation flows with
velocities up to 50 km/s occurred in the hot chromospheric plasma. We
interpreted the result in terms of the “gentle” evaporation that
occurs after accelerated particles heat the chromosphere.
Title: Dynamics of electric currents, magnetic field topology and
helioseismic response of a solar flare
Authors: Sharykin, Ivan; Kosovichev, Alexander
Bibcode: 2015IAUGA..2256452S
Altcode:
The solar flare on July 30, 2011 was of a modest X-ray class (M9.3),
but it made a strong photospheric impact and produced a "sunquake,"
observed with the Helioseismic and Magnetic Imager (HMI) on NASA's
Solar Dynamics Observatory (SDO). In addition to the helioseismic
waves (also observed with the SDO/AIA instrument), the flare caused
a large expanding area of white-light emission and was accompanied
by substantial restructuring of magnetic fields, leading to the rapid
formation of a sunspot structure in the flare region. The flare produced
no significant hard X-ray emission and no coronal mass ejection. This
indicates that the flare energy release was mostly confined to the lower
atmosphere. The absence of significant coronal mass ejection rules out
magnetic rope eruption as a mechanism of helioseismic waves. We discuss
the connectivity of the flare energy release with the electric currents
dynamics and show the potential importance of high-speed plasma flows
in the lower solar atmosphere during the flare energy release.
Title: Realistic Modeling of Local Dynamo Processes on the Sun
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Mansour, N. N.; Wray,
A. A.
Bibcode: 2015ApJ...809...84K
Altcode: 2015arXiv150608924K
Magnetic fields are usually observed in the quiet Sun as small-scale
elements that cover the entire solar surface (the “salt-and-pepper”
patterns in line-of-sight magnetograms). By using 3D radiative MHD
numerical simulations, we find that these fields result from a local
dynamo action in the top layers of the convection zone, where extremely
weak “seed” magnetic fields (e.g., from a 10-6 G) can
locally grow above the mean equipartition field to a stronger than 2000
G field localized in magnetic structures. Our results reveal that the
magnetic flux is predominantly generated in regions of small-scale
helical downflows. We find that the local dynamo action takes place
mostly in a shallow, about 500 km deep, subsurface layer, from which
the generated field is transported into the deeper layers by convective
downdrafts. We demonstrate that the observed dominance of vertical
magnetic fields at the photosphere and horizontal fields above the
photosphere can be explained by small-scale magnetic loops produced
by the dynamo. Such small-scale loops play an important role in the
structure and dynamics of the solar atmosphere and their detection in
observations is critical for understanding the local dynamo action on
the Sun.
Title: Dynamics of Electric Currents, Magnetic Field Topology,
and Helioseismic Response of a Solar Flare
Authors: Sharykin, I. N.; Kosovichev, A. G.
Bibcode: 2015ApJ...808...72S
Altcode: 2015arXiv150205190S
The solar flare on 2011 July 30 was of a modest X-ray class (M9.3),
but it made a strong photospheric impact and produced a “sunquake,”
which was observed with the Helioseismic and Magnetic Imager on board
NASA's Solar Dynamics Observatory. In addition to the helioseismic
waves, the flare caused a large expanding area of white-light emission
and was accompanied by the rapid formation of a sunspot structure in the
flare region. The flare produced hard X-ray (HXR) emission less then
300 keV and no coronal mass ejection (CME). The absence of CME rules
out magnetic rope eruption as a mechanism of helioseismic waves. The
sunquake impact does not coincide with the strongest HXR source,
which contradicts the standard beam-driven mechanism of sunquake
generation. We discuss the connectivity of the flare energy release
with the electric currents dynamics and show the potential importance
of high-speed plasma flows in the lower solar atmosphere during the
flare energy release.
Title: Sunquakes: Helioseismic response to solar flares
Authors: Kosovichev, A. G.
Bibcode: 2015exse.book..306K
Altcode: 2014arXiv1402.1249K
Sunquakes observed in the form of expanding wave ripples on the surface
of the Sun during solar flares represent packets of acoustic waves
excited by flare impacts and traveling through the solar interior. The
excitation impacts strongly correlate with the impulsive flare phase,
and are caused by the energy and momentum transported from the
energy release sites. The flare energy is released in the form of
energetic particles, waves, mass motions, and radiation. However,
the exact mechanism of the localized hydrodynamic impacts that
generate sunquakes is unknown. Solving the problem of the sunquake
mechanism will substantially improve our understanding of the flare
physics. In addition, sunquakes offer a unique opportunity for
studying the interaction of acoustic waves with magnetic fields and
flows in flaring active regions, and for developing new approaches to
helioseismic acoustic tomography.
Title: Energy Release and Initiation of a Sunquake in a C-class Flare
Authors: Sharykin, I. N.; Kosovichev, A. G.; Zimovets, I. V.
Bibcode: 2015ApJ...807..102S
Altcode: 2014arXiv1405.5912S
We present an analysis of the C7.0 solar flare from 2013 February
17, revealing a strong helioseismic response (sunquake) caused by a
compact impact observed with the Helioseismic and Magnetic Imager on
board the Solar Dynamics Observatory (SDO) in the low atmosphere. This
is the weakest known C-class flare generating a sunquake event. To
investigate the possible mechanisms of this event and understand
the role of accelerated charged particles and photospheric electric
currents, we use data from three space observatories: RHESSI, SDO,
and Geostationary Operational Environmental Satellite. We find that the
photospheric flare impact does not spatially correspond to the strongest
hard X-ray emission source, but both of these events are parts of the
same energy release. Our analysis reveals a close association of the
flare energy release with a rapid increase in the electric currents
and suggests that the sunquake initiation is unlikely to be caused
by the impact of high-energy electrons, but may be associated with
rapid current dissipation or a localized impulsive Lorentz force in
the lower layers of the solar atmosphere.
Title: Simulations of Stellar Magnetoconvection using the Radiative
MHD Code `StellarBox'
Authors: Wray, Alan A.; Bensassi, Khalil; Kitiashvili, Irina N.;
Mansour, Nagi N.; Kosovichev, Alexander G.
Bibcode: 2015arXiv150707999W
Altcode:
Realistic numerical simulations, i.e., those that make minimal use
of ad hoc modeling, are essential for understanding the complex
turbulent dynamics of the interiors and atmospheres of the Sun and
other stars and the basic mechanisms of their magnetic activity and
variability. The goal of this paper is to present a detailed description
and test results of a compressible radiative MHD code, `StellarBox',
specifically developed for simulating the convection zones, surface,
and atmospheres of the Sun and moderate-mass stars. The code solves the
three-dimensional, fully coupled compressible MHD equations using a
fourth-order Padé spatial differentiation scheme and a fourth-order
Runge-Kutta scheme for time integration. The radiative transfer
equation is solved using the Feautrier method for bi-directional
ray tracing and an opacity-binning technique. A specific feature
of the code is the implementation of subgrid-scale MHD turbulence
models. The data structures are automatically configured, depending
on the computational grid and the number of available processors, to
achieve good load balancing. We present test results and illustrate
the code's capabilities for simulating the granular convection on the
Sun and a set of main-sequence stars. The results reveal substantial
changes in the near-surface turbulent convection in these stars, which
in turn affect properties of the surface magnetic fields. For example,
in the solar case initially uniform vertical magnetic fields tend to
self-organize into compact (pore-like) magnetic structures, while in
more massive stars such structures are not formed and the magnetic
field is distributed more-or-less uniformly in the intergranular lanes.
Title: Oscillations in stellar superflares
Authors: Balona, L. A.; Broomhall, A. -M.; Kosovichev, A.; Nakariakov,
V. M.; Pugh, C. E.; Van Doorsselaere, T.
Bibcode: 2015MNRAS.450..956B
Altcode: 2015arXiv150401491B
Two different mechanisms may act to induce quasi-periodic pulsations
(QPP) in whole-disc observations of stellar flares. One mechanism may be
magnetohydromagnetic forces and other processes acting on flare loops
as seen in the Sun. The other mechanism may be forced local acoustic
oscillations due to the high-energy particle impulse generated by the
flare (known as `sunquakes' in the Sun). We analyse short-cadence Kepler
data of 257 flares in 75 stars to search for QPP in the flare decay
branch or post-flare oscillations which may be attributed to either
of these two mechanisms. About 18 per cent of stellar flares show a
distinct bump in the flare decay branch of unknown origin. The bump
does not seem to be a highly damped global oscillation because the
periods of the bumps derived from wavelet analysis do not correlate
with any stellar parameter. We detected damped oscillations covering
several cycles (QPP), in seven flares on five stars. The periods of
these oscillations also do not correlate with any stellar parameter,
suggesting that these may be a due to flare loop oscillations. We
searched for forced global oscillations which might result after
a strong flare. To this end, we investigated the behaviour of the
amplitudes of solar-like oscillations in eight stars before and after
a flare. However, no clear amplitude change could be detected. We
also analysed the amplitudes of the self-excited pulsations in two δ
Scuti stars and one γ Doradus star before and after a flare. Again,
no clear amplitude changes were found. Our conclusions are that a new
process needs to be found to explain the high incidence of bumps in
stellar flare light curves, that flare loop oscillations may have been
detected in a few stars and that no conclusive evidence exists as yet
for flare induced global acoustic oscillations (starquakes).
Title: Properties of Chromospheric Evaporation and Plasma Dynamics
of a Solar Flare from Iris
Authors: Sadykov, Viacheslav M.; Vargas Dominguez, Santiago;
Kosovichev, Alexander G.; Sharykin, Ivan N.; Struminsky, Alexei B.;
Zimovets, Ivan
Bibcode: 2015ApJ...805..167S
Altcode: 2014arXiv1412.0172S
The dynamics of hot chromospheric plasma of solar flares is a
key to understanding the mechanisms of flare energy release and
particle acceleration. A moderate M1.0 class flare of 2014 June 12,
(SOL2014-06-12T21:12) was simultaneously observed by NASA's Interface
Region Imaging Spectrograph (IRIS) and other spacecraft, and also by
the New Solar Telescope at the BBSO. This paper presents the first
part of our investigation focused on analysis of the IRIS data. Our
analysis of the IRIS data in different spectral lines reveals a strong
redshifted jet-like flow with a speed of ∼100 km s-1 of the
chromospheric material before the flare. Strong nonthermal emission
of the C ii k 1334.5 Å line, formed in the chromosphere-corona
transition region, is observed at the beginning of the impulsive
phase in several small (with a size of ∼1″) points. It is also
found that the C ii k line is redshifted across the flaring region
before, during, and after the impulsive phase. A peak of integrated
emission of the hot (1.1 · 107 K) plasma in the Fe xxi
1354.1 Å line is detected approximately five minutes after the
integrated emission peak of the lower temperature C ii k. A strong
blueshift of the Fe xxi line across the flaring region corresponds to
evaporation flows of the hot chromospheric plasma with a speed of 50
km s-1. Additional analysis of the RHESSI data supports
the idea that the upper chromospheric dynamics observed by IRIS has
features of “gentle” evaporation driven by heating of the solar
chromosphere by accelerated electrons and by a heat flux from the
flare energy release site.
Title: A Method for the Estimation of p-Mode Parameters from Averaged
Solar Oscillation Power Spectra
Authors: Reiter, J.; Rhodes, E. J., Jr.; Kosovichev, A. G.; Schou,
J.; Scherrer, P. H.; Larson, T. P.
Bibcode: 2015ApJ...803...92R
Altcode: 2015arXiv150407493R
A new fitting methodology is presented that is equally well suited for
the estimation of low-, medium-, and high-degree mode parameters from
m-averaged solar oscillation power spectra of widely differing spectral
resolution. This method, which we call the “Windowed, MuLTiple-Peak,
averaged-spectrum” or WMLTP Method, constructs a theoretical profile
by convolving the weighted sum of the profiles of the modes appearing
in the fitting box with the power spectrum of the window function of
the observing run, using weights from a leakage matrix that takes into
account observational and physical effects, such as the distortion of
modes by solar latitudinal differential rotation. We demonstrate that
the WMLTP Method makes substantial improvements in the inferences of
the properties of the solar oscillations in comparison with a previous
method, which employed a single profile to represent each spectral
peak. We also present an inversion for the internal solar structure,
which is based upon 6366 modes that we computed using the WMLTP method
on the 66 day 2010 Solar and Heliospheric Observatory/MDI Dynamics
Run. To improve both the numerical stability and reliability of the
inversion, we developed a new procedure for the identification and
correction of outliers in a frequency dataset. We present evidence
for a pronounced departure of the sound speed in the outer half of the
solar convection zone and in the subsurface shear layer from the radial
sound speed profile contained in Model S of Christensen-Dalsgaard and
his collaborators that existed in the rising phase of Solar Cycle 24
during mid-2010.
Title: Detection of Fast-Moving Waves Propagating Outward from
Sunspots in the Photosphere
Authors: Zhao, Junwei; Chen, Ruizhu; Hartlep, Thomas; Kosovichev,
Alexander
Bibcode: 2015TESS....110504Z
Altcode:
Helioseismic and magnetohydrodynamic waves are abundant in and above
sunspots. Through cross-correlating oscillation signals at various
locations, we are able to reconstruct how waves propagate away from
a wave source inside a sunspot in the photospheric level. Before
helioseismic waves are visible propagating away from the source,
a surprisingly fast-moving wave is detected traveling along the
sunspot's radial directionfrom inside of the sunspot to the outside,
extending about 15 Mm beyond the sunspot boundary. The wave has a
frequency range of 2.5 - 4.0 mHz, and appears dispersionless with a
phase velocity of 45.3 km/s, a few times faster than typical speeds of
sound and magnetohydrodynamic waves in the photosphere. The observed
wave is consistent with a magnetoacoustic wave, excited at approximately
5 Mm beneath the sunspot surface, sweeping across the photosphere,
although it is not clear how this wave is excited at that depth. If the
fast-moving wave is truly excited in the sunspot's subsurface area,
this will help open a new window to study the internal structure and
dynamics of sunspots.
Title: Preface: Cosmic magnetic fields
Authors: Kosovichev, Alexander
Bibcode: 2015AdSpR..55..779K
Altcode:
Recent advances in observations and modeling have opened new
perspectives for the understanding of fundamental dynamical processes
of cosmic magnetism, and associated magnetic activity on the Sun,
stars and galaxies. The goal of the Special Issue is to discuss
the progress in solar physics and astrophysics, similarities and
differences in phenomenology and physics of magnetic phenomena on the
Sun and other stars. Space observatories, ground-based telescopes,
and new observational methods have provided tremendous amount of
data that need to be analyzed and understood. The solar observations
discovered multi-scale organization of solar activity, dramatically
changing current paradigms of solar variability. On the other side,
stellar observations discovered new regimes of dynamics and magnetism
that are different from the corresponding solar phenomena, but described
by the same physics. Stars represent an astrophysical laboratory for
studying the dynamical, magnetic and radiation processes across a broad
range of stellar masses and ages. These studies allow us to look at
the origin and evolution of our Sun, whereas detailed investigations
of the solar magnetism give us a fundamental basis for interpretation
and understanding of unresolved stellar data.
Title: Flare Energy Release in the Low Solar Atmosphere: Fine
Structuring, Electric Currents and Sunquakes.
Authors: Sharykin, I.; Kosovichev, A. G.
Bibcode: 2014AGUFMSH41C4163S
Altcode:
Observations of C2.1 flare of August 15, 2011, obtained at NST/BBSO,
reveal fine structuring of H-alpha flare ribbons (with a characteristic
width ~100 km). Using SDO/HMI vector magnetic field measurements we
calculate vertical electric currents and discuss their role in the
flare energy release. We argue that the observed fine structuring of the
flare ribbons can be associated with dissipation of electric currents
in the partially ionized chromospheric plasma. We also discuss the
role of electric currents in two flares with sunquakes: C7.0 flare
of Febrary 17, 2013, and M9.2 flare of July 30, 2011. We find that
the observed sunquake impacts do not spatially correlate with the
strongest HXR emission sources but coincide with the intensive electric
currents. These observations suggest that the primary energy release
can be connected with rapid dissipation of electric currents in the low
atmosphere. We also conclude that the fine structuring of the electric
currents may play a very important role, as it leads to enhanced and
fast magnetic energy release, especially in the case of turbulent and
ambipolar resistivity which can take place in the low solar atmosphere.
Title: Large-scale flows and magnetic fields in solar-like stars
from global simulation with and without tachocline
Authors: Guerrero, G.; Kosovichev, A. G.; Smolarkiewicz, P. K.;
de Gouveia Dal Pino, E. M.
Bibcode: 2014AGUFMSH41B4150G
Altcode:
The large-scale flows patterns like differential rotation and
meridional circulation as well as the mean-field dynamo action in the
Sun and solar-like stars are thought to have their origin in helical
turbulent motions in the stellar convection zones. In this work we will
present recent results of HD and MHD global simulations of stars whose
stratification resemble that of the solar interior. The simulations are
performed with the EULAG code (Smolarkiewicz et al. 2001). They include
implicit modeling of the large-eddy contribution from the turbulent
scales to the resolved scales, thus, allowing higher turbulent levels
(e.g., Guerrero et al. 2013). In the HD regime, the value of the Rossby
(Ro) number defines large-scale flow patterns. Large values of Ro result
in an anti-solar differential rotation and a meridional circulation
consistent with a single circulation cell per hemisphere. Lower values
of Ro result in a solar-type differential rotation and a meridional
flow with multiple cells in radius and latitude. Due to the low
dissipation of the numerical scheme, the models are also able to
reproduce the tachocline and sustain it over a longer time scale. In
the MHD regime, both solutions are still allowed, however, the shift
from anti-solar to the solar-like rotation happens at a larger value
or Ro. A wide range of dynamo solutions is obtained for the magnetic
field, including steady and oscillating modes (see e.g., Fig. 1). We
also compare models with and without a stable stratified layer at
the bottom of the convection zone. We notice that the presence of a
naturally developed tachocline plays an important role in the dynamo
solution, modifying the morphology of the magnetic field, the cycles
period and influencing the large-scale flows.References:Smolarkiewicz,
P. K., Margolin, L. G., & Wyszogrodzki, A. A. 2001, JAtS, 58,
349; Guerrero, G., Smolarkiewicz, P. K., Kosovichev, A.K., Mansour,
N.N. 2013, ApJ, 779, 176.
Title: Three-Dimensional Magnetic Reconnection Under Low Chromospheric
Conditions Using a Two-Fluid Weakly Ionized Reactive Plasma Model
Authors: Alvarez Laguna, A.; Lani, A.; Poedts, S.; Mansour, N. N.;
Kosovichev, A. G.
Bibcode: 2014AGUFMSH23A4151A
Altcode:
Magnetic reconnection is a physical process enabling for the conversion
of so-called free (non-potential) magnetic energy into kinetic and
thermal energy by breaking the flux conservation law that exists for
ideal (i.e. perfectly conducting) plasmas. This ubiquitous phenomenon in
magnetized plasma plays an important role in the Sun's chromosphere as
likely being responsible for transient plasma phenomena such as solar
flares, spicules and chromospheric jets. In this work, we present
a computational model that simulates magnetic reconnection under
low chromospheric conditions using a two-fluid (plasma + neutrals)
approach introduced by Leake et al. (2012). This model considers
non-equilibrium partial ionization effects including ionization,
recombination reactions and scattering collisions while simulating
the interplay between the charged particles with the electromagnetic
field. Previous 2D simulations showed that the dynamics of ions
and neutrals are decoupled during the reconnection process. Also,
the effect of the chemical non-equilibrium in the reconnection region
plays a crucial role, yielding faster reconnection rates. We extended
these simulations to study different 3D configurations in order to
analyze the impact of non-equilibrium partial ionization effects on
the neutral sheet configuration(s) and the reconnection rate of more
realistic geometries. The results are compared with the two-dimensional
simulations.
Title: Swirling motions, fast plasma flows and small-scale
chromospheric eruptions in a sunspot light-bridge
Authors: Vargas Domínguez, S.; Kosovichev, A. G.
Bibcode: 2014AGUFMSH41C4159V
Altcode:
Multi-wavelength observations with the 1.6m New Solar Telescope (NST)
at Big Bear Solar Observatory (BBSO) have evidenced a complex dynamics
of sunspots. We present photospheric and chromospheric observations
of a sunspot light-bridge in AR 11850 taken on 29 September 2013. The
NST/BFI data in the TiO reveal strong shearing and high-speed swirling
flows in the light-bridge. Scanning of the H-alpha spectral line shows
strong chromospheric downflows in the red wing near the outermost
part of the light-bridge. Chromospheric structuring around the spot
appears to be more complex due to the presence of the light-bridge. We
detect small-scale explosive events that are likely to be boosted by
the interactions of magnetic field lines at the chromospheric level
and therefore triggered by the evolution of the light bridge in the
photosphere beneath. In particular a very localized explosive event
reaching the transition region and coronal temperatures is observed
from simultaneous IRIS and SDO data.
Title: Realistic Modeling of Multi-Scale MHD Dynamics of the Solar
Atmosphere
Authors: Kitiashvili, I.; Mansour, N. N.; Wray, A. A.; Yoon, S.;
Kosovichev, A. G.
Bibcode: 2014AGUFMSH41B4134K
Altcode:
Realistic 3D radiative MHD simulations open new perspectives for
understanding the turbulent dynamics of the solar surface, its
coupling to the atmosphere, and the physical mechanisms of generation
and transport of non-thermal energy. Traditionally, plasma eruptions
and wave phenomena in the solar atmosphere are modeled by prescribing
artificial driving mechanisms using magnetic or gas pressure forces
that might arise from magnetic field emergence or reconnection
instabilities. In contrast, our 'ab initio' simulations provide a
realistic description of solar dynamics naturally driven by solar
energy flow. By simulating the upper convection zone and the solar
atmosphere, we can investigate in detail the physical processes of
turbulent magnetoconvection, generation and amplification of magnetic
fields, excitation of MHD waves, and plasma eruptions. We present recent
simulation results of the multi-scale dynamics of quiet-Sun regions, and
energetic effects in the atmosphere and compare with observations. For
the comparisons we calculate synthetic spectro-polarimetric data to
model observational data of SDO, Hinode, and New Solar Telescope.
Title: Analysis of High-Resolution Observations of Sunspot
Oscillations and Waves
Authors: Kosovichev, A. G.
Bibcode: 2014AGUFMSH41C4164K
Altcode:
Oscillations of sunspots with a characteristic period of 3 minutes are
prominent features of the sunspot dynamics. In the sunspot chromosphere,
the oscillations are observed in the form of "running penumbral waves"
representing shocks with narrow compression fronts, traveling along the
expanding magnetic field lines. The origin of the sunspot oscillations
and waves is not understood, and is a subject of hot debates. It had
been suggested that the sunspot oscillations may be due to leakage of
high-frequency acoustic (p) modes excited in subphotospheric layers. An
alternative point of view is that the sources of sunspot oscillations
and waves are in the chromosphere. Using long series of high-resolution
sunspot images obtained with a narrow-band H-alpha filter at the
1.6m New Solar Telescope at Big Bear Solar Observatory, I identify
individual wave excitation events, investigate their properties and
discuss potential physical mechanisms.
Title: Using MHD simulations to model H-alpha and UV spectral lines
for interpretation of IRIS and NST data
Authors: Sadykov, Viacheslav M.; Kosovichev, Alexander G.
Bibcode: 2014arXiv1412.0288S
Altcode:
We present results of non-LTE modeling of H-alpha 6563 A and Mg
II k&h 2796 A and 2803 A lines. This modeling is important for
interpretation of coordinated observations from the recently launched
NASA's IRIS mission and from the New Solar Telescope at Big Bear
Solar Observatory. Among available codes for the non-LTE modeling,
the RH code is chosen as the most appropriate for modeling of the line
profiles. The most suitable Hydrogen and Magnesium atomic models are
selected by performing several tests of the code. The influence of the
ionization degree on the line profiles is also studied. Radiative-MHD
simulations of the solar atmosphere, obtained with the Bifrost code,
are used as input data for calculation of synthetic spectra of the
H-alpha and Mg II h&k lines for particular locations evolving with
time. The spectral line variations reveal the presence of MHD waves in
the simulation results. We construct oscillation power spectra of the
line intensity for different wavelength, and compare these with the
corresponding height-dependent power spectra of atmospheric parameters
from the simulations. We find correlations between the power spectra
of intensities of the line profiles at certain wavelengths and the
power spectra of the atmospheric parameters at the tau-unity heights
for these wavelengths. These results provide a new diagnostic method
of chromospheric oscillations; however, larger amounts of data are
needed to confirm these correlations.
Title: Multi-wavelength High-resolution Observations of a Small-scale
Emerging Magnetic Flux Event and the Chromospheric and Coronal
Response
Authors: Vargas Domínguez, Santiago; Kosovichev, Alexander;
Yurchyshyn, Vasyl
Bibcode: 2014ApJ...794..140V
Altcode: 2014arXiv1405.3550V
State-of-the-art solar instrumentation is now revealing magnetic
activity of the Sun with unprecedented temporal and spatial
resolutions. Observations with the 1.6 m aperture New Solar Telescope
(NST) of the Big Bear Solar Observatory are making next steps in our
understanding of the solar surface structure. Granular-scale magnetic
flux emergence and the response of the solar atmosphere are among the
key research topics of high-resolution solar physics. As part of a joint
observing program with NASA's Interface Region Imaging Spectrograph
(IRIS) mission on 2013 August 7, the NST observed active region
NOAA 11,810 in the photospheric TiO 7057 Å band with a resolution
of pixel size of 0.''034 and chromospheric He I 10830 Å and Hα
6563 Å wavelengths. Complementary data are provided by the Solar
Dynamics Observatory (SDO) and Hinode space-based telescopes. The
region displayed a group of solar pores, in the vicinity of which we
detect a small-scale buoyant horizontal magnetic flux tube causing
granular alignments and interacting with the preexisting ambient field
in the upper atmospheric layers. Following the expansion of distorted
granules at the emergence site, we observed a sudden appearance of an
extended surge in the He I 10830 Å data (bandpass of 0.05 Å). The
IRIS transition region imaging caught ejection of a hot plasma jet
associated with the He I surge. The SDO/HMI data used to study the
evolution of the magnetic and Doppler velocity fields reveal emerging
magnetic loop-like structures. Hinode/Ca II H and IRIS filtergrams
detail the connectivities of the newly emerged magnetic field in the
lower solar chromosphere. From these data, we find that the orientation
of the emerging magnetic field lines from a twisted flux tube formed
an angle of ~45° with the overlying ambient field. Nevertheless,
the interaction of emerging magnetic field lines with the pre-existing
overlying field generates high-temperature emission regions and boosts
the surge/jet production. The localized heating is detected before
and after the first signs of the surge/jet ejection. We compare the
results with previous observations and theoretical models and propose a
scenario for the activation of plasma jet/surges and confined heating
triggered by buoyant magnetic flux tubes rising up into a magnetized
upper environment. Such process may play a significant role in the
mass and energy flow from the interior to the corona.
Title: Control and operation of the 1.6 m New Solar Telescope in
Big Bear
Authors: Varsik, J.; Plymate, C.; Goode, P.; Kosovichev, A.; Cao,
W.; Coulter, R.; Ahn, K.; Gorceix, N.; Shumko, S.
Bibcode: 2014SPIE.9147E..5DV
Altcode:
The 1.6m New Solar Telescope (NST) has developed a modern and
comprehensive suite of instruments which allow high resolution
observations of the Sun. The current instrument package comprises
diffraction limited imaging, spectroscopic and polarimetric instruments
covering the wavelength range from 0.4 to 5.0 microns. The instruments
include broadband imaging, visible and near-infrared scanning
Fabry-Perot interferometers, an imaging spectropolarimeter,
a fast visible-light imaging spectrograph, and a unique new
scanning cryogenic infrared spectrometer/spectropolarimeter
that is nearing completion. Most instruments are operated with
a 308 subaperture adaptive optics system, while the thermal-IR
spectrometer has a correlation tracker. This paper reports on the
current observational programs and operational performance of the
telescope and instrumentation. The current control, data processing,
and archiving systems are also briefly discussed.
Title: Solar Meridional Flow in the Shallow Interior during the
Rising Phase of Cycle 24
Authors: Zhao, Junwei; Kosovichev, A. G.; Bogart, R. S.
Bibcode: 2014ApJ...789L...7Z
Altcode: 2014arXiv1406.2735Z
Solar subsurface zonal- and meridional-flow profiles during the
rising phase of solar cycle 24 are studied using the time-distance
helioseismology technique. The faster zonal bands in the
torsional-oscillation pattern show strong hemispheric asymmetries and
temporal variations in both width and speed. The faster band in the
northern hemisphere is located closer to the equator than the band
in the southern hemisphere and migrates past the equator when the
magnetic activity in the southern hemisphere is reaching maximum. The
meridional-flow speed decreases substantially with the increase of
magnetic activity, and the flow profile shows two zonal structures in
each hemisphere. The residual meridional flow, after subtracting a mean
meridional-flow profile, converges toward the activity belts and shows
faster and slower bands like the torsional-oscillation pattern. More
interestingly, the meridional-flow speed above latitude 30° shows
an anti-correlation with the poleward-transporting magnetic flux,
slower when the following-polarity flux is transported and faster when
the leading-polarity flux is transported. It is expected that this
phenomenon slows the process of magnetic cancellation and polarity
reversal in high-latitude areas.
Title: Studying Shallow Meridional Flow by Time-Distance
Helioseismology during the Rising Phase of Cycle 24
Authors: Zhao, Junwei; Kosovichev, Alexander G.; Bogart, Richard S.
Bibcode: 2014AAS...22421805Z
Altcode:
Using continuous SDO/HMI Doppler observations, we have studied the
solar subsurface torsional oscillation and meridional flow during the
rising phase of solar cycle 24. The faster bands of the torsional
oscillation pattern show clear temporal variations in both width
and strength, and the band in the northern hemisphere extends past
the equator into the southern hemisphere. The meridional-flow speed
drops substantially with the rise of magnetic activity, and at some
depths and for some briefperiods, the flow even reverses directions in
mid-latitudes. The residual meridional flow shows faster and slower
bands like the torsional oscillation patterns, but its faster bands
approach the equator earlier than the torsional oscillation bands. More
interestingly, the meridional flow speed in latitudes above 35 degree
slows down (speeds up) when the following-polarity (leading-polarity)
magnetic flux is transported poleward, essentially delaying the magnetic
polarity reversal and the onset of the next solar cycle.
Title: Transient Small-Scale Magnetic Flux Emergence and Atmospheric
Response Observed with New Solar Telescope and SDO
Authors: Vargas Domínguez, Santiago; Kosovichev, Alexander G.
Bibcode: 2014AAS...22412345V
Altcode:
State-of-the art solar instrumentation is now revealing the activity of
the Sun at the highest temporal and spatial resolution. Granular-scale
magnetic flux emergence and the response of the solar atmosphere is
one of the key topics. Observations with the 1.6m aperture New Solar
Telescope (NST) at Big Bear Solar Observatory (BBSO) are making next
steps in our understanding of the solar surface structure. On August 7,
2013, NST observed active region NOAA 11810 in different photospheric
and chromospheric wavelengths. The region displays a group of solar
pores, in the vicinity of which we detected a site of emerging magnetic
flux accompanied by intense and very confined abnormal granulation
dynamics, observed in the photospheric TiO 7057 A with a resolution
of 0.034 “/pix. Following the expansion of exploding granules in
this site, we observed a sudden appearance of an extended surge in the
HeI 10830A data (bandpass of 0.05 A). The SDO/HMI data used to study
the evolution of the magnetic field and Doppler velocities reveal a
short-lived emerging loop-like structure with strong upflows. We used
the SDO/AIA data to investigate the response of the transition region
and corona to the transient emerging flux phenomenon. We compare the
results with previous observations, and propose a scenario for the
production of plasma surges by the transient magnetic flux emergence
events.
Title: Fine Structure of Flare Ribbons and Evolution of Electric
Currents
Authors: Sharykin, I. N.; Kosovichev, A. G.
Bibcode: 2014ApJ...788L..18S
Altcode: 2014arXiv1404.5104S
Emission of solar flares across the electromagnetic spectrum is often
observed in the form of two expanding ribbons. The standard flare model
explains flare ribbons as footpoints of magnetic arcades, emitting
due to interaction of energetic particles with the chromospheric
plasma. However, the physics of this interaction and properties of
the accelerated particles are still unknown. We present results of
multiwavelength observations of the C2.1 flare of 2013 August 15,
observed with the New Solar Telescope of the Big Bear Solar Observatory,
and the Solar Dynamics Observatory, GOES, and Fermi spacecraft. The
observations reveal previously unresolved sub-arcsecond structure of
flare ribbons in regions of strong magnetic field consisting from
numerous small-scale bright knots. We observe a red-blue asymmetry
of Hα flare ribbons with a width as small as ~100 km. We
discuss the relationship between the ribbons and vertical electric
currents estimated from vector magnetograms, and show that Joule
heating can be responsible for energization of Hα knots
in the ribbons.
Title: Multiscale Properties of the Local Dynamo on the Sun
Authors: Kitiashvili, Irina; Kosovichev, Alexander G.; Mansour,
Nagi N; Wray, Alan A
Bibcode: 2014AAS...22410304K
Altcode:
Dynamics of the quiet Sun represents a background ('salt-and-pepper')
state for powerful manifestations of solar activity. Current numerical
simulations have shown that small-scale turbulent dynamics can
strongly couple with processes on larger scales, such as formation
of pores and sunspots. We perform 3D MHD radiative simulations of
top layers of the convection zone and the low atmosphere, taking
into account effects of turbulence, magnetic fields, ionization and
excitation of all abundant elements. To model the dynamo process we
carry a series of the simulations with various initial weak levels
of magnetic field perturbations. The results show that an initial,
randomly distributed ('seed') magnetic field of 1 micro-gauss, greatly
amplifies by subsurface turbulent dynamics. The self generated magnetic
field (dynamo) reaches 2 kG magnetic levels in the photosphere. The
local dynamo process primary operates 1 Mm below the surface
where the magnetic fields are amplified by helical flows. These
dynamo-generated magnetic fields are transported by downflows into
deeper layers. The process of the magnetic field amplification has
a substantially multiscale character, during which self-organized
turbulent helical flows work coherently on scales much larger then the
turbulent scales. We discuss the apparent contradiction of our results
with current paradigm that local dynamo can generate magnetic fields
only on the small turbulent scales. We compare our results with other
simulations and observations.
Title: Sub-arcsecond Structure and Dynamics of Flare Ribbons Observed
with New Solar Telescope
Authors: Sharykin, Ivan; Kosovichev, Alexander G.
Bibcode: 2014AAS...22412310S
Altcode:
Emission of solar flares across the electromagnetic spectrum is often
observed in the form of two expanding ribbons. The standard flare
model explains the flare ribbons as footpoints of magnetic arcades,
emitting due to the interaction of energetic particles with the
chromospheric plasma. However, the physics of this interaction and
properties of the accelerated particles are still unknown. We present
results of multiwavelength observations of C2.1 flare of August 15,
2011, observed with the 1.6-meter New Solar Telescope of Big Bear
Solar Observatory. These unique data are characterized by the great
spatial resolution reaching the telescope diffraction limit with good
spectral scanning of H-alpha line, and photospheric imaging. The
observations reveal previously unresolved sub-arcsecond structure
of the flare ribbons in regions of strong magnetic field. We discuss
the fine structure of the flare ribbons, their dynamics, and possible
mechanisms of the energy release and transport, using also data from
SDO, GOES and FERMI spacecraft.
Title: High Resolution Observations of Chromospheric Jets in
Sunspot Umbra
Authors: Yurchyshyn, Vasyl B.; Abramenko, Valentyna; Kosovichev,
Alexander G.; Goode, Philip R.
Bibcode: 2014AAS...22432301Y
Altcode:
Recent observations of sunspot's umbra suggested that it may be finely
structured at a sub-arcsecond scale representing a mix of hot and cool
plasma elements. In this study we report the first detailed observations
of the umbral spikes, which are cool jet-like structures seen in the
chromosphere of an umbra. The spikes are cone-shaped features with
a typical height of 0.5-1. Mm and a width of about 0. Mm. Their life
time ranges from 2 to 3 ~min and they tend to re-appear at the same
location. The preliminary analysis indicates that the spikes are not
associated with photospheric umbral dots and they rather tend to
occur above darkest parts of the umbra, where magnetic fields are
strongest. The spikes exhibit up and down oscillatory motions and
their spectral evolution suggests that they might be driven by upward
propagating shocks generated by photospheric oscillations. It is worth
noting that triggering of the running penumbral waves seems to occur
during the interval when the spikes reach their maximum height.
Title: Unusual Sunquake Events Challenge the Standard Model of
Solar Flares
Authors: Kosovichev, Alexander G.; Sharykin, Ivan; Zimovets, Ivan
Bibcode: 2014AAS...22410403K
Altcode:
"Sunquakes" represent helioseismic waves excited by solar
flares. According to the standard flare model, sunquakes are associated
with the hydrodynamic response of the low atmosphere to beams of
flare-accelerated particles. Observations with the HMI instrument
on Solar Dynamics Observatory have shown that sunquakes are a much
more common phenomenon than this was found from the previous SOHO/MDI
observations. The HMI observations reveal that sunquakes may occur not
only during strong X-class flare but also in relatively weak flares
of low M-class (as low as M1). It is particularly surprising that,
in some cases, the sunquake initiating impacts are observed in the
early impulsive or even pre-heating phase, prior to the main hard X-ray
impulse and even without a significant hard X-ray signal. We examine
properties of such sunquake events, present a detailed analysis of
M2.8 flare of February 17, 2013, using HMI, AIA, GOES and RHESSI data,
and discuss implications for the standard flare model.
Title: Photospheric and Chromospheric Dynamics of Sunspots Observed
with New Solar Telescope
Authors: Kosovichev, Alexander G.; Yurchyshyn, Vasyl B.
Bibcode: 2014AAS...22421811K
Altcode:
The 1.6m New Solar Telescope (NST) of Big Bear Solar Observatory
allows us to investigate the structure and dynamics of sunspots with
unprecedented spatial and temporal resolutions. We present results
of simultaneous observations of a sunspot in the photosphere with a
broad-band TiO-line filter and in the chromospheric H-alpha line with
Visible Imaging Spectrometer, and compare the observational results with
MHD models of sunspots. The observations reveal previously unresolved
features of the sunspot umbra and penumbra. In particular, the TiO data
clearly demonstrate highly twisted dynamics of penumbral filaments and
umbral dots and reveal strong shearing plasma flows in sunspot bridges,
not explained by the MHD simulations. The high-resolution H-alpha
spectroscopic data provide new views of the sunspot chromospheric
dynamics, including the fine structure of oscillations and waves,
penumbral jets, ubiquitous small-scale eruptions, and accretion
flows in a form of dense plasma sheets. The diffraction-limited NST
observations show that the sunspot dynamics is more complicated and
much richer than it is described by the current sunspot models.
Title: High Resolution Observations of Chromospheric Jets in
Sunspot Umbra
Authors: Yurchyshyn, V.; Abramenko, V.; Kosovichev, A.; Goode, P.
Bibcode: 2014ApJ...787...58Y
Altcode: 2014arXiv1404.7444Y
Recent observations of a sunspot's umbra have suggested that it may
be finely structured on a subarcsecond scale representing a mix of hot
and cool plasma elements. In this study, we report the first detailed
observations of umbral spikes, which are cool jet-like structures seen
in the chromosphere of an umbra. The spikes are cone-shaped features
with a typical height of 0.5-1.0 Mm and a width of about 0.1 Mm. Their
lifetime ranges from 2 to 3 minutes and they tend to re-appear at
the same location. The spikes are not associated with photospheric
umbral dots and they instead tend to occur above the darkest parts of
the umbra where magnetic fields are strongest. The spikes exhibit up
and down oscillatory motions and their spectral evolution suggests
that they might be driven by upward propagating shocks generated by
photospheric oscillations. It is worth noting that triggering of the
running penumbral waves seems to occur during the interval when the
spikes reach their maximum height.
Title: Verification of the Helioseismology Travel-time Measurement
Technique and the Inversion Procedure for Sound Speed Using
Artificial Data
Authors: Parchevsky, K. V.; Zhao, J.; Hartlep, T.; Kosovichev, A. G.
Bibcode: 2014ApJ...785...40P
Altcode:
We performed three-dimensional numerical simulations of the solar
surface acoustic wave field for the quiet Sun and for three models with
different localized sound-speed perturbations in the interior with
deep, shallow, and two-layer structures. We used the simulated data
generated by two solar acoustics codes that employ the same standard
solar model as a background model, but utilize different integration
techniques and different models of stochastic wave excitation. Acoustic
travel times were measured using a time-distance helioseismology
technique, and compared with predictions from ray theory frequently
used for helioseismic travel-time inversions. It is found that the
measured travel-time shifts agree well with the helioseismic theory
for sound-speed perturbations, and for the measurement procedure with
and without phase-speed filtering of the oscillation signals. This
testing verifies the whole measuring-filtering-inversion procedure
for static sound-speed anomalies with small amplitude inside the
Sun outside regions of strong magnetic field. It is shown that the
phase-speed filtering, frequently used to extract specific wave packets
and improve the signal-to-noise ratio, does not introduce significant
systematic errors. Results of the sound-speed inversion procedure show
good agreement with the perturbation models in all cases. Due to its
smoothing nature, the inversion procedure may overestimate sound-speed
variations in regions with sharp gradients of the sound-speed profile.
Title: Effects of Anisotropies in Turbulent Magnetic Diffusion in
Mean-field Solar Dynamo Models
Authors: Pipin, V. V.; Kosovichev, A. G.
Bibcode: 2014ApJ...785...49P
Altcode: 2013arXiv1307.6651P
We study how anisotropies of turbulent diffusion affect the
evolution of large-scale magnetic fields and the dynamo process
on the Sun. The effect of anisotropy is calculated in a mean-field
magnetohydrodynamics framework assuming that triple correlations provide
relaxation to the turbulent electromotive force (so-called the "minimal
τ-approximation"). We examine two types of mean-field dynamo models:
the well-known benchmark flux-transport model and a distributed-dynamo
model with a subsurface rotational shear layer. For both models,
we investigate effects of the double- and triple-cell meridional
circulation, recently suggested by helioseismology and numerical
simulations. To characterize the anisotropy effects, we introduce
a parameter of anisotropy as a ratio of the radial and horizontal
intensities of turbulent mixing. It is found that the anisotropy
affects the distribution of magnetic fields inside the convection
zone. The concentration of the magnetic flux near the bottom and top
boundaries of the convection zone is greater when the anisotropy is
stronger. It is shown that the critical dynamo number and the dynamo
period approach to constant values for large values of the anisotropy
parameter. The anisotropy reduces the overlap of toroidal magnetic
fields generated in subsequent dynamo cycles, in the time-latitude
"butterfly" diagram. If we assume that sunspots are formed in the
vicinity of the subsurface shear layer, then the distributed dynamo
model with the anisotropic diffusivity satisfies the observational
constraints from helioseismology and is consistent with the value of
effective turbulent diffusion estimated from the dynamics of surface
magnetic fields.
Title: Sunquakes and starquakes
Authors: Kosovichev, Alexander G.
Bibcode: 2014IAUS..301..349K
Altcode: 2014arXiv1401.8036K
In addition to well-known mechanisms of excitation of solar and
stellar oscillations by turbulent convection and instabilities, the
oscillations can be excited by an impulsive localized force caused
by the energy release in solar and stellar flares. Such oscillations
have been observed on the Sun (`sunquakes'), and created a lot of
interesting discussions about physical mechanisms of the impulsive
excitation and their relationship to the flare physics. The observation
and theory have shown that most of a sunquake's energy is released
in high-degree, high-frequency p modes. In addition, there have been
reports on helioseismic observations of low-degree modes excited by
strong solar flares. Much more powerful flares observed on other stars
can cause `starquakes' of substantially higher amplitude. Observations
of such oscillations can provide new asteroseismic information and
also constraints on mechanisms of stellar flares. I discuss the basic
properties of sunquakes, and initial attempts to detect flare-excited
oscillations in Kepler short-cadence data.
Title: Emergence of a small-scale magnetic flux tube and the response
of the solar atmosphere
Authors: Vargas Dominguez, S.; Kosovichev, A. G.; Yurchyshyn, V.
Bibcode: 2014CEAB...38...25V
Altcode:
Cutting-edge observations with the 1.6-meter telescope at Big Bear
Solar Observatory (BBSO) in California have taken research into the
activity of the Sun to new levels of understanding of the structure
and evolution of the solar atmosphere at high-resolution spatial and
temporal scales. On August 7, 2013 the NST observed active region NOAA
11810 in photospheric and chromospheric wavelengths. The observations
were performed as part of a program conducted jointly with NASA's
Interface Region Imaging Spectrograph (IRIS) mission, Solar Dynamics
Observatory (SDO) and Hinode satellite. These observations provided a
unique view on the emergence of a buoyant small-scale magnetic-flux
rope in the solar photosphere. The event is accompanied by response
of the solar atmosphere once the newly emerged field interacts with
the pre-existing overlying one. The reconnection process that takes
place in the region produces jet emission and high-temperature points
in the chromosphere and corona.
Title: Mechanism of local dynamo action on the Sun
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Mansour, N. N.; Wray,
A. A.
Bibcode: 2013arXiv1312.0982K
Altcode:
In the quiet Sun, magnetic fields are usually observed as small-scale
magnetic elements, `salt and pepper', covering the entire solar
surface. By using 3D radiative MHD numerical simulations we demonstrate
that these fields are a result of local dynamo action in the top layers
of the convection zone, where extremely weak `seed' magnetic fields can
locally grow above the mean equipartition field (e.g., from a $10^{-6}$
G `seed' field to more than 1000 G magnetic structures). We find that
the local dynamo action takes place only in a shallow, about 500 km
deep, subsurface layer, from which the generated field is transported
into deeper layers by convection downdrafts. We demonstrate that the
observed dominance of vertical magnetic fields at the photosphere
and the horizontal fields above the photosphere can be explained by
multi-scale magnetic loops produced by the dynamo.
Title: Turbulent Hydrodynamics and Oscillations of Moderate-Mass Stars
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Lele, S. K.; Mansour,
N. N.; Wray, A. A.
Bibcode: 2013ASPC..479..377K
Altcode:
The solar-type pulsators are characterized by acoustic oscillation
modes excited by turbulent convection in the upper convective boundary
layer. As the stellar mass increases the convection zone shrinks, the
scale and intensity of the turbulent motions increases, providing more
energy for excitation of acoustic modes. When the stellar mass reaches
about 1.6 solar masses the upper convection zone consists of two very
thin layers corresponding to H and He ionization, and in addition to
the acoustic modes the stars show strong internal gravity modes. The
thin convection zone is often considered insignificant for the stellar
dynamics and variability. We use three-dimensional (3D) numerical
radiative hydrodynamics simulations to study convective and oscillation
properties of Main Sequence stars from the solar-type stars to more
massive stars. We present simulation results for some of the target
stars selected for the Kepler Guest Observer project “Transition
in Variable Stars: From Solar-Type Stars to Gamma-Doradus Stars.”
For the moderate-mass (A-type) stars the simulations reveal supersonic
granular-type convection of a scale significantly larger than the solar
granulation scale, and strong overshooting plumes penetrating into the
stable radiative zone, that can affect the oscillation properties of
these stars.
Title: Differential Rotation in Solar-like Stars from Global
Simulations
Authors: Guerrero, G.; Smolarkiewicz, P. K.; Kosovichev, A. G.;
Mansour, N. N.
Bibcode: 2013ApJ...779..176G
Altcode: 2013arXiv1310.8178G
To explore the physics of large-scale flows in solar-like stars, we
perform three-dimensional anelastic simulations of rotating convection
for global models with stratification resembling the solar interior. The
numerical method is based on an implicit large-eddy simulation approach
designed to capture effects from non-resolved small scales. We obtain
two regimes of differential rotation, with equatorial zonal flows
accelerated either in the direction of rotation (solar-like) or in the
opposite direction (anti-solar). While the models with the solar-like
differential rotation tend to produce multiple cells of meridional
circulation, the models with anti-solar differential rotation result
in only one or two meridional cells. Our simulations indicate that the
rotation and large-scale flow patterns critically depend on the ratio
between buoyancy and Coriolis forces. By including a sub-adiabatic layer
at the bottom of the domain, corresponding to the stratification of a
radiative zone, we reproduce a layer of strong radial shear similar
to the solar tachocline. Similarly, enhanced super-adiabaticity at
the top results in a near-surface shear layer located mainly at lower
latitudes. The models reveal a latitudinal entropy gradient localized
at the base of the convection zone and in the stable region, which,
however, does not propagate across the convection zone. In consequence,
baroclinicity effects remain small, and the rotation isocontours
align in cylinders along the rotation axis. Our results confirm the
alignment of large convective cells along the rotation axis in the
deep convection zone and suggest that such "banana-cell" pattern can
be hidden beneath the supergranulation layer.
Title: Astrophysical processes on the Sun
Authors: Kosovichev, Alexander
Bibcode: 2013GApFD.107..717K
Altcode:
No abstract at ADS
Title: Helioseismic Constraints and a Paradigm Shift in the Solar
Dynamo
Authors: Kosovichev, Alexander G.; Pipin, Valery V.; Zhao, J.
Bibcode: 2013ASPC..479..395K
Altcode: 2014arXiv1402.1901K
Helioseismology provides important constraints for the solar dynamo
problem. However, the basic properties and even the depth of the dynamo
process, which operates also in other stars, are unknown. Most of the
dynamo models suggest that the toroidal magnetic field that emerges
on the surface and forms sunspots is generated near the bottom of the
convection zone, in the tachocline. However, there are a number of
theoretical and observational problems with justifying the deep-seated
dynamo models. This leads to the idea that the subsurface angular
velocity shear may play an important role in the solar dynamo. Using
helioseismology measurements of the internal rotation and meridional
circulation, we investigate a mean-field magneto-hydrodynamic model
of a dynamo distributed in the bulk of the convection zone but shaped
in a near-surface layer. We show that if the boundary conditions at
the top of the dynamo region allow the large-scale toroidal magnetic
fields to penetrate into the surface, then the dynamo wave propagates
along the isosurface of angular velocity in the subsurface shear layer,
forming the butterfly diagram in agreement with the Parker-Yoshimura
rule and solar-cycle observations. Unlike the flux-transport dynamo
models, this model does not depend on the transport of magnetic field
by meridional circulation at the bottom of the convection zone, and
works well when the meridional circulation forms two cells in radius,
as recently indicated by the deep-focus time-distance helioseismology
analysis of the SDO/HMI and SOHO/MDI data. We compare the new dynamo
model with various characteristics of the solar magnetic cycles,
including the cycle asymmetry (Waldmeier's relations) and magnetic
‘butterfly’ diagrams.
Title: The Mean-field Solar Dynamo with a Double Cell Meridional
Circulation Pattern
Authors: Pipin, V. V.; Kosovichev, A. G.
Bibcode: 2013ApJ...776...36P
Altcode: 2013arXiv1302.0943P
Recent helioseismology findings, as well as advances in direct
numerical simulations of global dynamics of the Sun, have indicated
that in each solar hemisphere meridional circulation may form more
than one cell along the radius in the convection zone. In particular,
recent helioseismology results revealed a double-cell structure of
the meridional circulation. We investigate properties of a mean-field
solar dynamo with such double-cell meridional circulation. The dynamo
model also includes the realistic profile of solar differential
rotation (including the tachocline and subsurface shear layer)
and takes into account effects of turbulent pumping, anisotropic
turbulent diffusivity, and conservation of magnetic helicity. Contrary
to previous flux-transport dynamo models, we find that the dynamo model
can robustly reproduce the basic properties of the solar magnetic cycles
for a wide range of model parameters and circulation speeds. The best
agreement with observations is achieved when the surface meridional
circulation speed is about 12 m s-1. For this circulation
speed, the simulated sunspot activity shows good synchronization with
the polar magnetic fields. Such synchronization was indeed observed
during previous sunspot Cycles 21 and 22. We compare theoretical and
observed phase diagrams of the sunspot number and the polar field
strength and discuss the peculiar properties of Cycle 23.
Title: Variability of Solar Five-Minute Oscillations in the Corona
as Observed by the Extreme Ultraviolet Spectrophotometer (ESP) on the
Solar Dynamics Observatory/Extreme Ultraviolet Variability Experiment
(SDO/EVE)
Authors: Didkovsky, L.; Kosovichev, A.; Judge, D.; Wieman, S.;
Woods, T.
Bibcode: 2013SoPh..287..171D
Altcode: 2012arXiv1211.0711D; 2012SoPh..tmp..307D
Solar five-minute oscillations have been detected in the power spectra
of two six-day time intervals from soft X-ray measurements of the Sun
observed as a star using the Extreme Ultraviolet Spectrophotometer
(ESP) onboard the Solar Dynamics Observatory (SDO)/Extreme Ultraviolet
Variability Experiment (EVE). The frequencies of the largest amplitude
peaks were found to match the known low-degree (ℓ=0 - 3) modes of
global acoustic oscillations within 3.7 μHz and can be explained
by a leakage of the global modes into the corona. Due to the strong
variability of the solar atmosphere between the photosphere and the
corona, the frequencies and amplitudes of the coronal oscillations
are likely to vary with time. We investigated the variations in the
power spectra for individual days and their association with changes
of solar activity, e.g. with the mean level of the EUV irradiance,
and its short-term variations caused by evolving active regions. Our
analysis of samples of one-day oscillation power spectra for a 49-day
period of low and intermediate solar activity showed little correlation
with the mean EUV irradiance and the short-term variability of the
irradiance. We suggest that some other changes in the solar atmosphere,
e.g., magnetic fields and/or inter-network configuration may affect
the mode leakage to the corona.
Title: Preface
Authors: Mansour, Nagi N.; Kosovichev, Alexander G.; Komm, Rudolf;
Longcope, Dana; Leibacher, John W.
Bibcode: 2013SoPh..287....1M
Altcode:
No abstract at ADS
Title: Detection of Equatorward Meridional Flow and Evidence of
Double-cell Meridional Circulation inside the Sun
Authors: Zhao, Junwei; Bogart, R. S.; Kosovichev, A. G.; Duvall,
T. L., Jr.; Hartlep, Thomas
Bibcode: 2013ApJ...774L..29Z
Altcode: 2013arXiv1307.8422Z
Meridional flow in the solar interior plays an important role in
redistributing angular momentum and transporting magnetic flux inside
the Sun. Although it has long been recognized that the meridional
flow is predominantly poleward at the Sun's surface and in its
shallow interior, the location of the equatorward return flow and the
meridional flow profile in the deeper interior remain unclear. Using
the first 2 yr of continuous helioseismology observations from the
Solar Dynamics Observatory/Helioseismic Magnetic Imager, we analyze
travel times of acoustic waves that propagate through different
depths of the solar interior carrying information about the solar
interior dynamics. After removing a systematic center-to-limb effect
in the helioseismic measurements and performing inversions for flow
speed, we find that the poleward meridional flow of a speed of 15 m
s-1 extends in depth from the photosphere to about 0.91 R
⊙. An equatorward flow of a speed of 10 m s-1
is found between 0.82 and 0.91 R ⊙ in the middle of the
convection zone. Our analysis also shows evidence of that the meridional
flow turns poleward again below 0.82 R ⊙, indicating an
existence of a second meridional circulation cell below the shallower
one. This double-cell meridional circulation profile with an equatorward
flow shallower than previously thought suggests a rethinking of how
magnetic field is generated and redistributed inside the Sun.
Title: Solar differential rotation: hints to reproduce a near-surface
shear layer in global simulations
Authors: Guerrero, G.; Smolarkiewicz, P. K.; Kosovichev, A.;
Mansour, N.
Bibcode: 2013IAUS..294..417G
Altcode: 2013arXiv1301.1330G
Convective turbulent motions in the solar interior, as well as the
mean flows resulting from them, determine the evolution of the solar
magnetic field. With the aim to get a better understanding of these
flows we study anelastic rotating convection in a spherical shell whose
stratification resembles that of the solar interior. This study is
done through numerical simulations performed with the EULAG code. Due
to the numerical formulation, these simulations are known as implicit
large eddy simulations (ILES), since they intrinsically capture the
contribution of, non-resolved, small scales at the same time maximizing
the effective Reynolds number. We reproduce some previous results and
find a transition between buoyancy and rotation dominated regimes which
results in anti-solar or solar like rotation patterns. Even thought the
rotation profiles are dominated by Taylor-Proudman columnar rotation,
we are able to reproduce the tachocline and a low latitude near-surface
shear layer. We find that simulations results depend on the grid
resolution as a consequence of a different sub-grid scale contribution.
Title: Effects of Double-Cell Meridional Circulation on the Solar
Dynamo and Activity Cycles
Authors: Pipin, Valery; Kosovichev, A. G.
Bibcode: 2013SPD....4440303P
Altcode:
It was long assumed that the meridional circulation on the Sun is
represented by a single cell occupying the whole convection zone,
with poleward flow at the top and with the return equator-ward flow
at the bottom. However, recent helioseismology observations and
numerical simulations provided clear evidence that the meridional
circulation has a double-cell structure with a return equator-ward
flow in the middle of the convection zone. This discovery requires
to re-examine the solar dynamo models. We discuss the properties of
a new mean-field solar dynamo that is coupled with the double-cell
meridional circulation pattern. It is found that such dynamo model
(which also includes the subsurface rotational shear layer, turbulent
pumping and other turbulence effects) can robustly reproduce the basic
properties of the solar magnetic activity within the wide range of
the dynamo parameters and amplitudes of the circulation speed. The
properties of the simulated sunspot activity migration are discussed
and compared with observations. It is found that the best agreement with
observation is achieved when the surface speed of circulation is about
12 m/s. Interesting that for this amplitude of the circulation speed the
simulated sunspot activity show the pretty good synchronization with
the polar magnetic field activity. Such synchronization was indeed
observed during the past Cycles 21 and 22. We compare our findings
with these observations.
Title: Solar Interior Meridional Flow from SDO/HMI
Authors: Zhao, Junwei; Bogart, R. S.; Kosovichev, A. G.; Duvall,
T. L.; Hartlep, T.
Bibcode: 2013SPD....4420402Z
Altcode:
Since the launch of Solar Dynamics Observatory, the Helioseismic
and Magnetic Imager has accumulated 3 years of continuous
observations. Using time-distance helioseismology, we have obtained
new results on both global and local scales. By analyzing the first two
years' observations, we were able to detect the equatorward meridional
flow at a depth of around 65 Mm, and detect the existence of a second
meridional circulation cell below about 120 Mm. This new profile of
interior meridional flow will pose challenges to the solar dynamo
models. At the shallower depths, we studied the temporal evolution of
the zonal and meridional flows. We found that both quantities showed
strong hemispherical asymmetries. Using global wavefield simulations
that have pre-set meridional flow profiles, we also assess the
capability of our analysis technique in recovering week flows in the
deep interior.
Title: Solar and Astrophysical Dynamos and Magnetic Activity
Authors: Kosovichev, Alexander G.; de Gouveia Dal Pino, Elisabete;
Yan, Yihua
Bibcode: 2013IAUS..294.....K
Altcode:
No abstract at ADS
Title: Ubiquitous Solar Eruptions Driven by Magnetized Vortex Tubes
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Lele, S. K.; Mansour,
N. N.; Wray, A. A.
Bibcode: 2013ApJ...770...37K
Altcode: 2013arXiv1301.0018K
The solar surface is covered by high-speed jets transporting mass
and energy into the solar corona and feeding the solar wind. The
most prominent of these jets have been known as spicules. However,
the mechanism initiating these eruption events is still unknown. Using
realistic numerical simulations we find that small-scale eruptions are
produced by ubiquitous magnetized vortex tubes generated by the Sun's
turbulent convection in subsurface layers. The swirling vortex tubes
(resembling tornadoes) penetrate into the solar atmosphere, capture and
stretch background magnetic field, and push the surrounding material up,
generating shocks. Our simulations reveal complicated high-speed flow
patterns and thermodynamic and magnetic structure in the erupting vortex
tubes. The main new results are: (1) the eruptions are initiated in
the subsurface layers and are driven by high-pressure gradients in the
subphotosphere and photosphere and by the Lorentz force in the higher
atmosphere layers; (2) the fluctuations in the vortex tubes penetrating
into the chromosphere are quasi-periodic with a characteristic period
of 2-5 minutes; and (3) the eruptions are highly non-uniform: the flows
are predominantly downward in the vortex tube cores and upward in their
surroundings; the plasma density and temperature vary significantly
across the eruptions.
Title: Recent Local Helioseismology Results from SDO/HMI
Authors: Zhao, Junwei; Bogart, R. S.; Kosovichev, A. G.; Duvall,
T. L., Jr.; Hartlep, Thomas
Bibcode: 2013enss.confE.118Z
Altcode:
Since the launch of Solar Dynamics Observatory, the Helioseismic
and Magnetic Imager has accumulated 2.5 years of continuous
observations. Using time-distance helioseismology, we have obtained
new results on both global and local scales. By analyzing the first
two years' observations, we were able to detect the equator-ward
meridional flow at a depth of around 70 Mm, and detect the existence
of a second meridional circulation below about 120 Mm. This new profile
of interior meridional flow will pose challenges to the current dynamo
models. At the shallower depth, we studied the temporal evolution of
zonal flows, as well as the residual meridional flow, which was obtained
by subtracting an averaged meridional flow profile. We found that both
quantities showed strong hemispherical asymmetries. On local scales,
we studied subsurface flows inside active regions and supergranules,
as well as the potential links between solar flares and these subsurface
dynamics.
Title: Approach to Integrate Global-Sun Models of Magnetic Flux
Emergence and Transport for Space Weather Studies
Authors: Mansour, Nagi Nicolas; Wray, A.; Mehrotra, P.; Henney, C.;
arge, N.; Manchester, C.; Godinez, H.; Koller, J.; Kosovichev, A.;
Scherrer, P.; Zhao, J.; Stein, R.; Duvall, T.; Fan, Y.
Bibcode: 2013enss.confE.125M
Altcode:
The Sun lies at the center of space weather and is the source of its
variability. The primary input to coronal and solar wind models is
the activity of the magnetic field in the solar photosphere. Recent
advancements in solar observations and numerical simulations provide
a basis for developing physics-based models for the dynamics of
the magnetic field from the deep convection zone of the Sun to the
corona with the goal of providing robust near real-time boundary
conditions at the base of space weather forecast models. The goal is
to develop new strategic capabilities that enable characterization
and prediction of the magnetic field structure and flow dynamics of
the Sun by assimilating data from helioseismology and magnetic field
observations into physics-based realistic magnetohydrodynamics (MHD)
simulations. The integration of first-principle modeling of solar
magnetism and flow dynamics with real-time observational data via
advanced data assimilation methods is a new, transformative step in
space weather research and prediction. This approach will substantially
enhance an existing model of magnetic flux distribution and transport
developed by the Air Force Research Lab. The development plan is to use
the Space Weather Modeling Framework (SWMF) to develop Coupled Models
for Emerging flux Simulations (CMES) that couples three existing models:
(1) an MHD formulation with the anelastic approximation to simulate
the deep convection zone (FSAM code), (2) an MHD formulation with
full compressible Navier-Stokes equations and a detailed description
of radiative transfer and thermodynamics to simulate near-surface
convection and the photosphere (Stagger code), and (3) an MHD
formulation with full, compressible Navier-Stokes equations and an
approximate description of radiative transfer and heating to simulate
the corona (Module in BATS-R-US). CMES will enable simulations of the
emergence of magnetic structures from the deep convection zone to the
corona. Finally, a plan will be summarized on the development of a
Flux Emergence Prediction Tool (FEPT) in which helioseismology-derived
data and vector magnetic maps are assimilated into CMES that couples
the dynamics of magnetic flux from the deep interior to the corona.
Title: Sunquake Observations from HMI: Time-Distance and Holography
Analyses
Authors: Kosovichev, Alexander; Zhao, Junwei
Bibcode: 2013enss.confE.124K
Altcode:
HMI observations of solar flares revealed new sunquake events. We
analyze and compare the results obtained using two different approaches:
time-distance analysis and holography. The time-distance analysis
provides detailed information about physical properties of the
flare-excited helioseismic waves, including their interaction with
strong magnetic field regions. On the other hand, the holography
approach, which measures the integrated signal, allows us to detect
weak events and investigate their relative power and its frequency
dependence. A remarkable features of the observed events are their
strong anisotropy, associated with the rapid motion of the flare
impacts in the low atmosphere, which are the source of the energy and
momentum of sunquakes. We identify the sunquake source spatial-temporal
characteristics and show that source locations determined from the
holographic method systematically differ from the HMI observed Doppler
impact locations. We discuss the potential mechanisms of sunquakes, and
their relationship to the energy release and transport in solar flares.
Title: Using realistic MHD simulations for modeling HMI observables
Authors: Kitiashvili, I.; Couvidat, S.; Mansour, N.; Wray, A.;
Kosovichev, A.
Bibcode: 2013enss.confE.127K
Altcode:
The solar atmosphere is extremely dynamic, and many important
phenomena which develop on small scales are unresolved in the
SDO/HMI observations. For correct calibration and interpretation of
HMI observations it is very important to investigate the effects of
small-scale structures and dynamics on the HMI observables. We use
radiative MHD simulations of the upper turbulent convective layer and
atmosphere of the Sun and spectro-polarimetric radiative transfer
codes to study Stokes profiles of the FeI 6173 line for different
conditions in the solar atmosphere, including quiet-Sun regions with
various background magnetic field strengths and sunspot umbrae and
penumbrae, and discuss effects on HMI observables and interpretation
of the HMI data.
Title: An analysis of apparent r-mode oscillations in solar activity,
the solar diameter, the solar neutrino flux, and nuclear decay
rates, with implications concerning the Sun's internal structure
and rotation, and neutrino processes
Authors: Sturrock, P. A.; Bertello, L.; Fischbach, E.; Javorsek, D.;
Jenkins, J. H.; Kosovichev, A.; Parkhomov, A. G.
Bibcode: 2013APh....42...62S
Altcode: 2012arXiv1211.6352S
This article presents a comparative analysis of solar activity data, Mt
Wilson diameter data, Super-Kamiokande solar neutrino data, and nuclear
decay data acquired at the Lomonosov Moscow State University (LMSU). We
propose that salient periodicities in all of these datasets may be
attributed to r-mode oscillations. Periodicities in the solar activity
data and in Super-Kamiokande solar neutrino data may be attributed to
r-mode oscillations in the known tachocline, with normalized radius
in the range 0.66-0.74, where the sidereal rotation rate is in the
range 13.7-14.6 year-1. We propose that periodicities
in the Mt Wilson and LMSU data may be attributed to similar r-mode
oscillations where the sidereal rotation rate is approximately
12.0 year-1, which we attribute to a hypothetical "inner"
tachocline separating a slowly rotating core from the radiative zone. We
also discuss the possible role of the Resonant Spin Flavor Precession
(RSFP) process, which leads to estimates of the neutrino magnetic
moment and of the magnetic field strength in or near the solar core.
Title: Solar Wave-field Simulation for Testing Prospects of
Helioseismic Measurements of Deep Meridional Flows
Authors: Hartlep, T.; Zhao, J.; Kosovichev, A. G.; Mansour, N. N.
Bibcode: 2013ApJ...762..132H
Altcode: 2012arXiv1209.4602H
The meridional flow in the Sun is an axisymmetric flow that is
generally directed poleward at the surface, and is presumed to be of
fundamental importance in the generation and transport of magnetic
fields. Its true shape and strength, however, are debated. We present
a numerical simulation of helioseismic wave propagation in the whole
solar interior in the presence of a prescribed, stationary, single-cell,
deep meridional circulation serving as synthetic data for helioseismic
measurement techniques. A deep-focusing time-distance helioseismology
technique is applied to the synthetic data, showing that it can in
fact be used to measure the effects of the meridional flow very deep
in the solar convection zone. It is shown that the ray approximation
that is commonly used for interpretation of helioseismology measurements
remains a reasonable approximation even for very long distances between
12° and 42° corresponding to depths between 52 and 195 Mm. From the
measurement noise, we extrapolate that time-resolved observations on
the order of a full solar cycle may be needed to probe the flow all
the way to the base of the convection zone.
Title: Initiation of Coronal Mass Ejections: A Comparison of AR11158
with a Simulation of Flux Cancellation
Authors: Manchester, W. B.; Fang, F.; Burns, C.; Kosovichev, A. G.;
Sun, X.; DeRosa, M. L.; Cheung, C.
Bibcode: 2012AGUFMSH53B..06M
Altcode:
We present a detailed comparison of an MHD simulation of magnetic
flux emergence with observations of a large-scale active region. The
simulation (Fang et al. 2012) addresses the buoyant rise of a flux
rope through the convection zone into the corona, which spontaneously
reproduces several features found in AR11158. We focus our study on
the central part of AR11158 from which an energetic CME was observed
on 2011 February 15. We examine AIA loops, HMI vector magnetograms,
photospheric flow patterns, and convection zone flow patterns to fully
characterize the active region and relate its dynamics to basic features
found in the MHD simulation. Salient features are the convergence of
flux concentrations of opposite polarity and strong shear flows along
the polarity inversion line observed prior to and during the CME. We
will show that such shear flows are readily explained as a response to
the Lorentz force, and the convergence are associated with convective
downdrafts that form over the polarity inversion line. We also compare
the brightening of coronal loops observed with AIA to tether-cutting
reconnection observed in or simulation. Together, these mechanisms
explain the buildup, concentration and release of energy necessary
for eruptive events.
Title: Investigation of Small-Scale Turbulent MHD Phenomena Using
Numerical Simulations and NST Observations
Authors: Kitiashvili, I.; Abramenko, V.; Goode, P. R.; Kosovichev,
A.; Mansour, N.; Wray, A.; Yurchyshyn, V.
Bibcode: 2012IAUSS...6E.104K
Altcode:
Recent progress in observational capabilities and numerical modeling
have provided unique high-resolution information demonstrating
complicated dynamics and structures of turbulent flows and magnetic
field on the Sun. The realistic approach to numerical simulations is
based on physical first principles and takes into account compressible
fluid flow in a highly stratified magnetized medium, 3D multi-bin
radiative energy transfer between fluid elements, a real-gas equation
of state, ionization, and excitation of all abundant species, magnetic
effects and sub-grid turbulence. We present new results of 3D radiative
MHD simulations of the upper solar convection zone and chromosphere
that reveal a fundamental role of small-scale vortex dynamics, and
compare the numerical results and predictions with observational
results from the 1.6 m clear aperture New Solar Telescope (NST) at
Big Bear Observatory. In particular, we investigate formation and
dynamics of ubiquitous small-scale vortex tubes mostly concentrated
in the intergranular lanes and their role in magnetic structuring
and acoustic emission of the Sun. These whirlpool-like flows are
characterized by very strong horizontal shear velocities (7 - 11 km/s)
and downflows (~7 km/s), and are accompanied by sharp decreases in
temperature, density and pressure at the surface. High-speed whirlpool
flows can attract and capture other vortices, penetrate into the low
chromosphere, and form stable magnetic flux tubes. The simulations also
reveal a strong connection between acoustic wave excitation events and
the dynamics of vortex tubes. In this talk, we will discuss different
aspects of small-scale turbulent dynamics of the low atmosphere from the
high-resolution simulations in comparison with recent NST observations,
and the strategy for future synergies of numerical simulations and
observations with large aperture solar telescopes.
Title: Numerical Simulation of Thermal and Magnetic Effects of MHD
Wave Propagation in Sunspot Models
Authors: Parchevsky, K.; Kosovichev, A. G.
Bibcode: 2012AGUFMSH13A2247P
Altcode:
Understanding of MHD wave propagation and transformation in sunspots
is very important for understanding helioseismic measurements and
improving helioseismic inversion procedures. Numerical simulations
help to reveal details of wave interaction with the non-uniform
background magnetic field and flows. Such simulations also provide
artificial data for testing and calibration techniques used for
analysis of data from space missions SOHO/MDI, SDO/HMI, HINODE, and
GONG network. There are three competing processes, which affect the
wave speed in sunspots: (i) thermal effects, (ii) magnetic field and
(iii) mass flows. Comparison of numerical simulations of the MHD wave
propagation in different models of sunspot helps to disentangle these
effects. We present simulation results of 3D MHD wave propagation
in sunspot models with separated and combined thermal and magnetic
effects. Simulations of MHD wave propagation from a single source in
the self-consistent magnetostatic sunspot model show flattening of
the wave front when the wave entrs the sunspot due to the reduced
background sound speed near the photosphere. At later moments of
time, the wave front is formed by waves propagating through deeper
layers. In these layers contribution of the magnetic field dominates
and the wave front restores its symmetric shape. The asymmetry of the
wave front decreases when the distance from the source to the axis
of the sunspot model increases. Separation of p- and f- modes shows
that mostly f-modes contribute to the wave front asymmetry while the
wave front of p-modes stays almost unperturbed. In the model with the
potential magnetic field configuration and quiet Sun background model,
the wave front accelerates from the moment when the wave enters the
magnetized region, forming a bulge toward the sunspot axis. For both
types of models simulations also show weak fast-to-slow conversion
of MHD waves near the surface where the plasma parameter beta equals
one. Mode conversion is stronger in the self-consistent magnetostatic
model than in the potential sunspot model, indicating that thermal
effects implicitly contribute to the mode conversion. We also present
simulations in realistic sunspot model calculated by M. Rempel.
Title: Detection of Equator-ward Meridional Flows in the Deep Solar
Interior
Authors: Zhao, J.; Bogart, R. S.; Kosovichev, A. G.; Duvall, T. L.
Bibcode: 2012AGUFMSH13C2266Z
Altcode:
The meridional flow observed on the solar surface is a slow plasma
motion from the equator to the poles. Flux-transport dynamo models
of the solar cycle assume that this flow transports magnetic field of
decaying active regions and causes polar field reversals. At what depth
the meridional flow turns to equator-ward and how fast is the return
flow are important questions for a better understanding of the dynamo
process, and are also long-time puzzles of helioseismology. A recent
finding of a systematic center-to-limb variation in the time-distance
helioseismology measurements allows us to develop an empirical
correction procedure for acoustic travel times, and improve the accuracy
of helioseismic inferences. Using the helioseismic data of two entire
years of SDO/HMI continuous observations and removing the systematic
effect, we have detected the equator-ward meridional flows. Inversion
of the travel times shows that the near-surface pole-ward meridional
flow starts turning equator-ward at approximately 0.92 R_sun at low
latitudes, and that the depth of the flow turning point increases
with latitude. The equator-ward flow has a speed of 10 m/s or so, and
extends from the surface to about 0.82 R_sun. Our analysis also shows
evidences for a second meridional circulation cell starting at about
0.82 R_sun and extending deep to near the tachocline area (0.7 R_sun).
Title: Sunquakes and Two Types of Flares
Authors: Kosovichev, A. G.
Bibcode: 2012AGUFMSH43B2167K
Altcode:
Uninterrupted observations from Solar Dynamics Observatory provide
unique opportunity for investigation of "sunquakes", helioseismic
waves caused by strong localized impacts in the low atmosphere during
impulsive phase of solar flares. The SDO observations show that these
events are more frequent than previously thought. They are observed
in solar flares from moderate M- to X-class. However, not all X-class
flares produce sunquakes, and it is puzzling why some moderate class
flares produce sunquakes, while significantly more powerful flares
do not. Using data from the HMI and AIA instruments I investigate
properties of sunquakes by detecting and analyzing the seismic wave
fronts and the sources of the flare impact, and discuss physical
mechanisms of the impact. By comparing energetic and morphological
characteristics of the flares with and without sunquakes, I present
arguments that this phenomenon reflects a division between two classes
of solar flares: confined and eruptive, which may be fundamentally
different in terms of the energy release mechanism.
Title: 3D MHD Simulations of Spontaneous Flow Ejections by Turbulent
Convection into the Chromosphere
Authors: Kitiashvili, I.; Kosovichev, A. G.; Mansour, N.; Wray, A.
Bibcode: 2012AGUFMSH51A2192K
Altcode:
Dynamical interaction of the highly turbulent subsurface and the low
atmosphere layers is a source of many observed phenomena on various
scales in the solar chromosphere. We investigate the energetic and
dynamical links between the turbulent convection and the chromosphere
by using 3D radiative MHD simulations. Our simulations of quiet-Sun
regions reveal ubiquitous formation of small-scale vortex tubes which
can drive flow ejections into the chromosphere. The vortex tubes are
formed through two basic mechanisms: convective instability inside
the granules and the Kelvin-Helmholtz instability in the intergranular
lanes. During their formation the vortex tubes become mostly vertical
and usually can be detected in the intergranular lanes. Dispute their
small scale the vortex tubes represent highly dynamical structures,
which can capture surrounding magnetic field and easily penetrate
into the atmosphere layers, producing quasi-periodic flow ejections,
shocks and Alfven waves, and contribute to the chromosphere heating.
Title: Helioseismic measurements of large-scale flows and meridional
circulation models using artificial data from numerical simulations
Authors: Hartlep, T.; Kosovichev, A. G.; Zhao, J.
Bibcode: 2012AGUFMSH13C2265H
Altcode:
The detection and measurement of flows in the solar interior is
of great importance for studying the dynamics of solar convection
zone. Over the years, helioseismology has been able to detect flows
in increasingly deeper layer of the Sun. Numerical simulations can
provide means for testing and calibrating such measurement techniques,
and can help increase our confidence in the inferences obtained
from observations. Here, we present numerical simulations of the 3D
helioseismic wave field in the presence of various flow models, and
time-distance helioseismology measurement using the artificial data from
the simulations. The simulations solve the linearized propagation of
helioseismic wave in the whole solar interior through stationary flow
fields. Recently, there has been evidence for a multi-cell meridional
flow. We have therefore simulated different models of meridional flows
-- single-cell, shallow and deep meridional flows and a multi-cell
meridional flow -- to evaluate if helioseismology measurements can
differentiate the models. Also, we have performed a simulation with
flows from a full-resolution snapshot of a solar convection simulation
in anelastic approximation using the "ASH" code, provided to us by
Mark Miesch. Time-distance helioseismology measurements and inversions
are performed.
Title: Dark Matter and Its Effects on Helioseismology
Authors: Hamerly, R.; Kosovichev, A. G.
Bibcode: 2012ASPC..462..537H
Altcode: 2011arXiv1110.1169H
Helioseismology can be used to place new constraints on the properties
of dark matter, allowing solar observations to complement more
conventional dark matter searches currently in operation. During the
course of its lifetime, the Sun accretes a sizeable amount of dark
matter. This accreted matter affects the heat transport of the stellar
core in ways that helioseismology can detect, or at least constrain. We
modify the CESAM stellar evolution code to take account of dark matter
and determine the effect of WIMP models on the stellar structure and
normal-mode oscillation frequencies.
Title: Self-organization of the Solar Turbulent Convection in
Magnetic Field
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Mansour, N. N.; Wray,
A. A.
Bibcode: 2012ASPC..462..382K
Altcode:
Observations of the solar surface show a highly turbulent behavior
of convection, and reveal coherent structures in distributed magnetic
fields. We present results of realistic radiation magneto-hydrodynamics
(MHD) simulations in three dimensions (3D) of the solar subsurface
layers, and investigate properties of magneto-convection for various
magnetic field topologies and strengths. In particular, we discuss the
filamentary structure and dynamics of sunspot penumbra, spontaneous
formation of pore-like and small-scale magnetic structures, and compare
the simulation results with observations.
Title: Radiation Hydrodynamics Simulations of Turbulent Convection
for Kepler Target Stars
Authors: Kitiashvili, I. N.; Guzik, J. A.; Kosovichev, A. G.; Mansour,
N. N.; Saio, H.; Shibahashi, H.; Wray, A. A.
Bibcode: 2012ASPC..462..378K
Altcode:
The solar-type pulsators are characterized by acoustic oscillation
modes excited by turbulent granular convection in the upper convective
boundary layer. As the stellar mass increases the convection zone
shrinks, the scale and intensity of the turbulent motions increases,
providing more energy for excitation of acoustic modes. When the stellar
mass reaches about 1.6 solar masses the upper convection zone consists
of two very thin layers corresponding to H and He ionization, and in
addition to the acoustic modes the stars show strong internal gravity
modes The thin convection zone is often considered insignificant for
the stellar dynamics and variability. We use numerical radiation
transfer simulations in three dimensions (3D) to study convective
and oscillation properties of main-sequence stars from the solar-type
stars to more massive stars. In the simulations we used models of the
stellar interior, calculated for individual Kepler mission targets. The
3D radiation hydrodynamics simulations reveal supersonic granular-type
convection on a scale significantly larger than the solar granulation,
and strong overshooting plumes penetrating into the stable radiative
zone, which can affect oscillation properties of a star.
Title: Verification of the Travel Time Measurement Technique and the
Helioseismic Inversion Procedure for Sound Speed Using Artificial Data
Authors: Parchevsky, Konstantin V.; Zhao, Junwei; Hartlep, Thomas;
Kosovichev, Alexander G.
Bibcode: 2012arXiv1209.4877P
Altcode:
We performed 3D numerical simulations of the solar surface wave
field for the quiet Sun and for three models with different localized
sound-speed variations in the interior with: (i) deep, (ii) shallow,
and (iii) two-layer structures. We used simulated data generated
by two different codes which use the same standard solar model as a
background model, but utilize two different integration techniques
and use different models of stochastic wave excitation. Acoustic
travel times were measured from all data sets using the time-distance
helioseismology technique and compared with the ray theory predictions,
frequently used for helioseismic travel-time inversions. It is
found that the measured travel-time shifts agree well with the ray
theory in both cases with and without phase-speed filtering for
the shallow and deep perturbations. This testing verifies the whole
measuring-filtering-inversion procedure for sound-speed anomalies inside
the Sun. It is shown, that the phase-speed filtering, frequently used
to improve the signal-to-noise ratio does not introduce significant
systematic errors. Results of the sound-speed inversion procedure
show good agreement with the background sound-speed profiles in
all cases. Due to its smoothing nature, the inversion procedure
overestimates sound speed variations in areas with sharp gradients of
the sound-speed profile.
Title: Interaction of Helioseismic Waves with Sunspots: Observations
and Numerical MHD Simulations
Authors: Zhao, J.; Parchevsky, K. V.; Kosovichev, A. G.
Bibcode: 2012ASPC..462..277Z
Altcode:
We investigate how helioseismic waves that originate from effective
point sources interact with sunspots. For observations, the waves
from point sources are reconstructed by cross-correlating observed
photospheric Doppler signals. For numerical simulations, the waves are
generated by simulating perturbation propagation from a pulse near the
surface and propagate through magnetostatic and magnetohydrodynamic
(MHD) sunspot models. For both cases, we study f-mode and p-mode waves
separately. We also study different cases when the point source is
located outside the sunspot, and when the source is located inside the
sunspot. Our results nicely visualize how the waves, including both wave
amplitudes and phases, interact with the magnetic field, thermodynamic
and flow structure of sunspots, and how the waveform evolves before,
during, and after the propagation through sunspots. Our analysis of
wave-magnetic-field interactions also extends below the sunspot's
surface. This brings us new rich information of how the waves respond
to the magnetic field below the surface, but also poses new challenges
for local helioseismology to infer the sunspot's interior properties
from waveform observations.
Title: Helioseismic Detection of Emerging Magnetic Flux
Authors: Ilonidis, S.; Zhao, J.; Kosovichev, A. G.
Bibcode: 2012ASPC..462..283I
Altcode: 2012arXiv1203.2546I
Investigating the properties of magnetic flux emergence is one of the
most important problems of solar physics. In this study we present a
newly developed deep-focus time-distance measurement scheme which is
able to detect strong emerging flux events in the deep solar interior,
before the flux becomes visible on the surface. We discuss in detail the
differences between our method and previous methods, and demonstrate
step-by-step how the signal-to-noise (S/N) ratio is increased. The
method is based on detection of perturbations in acoustic phase travel
times determined from cross-covariances of solar oscillations observed
on the surface. We detect strong acoustic travel-time reductions of
an order of 12 - 16 seconds at a depth of 42 - 75 Mm. These acoustic
anomalies are detected 1 - 2 days before high peaks in the photospheric
magnetic flux rate implying that the average emerging speed is 0.3 -
0.6 km s-1. The results of this work contribute to our
understanding of solar magnetism and benefit space weather forecasting.
Title: Detection of Small-scale Granular Structures in the Quiet
Sun with the New Solar Telescope
Authors: Abramenko, V. I.; Yurchyshyn, V. B.; Goode, P. R.;
Kitiashvili, I. N.; Kosovichev, A. G.
Bibcode: 2012ApJ...756L..27A
Altcode: 2012arXiv1208.4337A
Results of a statistical analysis of solar granulation are presented. A
data set of 36 images of a quiet-Sun area on the solar disk center was
used. The data were obtained with the 1.6 m clear aperture New Solar
Telescope at Big Bear Solar Observatory and with a broadband filter
centered at the TiO (705.7 nm) spectral line. The very high spatial
resolution of the data (diffraction limit of 77 km and pixel scale of
0farcs0375) augmented by the very high image contrast (15.5% ± 0.6%)
allowed us to detect for the first time a distinct subpopulation of
mini-granular structures. These structures are dominant on spatial
scales below 600 km. Their size is distributed as a power law with an
index of -1.8 (which is close to the Kolmogorov's -5/3 law) and no
predominant scale. The regular granules display a Gaussian (normal)
size distribution with a mean diameter of 1050 km. Mini-granular
structures contribute significantly to the total granular area. They are
predominantly confined to the wide dark lanes between regular granules
and often form chains and clusters, but different from magnetic bright
points. A multi-fractality test reveals that the structures smaller
than 600 km represent a multi-fractal, whereas on larger scales the
granulation pattern shows no multi-fractality and can be considered
as a Gaussian random field. The origin, properties, and role of the
population of mini-granular structures in the solar magnetoconvection
are yet to be explored.
Title: Local-Helioseismology Study of Supergranulation in the
Polar Region
Authors: Nagashima, K.; Zhao, J.; Kosovichev, A. G.; Sekii, T.
Bibcode: 2012ASPC..454...19N
Altcode:
Hinode/SOT data have been used to study supergranulation in the polar
region. Although foreshortening generally makes it difficult to observe
the polar region in detail, to partially overcome the difficulty we
use the high-resolution Hinode/SOT observations of the polar regions
during the period of the highest inclination of the solar axis to the
ecliptic. By time-distance helioseismology we have found 'alignment'
of the supergranular cells peculiar in the polar region. This
might be an indication of the giant-cell structure in the polar region.
Title: Fine-scale Magnetic Structures and Flows in Sunspot Simulations
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Wray, A. A.; Mansour,
N. N.
Bibcode: 2012ASPC..454..253K
Altcode:
One of most intriguing processes on the Sun is the formation and
dynamics of sunspots and active regions, where magneto-convective
conditions are very different from quiet Sun regions. High-resolution
observations from Hinode and numerical simulations shed light into these
processes. In our 3D radiative MHD simulations we take into account
real-gas equation of state, ionization and excitation of all abundant
spices, turbulent and magnetic effects. We present recent results of
numerical simulations of a top layer of the convective zone and the
photosphere in the presence of magnetic field of various strength and
inclination. The simulation results explain the filamentary organization
of penumbra, reveal the mechanisms of the Evershed effect and the
sea-serpent behavior of magnetic field lines, and show the subsurface
dynamics of umbral dots as a natural consequence of magnetoconvection
processes.
Title: Local Helioseismology of Sunspots: Current Status and
Perspectives
Authors: Kosovichev, Alexander G.
Bibcode: 2012SoPh..279..323K
Altcode: 2010arXiv1010.4927K
Mechanisms of the formation and stability of sunspots are among
the longest-standing and intriguing puzzles of solar physics and
astrophysics. Sunspots are controlled by subsurface dynamics, hidden
from direct observations. Recently, substantial progress in our
understanding of the physics of the turbulent magnetized plasma in
strong-field regions has been made by using numerical simulations
and local helioseismology. Both the simulations and helioseismic
measurements are extremely challenging, but it is becoming clear that
the key to understanding the enigma of sunspots is a synergy between
models and observations. Recent observations and radiative MHD numerical
models have provided a convincing explanation for the Evershed flows in
sunspot penumbrae. Also, they lead to the understanding of sunspots as
self-organized magnetic structures in the turbulent plasma of the upper
convection zone, which are maintained by a large-scale dynamics. Local
helioseismic diagnostics of sunspots still have many uncertainties,
some of which are discussed in this review. However, there have
been significant achievements in resolving these uncertainties,
verifying the basic results by new high-resolution observations,
testing the helioseismic techniques by numerical simulations, and
comparing results obtained by different methods. For instance, a recent
analysis of helioseismology data from the Hinode space mission has
successfully resolved several uncertainties and concerns (such as the
inclined-field and phase-speed filtering effects) that might affect
the inferences of the subsurface wave-speed structure of sunspots
and the flow pattern. It is becoming clear that for the understanding
of the phenomenon of sunspots it is important to further improve the
helioseismology methods and investigate the whole life cycle of active
regions, from magnetic flux emergence to dissipation. The Solar Dynamics
Observatory mission has started to provide data for such investigations.
Title: Vortex tubes of turbulent solar convection
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Mansour, N. N.; Lele,
S. K.; Wray, A. A.
Bibcode: 2012PhyS...86a8403K
Altcode: 2011arXiv1112.5925K
The investigation of the turbulent properties of solar convection is
important for understanding the multi-scale dynamics observed on the
solar surface. In particular, recent high-resolution observations have
revealed ubiquitous vortical structures, and numerical simulations have
demonstrated links between vortex tube dynamics and the magnetic field
organization. Simulations have shown the importance of vortex tube
interactions in mechanisms of acoustic wave excitation on the Sun. In
this paper, we investigate the mechanisms of formation of vortex tubes
in highly turbulent convective flows near the solar surface by using
realistic radiative hydrodynamic large-eddy simulations. Analysis of
data from the simulations indicates two basic processes of vortex tube
formation: (i) the development of small-scale convective instability
inside convective granules and (ii) a Kelvin-Helmholtz-type instability
of shearing flows in intergranular lanes. Our analysis shows that
vortex stretching during these processes is a primary source of the
generation of small-scale vorticity on the Sun.
Title: Special issue on current research in astrophysical magnetism
Authors: Kosovichev, Alexander; Lundstedt, Henrik; Brandenburg, Axel
Bibcode: 2012PhyS...86a0201K
Altcode:
Much of what Hannes Alfvén envisaged some 70 years ago has now
penetrated virtually all branches of astrophysical research. Indeed,
magnetic fields can display similar properties over a large range
of scales. We have therefore been able to take advantage of the
transparency of galaxies and the interstellar medium to obtain
measurements inside them. On the other hand, the Sun is much closer,
allowing us to obtain a detailed picture of the interaction of flows
and magnetic fields at the surface, and more recently in the interior
by helioseismology. Moreover, the solar timescales are generally
much shorter, making studies of dynamical processes more direct. This special issue on current research in astrophysical magnetism
is based on work discussed during a one month Nordita program Dynamo,
Dynamical Systems and Topology and comprises papers that fall into four
different categories (A)-(D). (A) Papers on small-scale magnetic
fields and flows in astrophysics 1. E M de Gouveia Dal Pino, M R M
Leão, R Santos-Lima, G Guerrero, G Kowal and A Lazarian Magnetic flux
transport by turbulent reconnection in astrophysical flows 2. Philip
R Goode, Valentyna Abramenko and Vasyl Yurchyshyn New solar telescope
in Big Bear: evidence for super-diffusivity and small-scale solar
dynamos? 3. I N Kitiashvili, A G Kosovichev, N N Mansour, S K Lele
and A A Wray Vortex tubes of turbulent solar convection The above
collection of papers begins with a review of astrophysical reconnection
and introduces the concept of dynamos necessary to explain the existence
of contemporary magnetic fields both on galactic and solar scales
(paper 1). This is complemented by observations with the new Big
Bear Solar Observatory telescope, allowing us to see magnetic field
amplification on small scales (paper 2). This in turn is complemented
by realistic simulations of subsurface and surface flow patterns
(paper 3). (B) Papers on theoretical approaches to turbulent
fluctuations 4. Nathan Kleeorin and Igor Rogachevskii Growth rate
of small-scale dynamo at low magnetic Prandtl numbers 5. Erico L
Rempel, Abraham C-L Chian and Axel Brandenburg Lagrangian chaos in
an ABC-forced nonlinear dynamo 6. J E Snellman, M Rheinhardt, P J
Käpylä, M J Mantere and A Brandenburg Mean-field closure parameters
for passive scalar turbulence Research in dynamo theory has been
actively pursued for over half a century. It started by trying to
understand the large-scale magnetic fields of the Sun and the Earth,
and subsequently also in galaxies. Such large-scale fields can nowadays
be understood in terms of mean-field dynamo theory that explains the
possibility of large-scale field generation under anisotropic conditions
lacking mirror symmetry. However, even when none of this is the case,
dynamos can still work, and they are called small-scale dynamos that
were referred to in paper 2. This was studied originally under the
assumption that the flow is smooth compared with the magnetic field,
but in the Sun the opposite is the case. This is because viscosity
is much smaller than magnetic diffusivity, i.e., their ratio, which
is the magnetic Prandtl number, is small. In that case the physics
of small-scale dynamos changes, but dynamos still exist even then
(paper 4). Tracing the flow lines in nonlinear small-scale dynamos is
important for understanding their mixing properties (paper 5). Turbulent
mixing is a generic concept that applies not only to magnetic field,
but also to passive scalars which are often used as a prototype for
studying this. Turbulence simulations have helped tremendously in
quantifying the ability of turbulent flows to mix, but the more we
know, the more complicated it becomes. It turns out that spatial and
temporal coupling is an important consideration for allowing accurate
comparison between numerical simulations and mean-field theory (paper
6). (C) The large-scale solar cycle 7. V V Pipin and D D Sokoloff
The fluctuating α-effect and Waldmeier relations in the nonlinear
dynamo models1 8. Radostin D Simitev and Friedrich H Busse
Solar cycle properties described by simple convection-driven dynamos
The mean-field concept has helped us constructing detailed models
of the solar cycle and to make comparison with observed features of
the solar 11-year cycle. One such feature is the Waldmeier relation
between growth time and amplitude of the cycle, and there is another
relation for the declining part of the cycle. These relations
reflect nonlinear aspects of the model and therefore constitute an
important test of the model (paper 7). While mean-field theory is
a useful concept for modeling solar activity, it must eventually be
tested against fully three-dimensional simulations. At present, such
simulations are often quite idealized, because only the large scales
of the turbulent convection of stars can be resolved. Nevertheless,
numerical simulations begin to show many properties that are also seen
in the Sun (paper 8). (D) Flow and dynamo properties in spherical
shells 9. Maxim Reshetnyak and Pavel Hejda Kinetic energy cascades
in quasi-geostrophic convection in a spherical shell 10. Radostin
D Simitev and Friedrich H Busse Bistable attractors in a model of
convection-driven spherical dynamos As the rotation speed is
increased, the flow becomes more strongly constrained by the Coriolis
force. In a spherical shell, such a flow is additionally constrained
by gravity, or at least by the geometry of the domain. Such flows
are called geostrophic. Only now are we beginning to learn about the
subtle properties of the kinetic energy cascade in such flows (paper
9). Turbulent systems are highly nonlinear and it is in principle
possible to find multiple solutions of the equations even for the same
boundary and initial conditions. For turbulent systems, we can only ask
about the statistical properties of the solutions, and the question
of multiple solutions is then less obvious. However, in turbulent
dynamos in convective shells, a nice example has been found where
this is possible. A detailed account of this is given in paper 10. Most of the participants of the Nordita program were able to stay for
the full month of the program, allowing them to think about new ideas
that will be reflected not only in papers on the short term, but also
in new projects and collaborations on a larger scale in the years to
come. We therefore thank Nordita for providing a stimulating atmosphere
and acknowledge the generous support. 1This paper
has been published as V V Pipin and D D Sokoloff 2011 Phys. Scr. 84 065903.
Title: Diagnostics of Magnetic Flux Emergence and Evolution by
Local Helioseismology
Authors: Kosovichev, Alexander; Ilonidis, Stathis; Zhao, Junwei;
Kholikov, Shukur
Bibcode: 2012shin.confE..48K
Altcode:
We investigate the capability of local (time-distance)helioseismology
for forecasting the emergence and developmentof major active regions
and eruptive events, flares and CMEs,as well as for data driven MHD
modeling of CME events. Therecent successful detection of several
large solar activeregions in acoustic travel-time signals before
the emergence ofactive regions has opened a new opportunity for
predictingmajor magnetic flux emergence events that are a primary
sourceof solar flares and CMEs, from several hours to two days
inadvance. In addition, time-distance helioseismology has becomea
useful tool to map solar subsurface flow fields up to morethan
20 Mm below the photosphere, and discovered strongshearing flows
associated with flaring and CME activity. Wediscuss the current
status of these measurements, theirsensitivity and uncertainties,
implementation for the SDO/HMIpipeline processing, and the data
products available for thedata-driven MHD modeling. This work can make
a substantialcontribution to the primary goals of the SHINE program
toinvestigate the connections between eruptive events andmagnetic
phenomena on the Sun, their ultimate origins andprecursors.
Title: Energy Release in Solar Flares: Catastrophic Magnetic Field
Changes, Sunquakes and Coronal Eruptions
Authors: Kosovichev, Alexander
Bibcode: 2012shin.confE.101K
Altcode:
Understanding the mechanisms of energy release in solar flaresis
critical for quantitative data-driven modeling of coronaleruptions
and CMEs. Uninterrupted observations of the Sun fromSDO and other
space missions provide unique opportunities fordiagnostics of solar
flares from the low atmosphere to thecorona. These multi-wavelength
observations indicate that thetwo basic classes of solar flares:
confined and eruptive, areprobably related to two different
mechanisms of the energyrelease. In confined (compact) flares, the
energy is mostlyreleased in the low atmosphere. Typically, such flares
areaccompanied by catastrophic changes of photospheric magneticfields,
produce helioseismic waves ('sunquakes') and coronalshocks, but no
substantial CMEs. Contrary, in eruptive flareswith large CME events,
the energy is released in the highcorona, making little impact in the
lower atmosphere. Usingdata from the HMI and AIA SDO instruments,
Hinode, RHESSI andGOES, I focus on analysis of energy release in
confined flares,which represent particular challenge for flare theories
andacceleration physics. The results show that contrary to thestandard
flare model the initial energy release, which triggersthe whole chain
of flare events, probably occurs in the loweratmosphere during the
flare pre-heating phase.
Title: Turbulent Kinetic Energy Spectra of Solar Convection from
NST Observations and Realistic MHD Simulations
Authors: Kitiashvili, I. N.; Abramenko, V. I.; Goode, P. R.;
Kosovichev, A. G.; Lele, S. K.; Mansour, N. N.; Wray, A. A.;
Yurchyshyn, V. B.
Bibcode: 2012arXiv1206.5300K
Altcode:
Turbulent properties of the quiet Sun represent the basic state of
surface conditions, and a background for various processes of solar
activity. Therefore understanding of properties and dynamics of this
`basic' state is important for investigation of more complex phenomena,
formation and development of observed phenomena in the photosphere and
atmosphere. For characterization of the turbulent properties we compare
kinetic energy spectra on granular and sub-granular scales obtained
from infrared TiO observations with the New Solar Telescope (Big Bear
Solar Observatory) and from 3D radiative MHD numerical simulations
('SolarBox' code). We find that the numerical simulations require a high
spatial resolution with 10 - 25 km grid-step in order to reproduce the
inertial (Kolmogorov) turbulence range. The observational data require
an averaging procedure to remove noise and potential instrumental
artifacts. The resulting kinetic energy spectra show a good agreement
between the simulations and observations, opening new perspectives for
detailed joint analysis of more complex turbulent phenomena on the Sun,
and possibly on other stars. In addition, using the simulations and
observations we investigate effects of background magnetic field,
which is concentrated in self-organized complicated structures in
intergranular lanes, and find an increase of the small-scale turbulence
energy and its decrease at larger scales due to magnetic field effects.
Title: Helioseismic Measurements of Emerging Magnetic Flux in the
Solar Convection Zone
Authors: Ilonidis, Stathis; Zhao, J.; Kosovichev, A.; Hartlep, T.
Bibcode: 2012AAS...22010902I
Altcode:
Solar magnetic fields are probably generated deep inside the
convection zone and then emerge to the surface and form active
regions. Helioseismology is capable of probing acoustic perturbations in
the solar interior by cross-correlating oscillation signals observed at
the surface. In this study, we employ the time-distance helioseismology
technique, and for several active regions observed with SOHO/MDI,
SDO/HMI and GONG instruments investigate variations of the acoustic
cross-correlation signals and phase travel-time shifts caused by
emerging magnetic structures as deep as 65,000 km and 1-2 days before
these structures appear at the surface. We discuss optimization of
the time-distance method for the detection of emerging flux, present
characteristics of the helioseismic signatures, and make comparisons
with numerical simulations. We also investigate the relationship between
the helioseismic signals and properties of the emerged active regions,
and discuss perspectives for utilizing this method to improve space
weather forecast.
Title: Searching For Equator-ward Meridional Flows In The Solar
Interior
Authors: Zhao, Junwei; Bogart, R. S.; Kosovichev, A. G.; Duvall,
T. L., Jr.
Bibcode: 2012AAS...22010905Z
Altcode:
At what depth the equator-ward meridional flow exists and what is
its speed are important questions for a better understanding of
solar dynamo and a better prediction of how active a solar cycle
is. However, the depth and the speed of the equator-ward flow are
still not determined from helioseismology after studies of more than
one decade. The new high-resolution observations from HMI has offered
us a new chance to tackle these problems. Moreover, the systematic
center-to-limb variation that was recently found in time-distance
helioseismology analysis probably implies that the equator-ward flows
exist in areas much shallower than the tachocline. Utilizing two years'
HMI observations, we examine how well we can determine the depth and
speed of the returning meridional flow.
Title: Dynamics of Magnetized Vortex Tubes in the Solar Chromosphere
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Mansour, N. N.; Wray,
A. A.
Bibcode: 2012ApJ...751L..21K
Altcode: 2012arXiv1201.5442K
We use three-dimensional radiative MHD simulations to investigate the
formation and dynamics of small-scale (less than 0.5 Mm in diameter)
vortex tubes spontaneously generated by turbulent convection in
quiet-Sun regions with an initially weak (10 G) mean magnetic
field. The results show that the vortex tubes penetrate into the
chromosphere and substantially affect the structure and dynamics
of the solar atmosphere. The vortex tubes are mostly concentrated
in intergranular lanes and are characterized by strong (near sonic)
downflows and swirling motions that capture and twist magnetic field
lines, forming magnetic flux tubes that expand with height and attain
magnetic field strengths ranging from 200 G in the chromosphere to more
than 1 kG in the photosphere. We investigate in detail the physical
properties of these vortex tubes, including thermodynamic properties,
flow dynamics, and kinetic and current helicities, and conclude that
magnetized vortex tubes provide an important path for energy and
momentum transfer from the convection zone into the chromosphere.
Title: Convection-Chromosphere Coupling due to Vortex Tube Dynamic
Authors: Kitiashvili, Irina; Kosovichev, A.; Mansour, N.; Wray, A.
Bibcode: 2012AAS...22012405K
Altcode:
Ubiquitous distribution of small-scale vortex tubes on the solar surface
was found both in observations and simulations. Complicated dynamics
of the turbulent vortex tubes is a source of various observed effects
such as acoustic waves excitation and processes of self-organization
in magnetized solar plasma. We use realistic-type radiative 3D MHD
simulations to investigate in detail different mechanisms of the
vortex tube formation by granular flows in the upper convection zone,
appearance of vortex tubes on the surface and their interaction with
the atmosphere. The simulation results reveal new interesting effects of
penetration of the vortex tubes from the turbulent subphotosphere into
the chromosphere, interaction between these layers, and influence of
the helical motions on thermodynamic properties of the chromosphere. In
the presence of background magnetic field, the vortex tubes lead to
formation of compact magnetic flux tubes, playing important role in
the mass and energy flux into the chromosphere. We discuss implication
of the simulation results for future high-resolution observations.
Title: Physics of Sunquakes Events Observed with SDO
Authors: Kosovichev, Alexander G.
Bibcode: 2012AAS...22010903K
Altcode:
Sunquake events representing helioseismic response to solar flares
are caused by strong localized impacts in the low atmosphere during
the flare impulsive phase. Several mechanisms of the impact have
been debated, but there is no clear understanding of how energy
and momentum are transported from the magnetic energy release site
(presumably located in the higher atmosphere) to the solar surface. It
is also puzzling why some moderate class flares produce sunquakes,
while significantly more powerful flares do not. Observations with SDO
have substantially improved our ability to investigate details of the
helioseismic response and the impact source properties and dynamics,
providing data with high spatial and temporal resolutions, as well as
spectro-polarimetric properties. I will present new results on several
sunquake events observed with the HMI and AIA instruments and discuss
the basic properties of the helioseismic waves, their interaction with
active regions, the source dynamics and its relation to the amplitude
and direction of the waves. The observations also reveal interesting
relationships between the sunquake impacts, X-ray and white-light
emissions and magnetic field changes in solar flares. I will compare
the observational results with the physical models of sunquakes
(thick-target model, McClymont jerk, backwarming, mass eruption),
and discuss model constraints from the new observations.
Title: Magnetic and Thermal Effects of MHD Wave Propagation in
Different Models of Sunspots
Authors: Parchevsky, Konstantin; Kosovichev, A. G.
Bibcode: 2012AAS...22020619P
Altcode:
Understanding of MHD wave propagation and transformation in sunspots
is very important for understanding helioseismic measurements and
improving helioseismic inversion procedures. Numerical simulations help
to reveal details of wave interaction with the non-uniform background
magnetic field and flows. Such simulations also provide artificial
data for testing and calibration techniques used for analysis of data
from space missions SOHO/MDI, SDO/HMI, HINODE, and GONG network. There are three competing processes, which affect the wave speed
in sunspots: (i) thermal effects, (ii) magnetic field and (iii) mass
flows. Comparison of numerical simulations of the MHD wave propagation
in different models of sunspot helps to disentangle these effects. We
present simulation results of 3D MHD wave propagation in sunspot models
with separated and combined thermal and magnetic effects. When an MHD
wave enters a self-consistent magnetostatic model of the sunspot, the
wave front flattens due to the reduced background sound speed near the
photosphere. Later, when the wave propagates further, the wave front
restores it's original shape, because waves propagate through deeper
regions where contribution of the magnetic field dominates. In the
model with the potential magnetic field configuration and quiet Sun
background model, the wave front accelerates from the moment when the
wave enters the magnetized region, forming a bulge toward the sunspot
axis. Simulations also show weak fast-to-slow conversion of MHD waves
near the surface where the plasma parameter beta equals one. We also
present simulations in realistic sunspot model calculated by M. Rempel.
Title: Effects of Spectral Line Formation Height in Time-Distance
Helioseismology
Authors: Nagashima, K.; Parchevsky, K. V.; Zhao, J.; Duvall, T. L.,
Jr.; Kosovichev, A. G.; Sekii, T.
Bibcode: 2012ASPC..456...57N
Altcode:
To understand the effect of the formation-height difference in
time-distance helioseismology analyses, we consider the wave behavior
above the surface. We show that by using the numerically-simulated
wavefields at two different heights this difference may cause
travel-time shifts due to the non-stationary character of waves excited
by near-surface acoustic sources. This needs to be taken into account
in multi-wavelength helioseismology and measurements close to the
solar limb.
Title: Testing Helioseismic Measurements Of Subsurface Meridional
And Large-scale Flows Using Artificial Data From Numerical Simulations
Authors: Hartlep, Thomas; Zhao, J.; Kosovichev, A. G.; Mansour, N. N.
Bibcode: 2012AAS...22010904H
Altcode:
Numerical simulations of the solar acoustic wave-field have become an
important tool for validating helioseismic measurement and inversion
techniques. Here, we present results from simulating linearized
wave propagation in the full 3D solar interior through stationary
flow fields, and measurements applied to this artificial data. The
background flow include different meridional flow models with deep
and shallow return flows, as well as a full-resolution snapshot from a
solar convection simulation in anelastic approximation using the "ASH"
code, provided to us by Mark Miesch. Time-distance helioseismology
measurements and inversions are carried using this artificial data
and the results are compared to the models. We determine and discuss
the accuracy and resolution of the subsurface flow patterns.
Title: A First Look at Magnetic Field Data Products from SDO/HMI
Authors: Liu, Y.; Scherrer, P. H.; Hoeksema, J. T.; Schou, J.; Bai,
T.; Beck, J. G.; Bobra, M.; Bogart, R. S.; Bush, R. I.; Couvidat,
S.; Hayashi, K.; Kosovichev, A. G.; Larson, T. P.; Rabello-Soares,
C.; Sun, X.; Wachter, R.; Zhao, J.; Zhao, X. P.; Duvall, T. L., Jr.;
DeRosa, M. L.; Schrijver, C. J.; Title, A. M.; Centeno, R.; Tomczyk,
S.; Borrero, J. M.; Norton, A. A.; Barnes, G.; Crouch, A. D.; Leka,
K. D.; Abbett, W. P.; Fisher, G. H.; Welsch, B. T.; Muglach, K.;
Schuck, P. W.; Wiegelmann, T.; Turmon, M.; Linker, J. A.; Mikić,
Z.; Riley, P.; Wu, S. T.
Bibcode: 2012ASPC..455..337L
Altcode:
The Helioseismic and Magnetic Imager (HMI; Scherrer & Schou 2011)
is one of the three instruments aboard the Solar Dynamics Observatory
(SDO) that was launched on February 11, 2010 from Cape Canaveral,
Florida. The instrument began to acquire science data on March 24. The
regular operations started on May 1. HMI measures the Doppler velocity
and line-of-sight magnetic field in the photosphere at a cadence of
45 seconds, and the vector magnetic field at a 135-second cadence,
with a 4096× 4096 pixels full disk coverage. The vector magnetic
field data is usually averaged over 720 seconds to suppress the p-modes
and increase the signal-to-noise ratio. The spatial sampling is about
0".5 per pixel. HMI observes the Fe i 6173 Å absorption line, which
has a Landé factor of 2.5. These data are further used to produce
higher level data products through the pipeline at the HMI-AIA Joint
Science Operations Center (JSOC) - Science Data Processing (Scherrer et
al. 2011) at Stanford University. In this paper, we briefly describe the
data products, and demonstrate the performance of the HMI instrument. We
conclude that the HMI is working extremely well.
Title: Response to Comment on “Detection of Emerging Sunspot
Regions in the Solar Interior”
Authors: Ilonidis, Stathis; Zhao, Junwei; Kosovichev, Alexander
Bibcode: 2012Sci...336..296I
Altcode:
Braun claims that his analysis using helioseismic holography does
not confirm the detection of emerging sunspot regions. We examine
his measurement procedure and explain why his method has different
sensitivity than our method. We also discuss possible physical processes
that may cause the detected phase travel-time shifts.
Title: Systematic Center-to-limb Variation in Measured Helioseismic
Travel Times and its Effect on Inferences of Solar Interior Meridional
Flows
Authors: Zhao, Junwei; Nagashima, Kaori; Bogart, R. S.; Kosovichev,
A. G.; Duvall, T. L., Jr.
Bibcode: 2012ApJ...749L...5Z
Altcode: 2012arXiv1203.1904Z
We report on a systematic center-to-limb variation in measured
helioseismic travel times, which must be taken into account for
an accurate determination of solar interior meridional flows. The
systematic variation, found in time-distance helioseismology analysis
using SDO/HMI and SDO/AIA observations, is different in both travel-time
magnitude and variation trend for different observables. It is not clear
what causes this systematic effect. Subtracting the longitude-dependent
east-west travel times, obtained along the equatorial area, from
the latitude-dependent north-south travel times, obtained along the
central meridian area, gives remarkably similar results for different
observables. We suggest this as an effective procedure for removing
the systematic center-to-limb variation. The subsurface meridional
flows obtained from inversion of the corrected travel times are
approximately 10 m s-1 slower than those obtained without
removing the systematic effect. The detected center-to-limb variation
may have important implications in the derivation of meridional flows
in the deep interior and needs to be better understood.
Title: Commission 12: Solar Radiation and Structure
Authors: Kosovichev, Alexander; Cauzzi, Gianna; Pillet, Valentin
Martinez; Asplund, Martin; Brandenburg, Axel; Chou, Dean-Yi;
Christensen-Dalsgaard, Jorgen; Gan, Weiqun; Kuznetsov, Vladimir D.;
Rovira, Marta G.; Shchukina, Nataliya; Venkatakrishnan, P.
Bibcode: 2012IAUTA..28...81K
Altcode: 2012IAUTB..28...81K
Commission 12 of the International Astronomical Union encompasses
investigations of the internal structure and dynamics of the Sun, mostly
accessible through the techniques of local and global helioseismology,
the quiet solar atmosphere, solar radiation and its variability, and
the nature of relatively stable magnetic structures like sunspots,
faculae and the magnetic network. The Commission sees participation
of over 350 scientists worldwide.
Title: Division II: Sun and Heliosphere
Authors: Martínez Pillet, Valentín; Klimchuk, James A.; Melrose,
Donald B.; Cauzzi, Gianna; van Driel-Gesztelyi, Lidia; Gopalswamy,
Natchimuthuk; Kosovichev, Alexander; Mann, Ingrid; Schrijver,
Carolus J.
Bibcode: 2012IAUTA..28...61M
Altcode: 2012IAUTA..28...61P
The solar activity cycle entered a prolonged quiet phase that started
in 2008 and ended in 2010. This minimum lasted for a year longer
than expected and all activity proxies, as measured from Earth and
from Space, reached minimum values never observed before (de Toma,
2012). The number of spotless days from 2006 to 2009 totals 800, the
largest ever recorded in modern times. Solar irradiance was at historic
minimums. The interplanetary magnetic field was measured at values as
low as 2.9 nT and the cosmic rays were observed at records-high. While
rumors spread that the Sun could be entering a grand minimum quiet
phase (such as the Maunder minimum of the XVII century), activity
took over in 2010 and we are now well into Solar Cycle 24 (albeit,
probably, a low intensity cycle), approaching towards a maximum due
by mid 2013. In addition to bringing us the possibility to observe
a quiet state of the Sun and of the Heliosphere that was previously
not recorded with modern instruments, the Sun has also shown us how
little we know about the dynamo mechanism that drives its activity as
all solar cycle predictions failed to see this extended minimum coming.
Title: Effects of vortex tube dynamics in the chromosphere
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Lele, S. K.; Mansour,
N. N.; Wray, A. A.
Bibcode: 2012decs.confE..96K
Altcode:
Investigation of the solar atmosphere dynamics cannot be complete
without understanding coupling, and mass and energy exchange between
the strongly-turbulent subphotosphere and the chromosphere. Modern
computational capabilities allow us to construct realistic dynamical
models, which take into account dynamical, chemical and radiative
properties of the solar plasma. Such simulations based on first physical
principles and accurate modeling of effects of magnetic field and
small-scale turbulence, coupled with spectro-polarimetric line formation
calculations, provide synthetic multi-wavelength observables, and are
very important for interpretation of observational data. The simulations
allow us to study physical processes and phenomena that have not been
resolved in observations. In this talk we will present our recent
results of high-resolution 3D radiative MHD numerical simulations of
top layers of the convective zone and the chromosphere. The simulations
reveal ubiquitous distribution of small-scale swirling motions in
quiet-Sun and magnetic regions, forming vortex tubes extending from
the subphosphere into the chromosphere. Our results show that these
small-scale vortex tubes that originally formed in subsurface layer
and penetrate into the chromosphere provide an efficient coupling
of the turbulent convective layers with the atmosphere. They play
important role in various processes, such as shearing instabilities,
wave excitation, formation of magnetic flux tubes and transport
of energy, mass, momentum and also turbulent properties from the
convection zone into the chromosphere. In the presentation, we will
focus on the physical aspects of the vortex tube formation, penetration
into the atmosphere, interaction with magnetic fields, their role in
the energy exchange, and on observational diagnostics and comparison
with observational data.
Title: Links between photospheric and chromospheric oscillations
Authors: Kosovichev, A. G.; Kitiashvili, I. N.; Mitra-Kraev, U.;
Sekii, T.
Bibcode: 2012decs.confE..97K
Altcode:
Oscillations excited by turbulent convection play important in the
dynamics and energetics of the solar atmosphere. Oscillations below the
acoustic cut-off frequency form photospheric resonant modes trapped
in the interior but also penetrating into the chromosphere. Above
the frequency cut-off, the oscillations represent traveling waves
in the chromosphere that form pseudo-modes due to interference with
waves coming from the interior. The physics of the chromospheric
oscillations, their coupling to the photospheric oscillations, and
their role in the chromospheric dynamics and energetics are not fully
understood. The observed oscillation properties strongly depend on
the excitation mechanism, interaction with turbulence and radiation,
and local structure and dynamics of the chromosphere. Significant
advances can be made through multi-wavelength observations of
atmospheric oscillations and realistic numerical radiative hydrodynamics
simulations. Using Hinode/SOT data we investigate the basic properties
of solar oscillations observed at two levels in the solar atmosphere,
in the G-band (formed in the photosphere) and in the CaII H line
(chromospheric emission). We analyzed the data by calculating the
individual power spectra as well as the cross-spectral properties,
i.e., coherence and phase shift. The observational properties are
compared with theoretical models and numerical simulations. The
results reveal significant frequency shifts between the CaII H and
G-band spectra, in particular above the acoustic cutoff frequency for
pseudo-modes. The cross-spectrum phase shows peaks associated with
the acoustic oscillation (p-mode) lines, and begins to increase with
frequency around the acoustic cut-off. However, we find no phase shift
for the (surface gravity wave) f-mode. The observed properties for
the p-modes are qualitatively reproduced in a model that includes a
correlated background due to radiative effects. Our results show that
multi-wavelength observations of solar oscillations, in combination
with radiative hydrodynamics modeling, help to understand the coupling
between photospheric and chromospheric oscillations.
Title: Bipolar Magnetic Regions on the Sun: Global Analysis of the
SOHO/MDI Data Set
Authors: Stenflo, J. O.; Kosovichev, A. G.
Bibcode: 2012ApJ...745..129S
Altcode: 2011arXiv1112.5226S
The magnetic flux that is generated by dynamo processes inside the
Sun emerges in the form of bipolar magnetic regions. The properties
of these directly observable signatures of the dynamo can be extracted
from full-disk solar magnetograms. The most homogeneous, high-quality
synoptic data set of solar magnetograms has been obtained with the
Michelson Doppler Imager (MDI) instrument on the Solar and Heliospheric
Observatory spacecraft during 1995-2011. We have developed an IDL
program that has, when applied to the 73,838 magnetograms of the MDI
data set, automatically identified 160,079 bipolar magnetic regions that
span a range of scale sizes across nearly four orders of magnitude. The
properties of each region have been extracted and statistically
analyzed, in particular with respect to the polarity orientations of
the bipolar regions, including their tilt-angle distributions and
their violations of Hale's polarity law. The latitude variation of
the average tilt angles (with respect to the E-W direction), which is
known as Joy's law, is found to closely follow the relation 32fdg1 ×
sin (latitude). There is no indication of a dependence on region size
that one may expect if the tilts were produced by the Coriolis force
during the buoyant rise of flux loops from the tachocline region. A few
percent of all regions have orientations that violate Hale's polarity
law. We show explicit examples, from different phases of the solar
cycle, where well-defined medium-size bipolar regions with opposite
polarity orientations occur side by side in the same latitude zone in
the same magnetogram. Such oppositely oriented large bipolar regions
cannot be part of the same toroidal flux system, but different flux
systems must coexist at any given time in the same latitude zones. These
examples are incompatible with the paradigm of coherent, subsurface
toroidal flux ropes as the source of sunspots, and instead show that
fluctuations must play a major role at all scales for the turbulent
dynamo. To confirm the profound role of fluctuations at large scales,
we show explicit examples in which large bipolar regions differ from the
average Joy's law orientation by an amount between 90° and 100°. We
see no observational support for a separation of scales or a division
between a global and a local dynamo, since also the smallest scales in
our sample retain a non-random component that significantly contributes
to the accumulated emergence of a north-south dipole moment that will
lead to the replacement of the old global poloidal field with a new
one that has the opposite orientation.
Title: Implementation and Comparison of Acoustic Travel-Time
Measurement Procedures for the Solar Dynamics Observatory/Helioseismic
and Magnetic Imager Time - Distance Helioseismology Pipeline
Authors: Couvidat, S.; Zhao, J.; Birch, A. C.; Kosovichev, A. G.;
Duvall, T. L.; Parchevsky, K.; Scherrer, P. H.
Bibcode: 2012SoPh..275..357C
Altcode:
The Helioseismic and Magnetic Imager (HMI) instrument onboard the
Solar Dynamics Observatory (SDO) satellite is designed to produce
high-resolution Doppler-velocity maps of oscillations at the solar
surface with high temporal cadence. To take advantage of these
high-quality oscillation data, a time - distance helioseismology
pipeline (Zhao et al., Solar Phys. submitted, 2010) has been
implemented at the Joint Science Operations Center (JSOC) at Stanford
University. The aim of this pipeline is to generate maps of acoustic
travel times from oscillations on the solar surface, and to infer
subsurface 3D flow velocities and sound-speed perturbations. The wave
travel times are measured from cross-covariances of the observed
solar oscillation signals. For implementation into the pipeline we
have investigated three different travel-time definitions developed in
time - distance helioseismology: a Gabor-wavelet fitting (Kosovichev
and Duvall, SCORE'96: Solar Convection and Oscillations and Their
Relationship, ASSL, Dordrecht, 241, 1997), a minimization relative to a
reference cross-covariance function (Gizon and Birch, Astrophys. J.571,
966, 2002), and a linearized version of the minimization method (Gizon
and Birch, Astrophys. J.614, 472, 2004). Using Doppler-velocity data
from the Michelson Doppler Imager (MDI) instrument onboard SOHO,
we tested and compared these definitions for the mean and difference
travel-time perturbations measured from reciprocal signals. Although
all three procedures return similar travel times in a quiet-Sun region,
the method of Gizon and Birch (Astrophys. J.614, 472, 2004) gives travel
times that are significantly different from the others in a magnetic
(active) region. Thus, for the pipeline implementation we chose the
procedures of Kosovichev and Duvall (SCORE'96: Solar Convection and
Oscillations and Their Relationship, ASSL, Dordrecht, 241, 1997)
and Gizon and Birch (Astrophys. J.571, 966, 2002). We investigated
the relationships among these three travel-time definitions, their
sensitivities to fitting parameters, and estimated the random errors
that they produce.
Title: Time-Distance Helioseismology Data-Analysis Pipeline for
Helioseismic and Magnetic Imager Onboard Solar Dynamics Observatory
(SDO/HMI) and Its Initial Results
Authors: Zhao, J.; Couvidat, S.; Bogart, R. S.; Parchevsky, K. V.;
Birch, A. C.; Duvall, T. L.; Beck, J. G.; Kosovichev, A. G.; Scherrer,
P. H.
Bibcode: 2012SoPh..275..375Z
Altcode: 2011SoPh..tmp...86Z; 2011SoPh..tmp..163Z; 2011arXiv1103.4646Z;
2011SoPh..tmp..232Z
The Helioseismic and Magnetic Imager onboard the Solar Dynamics
Observatory (SDO/HMI) provides continuous full-disk observations
of solar oscillations. We develop a data-analysis pipeline based
on the time-distance helioseismology method to measure acoustic
travel times using HMI Doppler-shift observations, and infer solar
interior properties by inverting these measurements. The pipeline
is used for routine production of near-real-time full-disk maps of
subsurface wave-speed perturbations and horizontal flow velocities
for depths ranging from 0 to 20 Mm, every eight hours. In addition,
Carrington synoptic maps for the subsurface properties are made from
these full-disk maps. The pipeline can also be used for selected target
areas and time periods. We explain details of the pipeline organization
and procedures, including processing of the HMI Doppler observations,
measurements of the travel times, inversions, and constructions of the
full-disk and synoptic maps. Some initial results from the pipeline,
including full-disk flow maps, sunspot subsurface flow fields, and
the interior rotation and meridional flow speeds, are presented.
Title: Magnetohydrodynamic simulations of flows around rotating and
non-rotating axisymmetric magnetic flux concentrations
Authors: Hartlep, T.; Busse, F. H.; Hurlburt, N. E.; Kosovichev, A. G.
Bibcode: 2012MNRAS.419.2325H
Altcode: 2011MNRAS.tmp.1880H
We present results on modelling magnetic flux tubes in an unstably
stratified medium and the flows around them using 2D axisymmetric
magnetohydrodynamic (MHD) simulations. The study is motivated by the
formation of magnetic field concentrations at the solar surface in
sunspots and magnetic pores and the large-scale flow patterns associated
with them. The simulations provide consistent, self-maintained models of
concentrated magnetic field in a convective environment, although they
are not fully realistic or directly applicable to the solar case. In
this paper, we explore under which conditions the associated flows near
the surface are converging (towards the spot centre) or diverging (away
from the axis) in nature. It is found that, depending on the parameters
of the problem, the results can depend on the initial conditions, in
particular for zero or low rotation rates and Prandtl numbers smaller
than unity. The solutions with a converging flow generally produce
more strongly confined magnetic flux tubes.
Title: The Helioseismic and Magnetic Imager (HMI) Investigation for
the Solar Dynamics Observatory (SDO)
Authors: Scherrer, P. H.; Schou, J.; Bush, R. I.; Kosovichev, A. G.;
Bogart, R. S.; Hoeksema, J. T.; Liu, Y.; Duvall, T. L.; Zhao, J.;
Title, A. M.; Schrijver, C. J.; Tarbell, T. D.; Tomczyk, S.
Bibcode: 2012SoPh..275..207S
Altcode:
The Helioseismic and Magnetic Imager (HMI) instrument and investigation
as a part of the NASA Solar Dynamics Observatory (SDO) is designed
to study convection-zone dynamics and the solar dynamo, the origin
and evolution of sunspots, active regions, and complexes of activity,
the sources and drivers of solar magnetic activity and disturbances,
links between the internal processes and dynamics of the corona and
heliosphere, and precursors of solar disturbances for space-weather
forecasts. A brief overview of the instrument, investigation objectives,
and standard data products is presented.
Title: Turbulent Cross-helicity in the Mean-field Solar Dynamo Problem
Authors: Pipin, V. V.; Kuzanyan, K. M.; Zhang, H.; Kosovichev, A. G.
Bibcode: 2011ApJ...743..160P
Altcode: 2011arXiv1105.4285P
We study the dynamical and statistical properties of turbulent
cross-helicity (correlation of the aligned fluctuating velocity
and magnetic field components). We derive an equation governing
generation and evolution of the turbulent cross-helicity and
discuss its meaning for the dynamo. Using the symmetry properties
of the problem we suggest a general expression for the turbulent
cross-helicity. Effects of the density stratification, large-scale
magnetic fields, differential rotation, and turbulent convection
are taken into account. We investigate the relative contribution
of these effects to the cross-helicity evolution for two kinds of
dynamo models of the solar cycle: a distributed mean-field model and
a flux-transport dynamo model. We show that the contribution from the
density stratification follows the evolution of the radial magnetic
field, while large-scale electric currents produce a more complicated
pattern of the cross-helicity of comparable magnitude. The pattern
of the cross-helicity evolution strongly depends on details of the
dynamo mechanism. Thus, we anticipate that direct observations of the
cross-helicity on the Sun may serve for the diagnostic purpose of the
solar dynamo process.
Title: Testing Helioseismic Measurements of the Solar Meridional
Flow with Numerical Simulations
Authors: Hartlep, T.; Zhao, J.; Kosovichev, A. G.; Mansour, N. N.;
Rempel, M.; Pipin, V.
Bibcode: 2011AGUFMSH52B..03H
Altcode:
The meridional flow is of fundamental importance for understanding
magnetic flux transport in the solar interior. Reliable measurements of
the flow could provide important constraints for dynamo theories. The
actual shape and strength of the meridional flow, particularly in the
deep interior, remains unknown. Detecting such weak flows with a speed
of 10-20 m/s in the deep solar interior is a challenging problem for
helioseismology. Numerical simulations of helioseismic wave propagation
provide means for testing and calibrating measurement techniques,
and can help increase our confidence in the inferences obtained from
helioseismic inversions. We have developed a 3D numerical spectral code
to simulate the propagation of acoustic waves in the whole-Sun. With
this code, we simulate the propagation of stochastic wave fields given
mean meridional flows of different strength and circulation patterns
(including flow models with deep and shallow stagnation points). Our
helioseismic measurement techniques are based on estimating acoustic
travel times from wave-field cross-correlations (time-distance
helioseismology method). We investigate various cross-correlation
schemes, and study the sensitivity of acoustic travel times to the
depth and speed of the meridional flow. Using the numerical simulation
results we discuss the prospects of measuring the Sun's meridional
flow from Solar Dynamics Observatory (SDO/HMI) data.
Title: Vortex tubes of solar convection: formation, properties
and dynamics
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Lele, S. K.; Mansour,
N. N.; Wray, A. A.
Bibcode: 2011AGUFMSH43A1927K
Altcode:
Turbulent convection of the Sun demonstrates very complicated dynamics,
which is often associated with different scales of self-organization. In
particular, vortex tube structures have been identified initially in
numerical simulations and then in high-resolution observations. We
present new results of 3D radiative MHD simulations of a top layer of
the convective zone that reveal the important role of turbulent vortex
tubes in various solar processes: acoustic waves excitation, convective
downdrafts, spontaneous formation of stable magnetic structures and
others. We consider various aspects of the vortex tubes dynamics,
including their formation, identification, physical characteristics, and
links to phenomena observed in the quiet Sun and magnetic regions. We
compare the simulation results with observational data from SDO/HMI,
Hinode and large ground-based telescope.
Title: Detection of Emerging Sunspot Regions in the Solar Interior
by Helioseismology
Authors: Ilonidis, S.; Zhao, J.; Kosovichev, A. G.
Bibcode: 2011AGUFMSH54A..01I
Altcode:
Predicting solar magnetic activity is one of the most important
problems of solar and space physics. Helioseismology allows us to
probe conditions inside the Sun by observing acoustic oscillations
on the solar surface. In this study, we analyze Doppler observations
from the MDI and HMI instruments on SOHO and SDO spacecraft using a
time-distance helioseismology technique specifically developed for
measuring acoustic travel time anomalies caused by perturbations in
the deep interior. We have detected strong perturbations associated
with emerging active regions at a depth of about 60Mm, about 1-2 days
before these regions are formed in the photosphere. We compare our
results with numerical simulation models of magnetic flux emergence,
discuss implications for solar magnetism studies, and suggest potential
applications for space weather forecast.
Title: The Waldmeier relations as due to nonlinear surface-shear
shaped dynamo
Authors: Pipin, V.; Kosovichev, A. G.
Bibcode: 2011AGUFMSH43A1922P
Altcode:
We present a study of a solar dynamo model operating in the bulk of
the convection zone with the toroidal magnetic field flux concentrated
in the subsurface rotational shear layer. We explore how this type of
dynamo may depend on spatial variations of turbulent parameters and on
the differential rotation near the surface. The mean-field dynamo model
takes into account the evolution of magnetic helicity and describes its
nonlinear feedback on the generation of large-scale magnetic field by
the α -effect. We compare the magnetic cycle characteristics predicted
by the model, including the cycle asymmetry (associated with the growth
and decay times) and the duration - amplitude relation (Waldmeier's
effects), with the observed sunspot cycle properties. We show that the
model qualitatively reproduces the basic properties of the solar cycles.
Title: Amazing M9-Flare: Sunquakes, White Light Emission and Magnetic
Restructuring
Authors: Kosovichev, A.; Desai, P.; Hayashi, K.
Bibcode: 2011AGUFMSH33A2042K
Altcode:
The solar flare of July 30, 2011 (start time 2:04UT), in active
region NOAA 11261, observed with the Helioseismic and Magnetic Imager
(HMI) on Solar Dynamics Observatory, had a modest X-ray class (M9),
but it made a very strong photospheric impact. The flare generated
helioseismic waves, "sunquakes" (also observed with the SDO/AIA
instrument), caused a large expanding area of white-light emission,
and was accompanied by substantial restructuring of magnetic field
around the magnetic neutral line in the flare region. Surprisingly,
there was no significant hard X-ray emission and coronal mass ejection,
associated with this flare. This provides an indication that the
flare energy release was probably confined in the lower atmosphere. We
present results of initial analysis of the SDO data, and discuss new
challenges from these observations for the standard flare model.
Title: Measuring Acoustic Travel Times in Higher-Latitude Regions
of the Sun using Hinode and SDO Data
Authors: Nagashima, K.; Duvall, T.; Zhao, J.; Kosovichev, A. G.;
Parchevsky, K.; Sekii, T.
Bibcode: 2011AGUFMSH51B2016N
Altcode:
The interior structure and dynamics of the Sun can be probed by
measuring and inverting travel times of acoustic waves, widely
known as time-distance helioseismology. Recent high-resolution
observations of solar oscillations with Hinode/SOT and SDO/HMI
provide us with an opportunity to investigate the flow dynamics in
higher-latitude regions of the Sun. Of particular interest is the
meridional circulation flow, which is crucial for understanding the
solar dynamo mechanism and predicting the solar activity cycles. We
investigate systematic uncertainties of the travel times due to
the center-to-limb variations, which may significantly affect the
helioseismic inferences of the meridional flows. We present the results
of analysis of the cross-correlations in the Doppler velocity, line
core and intensity observations, and the corresponding travel-time fits
for various positions on the solar disk. We discuss the origin of the
center-to-limb variations, including the foreshortening effect, the
difference in the line formation height, and other effects. For better
understanding of the relative role of these effects we use 3D numerical
simulations of solar oscillations in a realistic model of the Sun.
Title: The Asymmetry of Sunspot Cycles and Waldmeier Relations as
a Result of Nonlinear Surface-shear Shaped Dynamo
Authors: Pipin, V. V.; Kosovichev, A. G.
Bibcode: 2011ApJ...741....1P
Altcode: 2011arXiv1105.1828P
The paper presents a study of a solar dynamo model operating in the
bulk of the convection zone with the toroidal magnetic field flux
concentrated in the subsurface rotational shear layer. We explore how
this type of dynamo may depend on spatial variations of turbulent
parameters and on the differential rotation near the surface. The
mean-field dynamo model takes into account the evolution of magnetic
helicity and describes its nonlinear feedback on the generation
of large-scale magnetic field by the α-effect. We compare the
magnetic cycle characteristics predicted by the model, including
the cycle asymmetry (associated with the growth and decay times)
and the duration-amplitude relation (Waldmeier's effects), with the
observed sunspot cycle properties. We show that the model qualitatively
reproduces the basic properties of the solar cycles.
Title: Excitation of Solar Acoustic Waves and Vortex Tube Dynamics
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Lele, S. K.; Mansour,
N. N.; Wray, A. A.
Bibcode: 2011sdmi.confE..26K
Altcode:
Oscillatory behavior is one of the basic properties of the solar
surface. Therefore understanding the mechanism of acoustic waves
excitation in the turbulent near-surface layer is very important for
the interpretation of helioseismology data and development of new
methods of helioseismic diagnostics of the solar interior, as well
as for understanding of the role of the acoustic flux in the energy
transport. Observations of individual impulsive events generating
acoustic waves have been mostly detected in the intergranular lanes
and are associated with local strong cooling of fluid elements. Also,
the modern high-resolution observations revealed a process of dragging
of small-scale magnetic concentrations toward the center of a convective
vortex motion in the photosphere. A substantial progress is being made
from the analysis of high-resolution observational data, particularly
from Hinode, Sunrise, NST, and SDO/HMI, and also from high-resolution
realistic numerical simulations. The simulations take into account
all essential turbulent and other physical properties of the solar
plasma, and allow us to look at the scales that cannot be resolved
in observations, and also compare the data and models. We present new
results of 3D radiative MHD simulations of the upper convection zone and
atmosphere, and show that one of the possible mechanisms of the acoustic
waves generation is a result of interaction two and more vortex tubes
in the intergranular lanes. The process of a vortex annihilation,
which produces acoustic waves, the properties of these waves and
vortices, magnetic influence on the efficiency of acoustic emission,
and comparison with the available observational data will be discussed.
Title: Multi-wavelength time-distance helioseismology analyses
Authors: Nagashima, Kaori; Zhao, Junwei; Duvall, Thomas, Jr.;
Kosovichev, Alexander G.; Parchevsky, Konstantin; Sekii, Takashi
Bibcode: 2011sdmi.confE..37N
Altcode:
Travel times of the acoustic waves in the Sun tell us the structure
and the dynamics of the Sun. This information have been used to probe
the solar interior. If we exploit multi-layer observation datasets,
however, it will provide us with means to study the wave propagation
between the layers as well (Nagashima et al. 2009). In this study, using
multi-wavelength datasets obtained by Hinode/SOT, SDO/HMI, and SDO/AIA
we calculate the cross-correlation function of the wavefield and carry
out time-distance helioseismology analyses. Our preliminary results show
that when we cross-correlate the wavefields of two different layers
the cross-correlation functions between these layers are different
from the cross-correlation functions of both single layers, and this
provides us with an insight of wave propagation properties. We also
use numerical simulations of solar oscillations to help interpret our
observational results.
Title: Strong photospheric impact of M-class flare: helioseismic
response, white light emission and magnetic restructuring
Authors: Kosovichev, Alexander; Desai, Priya; Hayashi, Keiji
Bibcode: 2011sdmi.confE.101K
Altcode:
Solar flare of July 30, 2011, had a modest X-ray class (M9), but
made a very strong photospheric impact, observed by the Helioseismic
and Magnetic Imager (HMI) on Solar Dynamics Observatory. The flare
generated helioseismic waves, (also observed with the SDO/AIA
instrument), caused a large expanding area of white-light emission,
and was accompanied by substantial restructuring of magnetic fields in
the flare region. There was no significant hard X-ray emission and no
coronal mass ejection. This indicates that the flare energy release
was probably confined in the lower atmosphere. We present results of
initial analysis of the SDO data.
Title: Helioseismic Frechet Traveltime Kernels in Spherical
Coordinates
Authors: Schlottmann, R. B.; Kosovichev, A. G.
Bibcode: 2011sdmi.confE..29S
Altcode: 2011arXiv1105.4619S
Seismic traveltime measurements are a crucial tool in the investigation
of the solar interior, particularly in the examination of fine-scale
structure. Traditional analysis of traveltimes relies on a geometrical
ray picture of acoustic wave propagation, which assumes high
frequencies. However, it is well-known that traveltimes obtained
from finite-frequency waves are sensitive to variations of medium
parameters in a wide Fresnel zone around the ray path. To address this
problem, Frechet traveltime sensitivity kernels have previously been
developed. These kernels use a more realistic approximation of the wave
propagation to obtain a linear relationship between traveltimes and
variations in medium parameters. Frechet kernels take into account the
actual frequency content of the measured waves and, thus, reproduce the
Fresnel zone. Kernel theory has been well-developed in previous work on
plane-parallel models of the Sun for use in local helioseismology. Our
primary purpose is to apply kernel theory to much larger scales and in
a spherical geometry. We also present kernel theory in a different way,
using basic functional analytic methods, in the hope that this approach
provides an even clearer understanding of the theory, as well as a
set of tools for calculating kernels. Our results are very general
and can be used to develop kernels for sensitivity to sound speed,
density, magnetic fields, fluid flows, and any other medium parameter
which can affect wave propagation.
Title: Variability of five-minute solar oscillations in the corona
as observed by the Extreme Ultraviolet Spectrophotometer (ESP) on the
Solar Dynamics Observatory Extreme Ultraviolet Variability Experiment
(SDO/EVE)
Authors: Didkovsky, Leonid; Judge, Darrell; Kosovichev, Alexander;
Wieman, Seth; Woods, Tom
Bibcode: 2011sdmi.confE..98D
Altcode:
Solar oscillations in the corona were detected in the frequency
range corresponding to five-minute acoustic modes of the Sun. The
oscillations have been observed using soft Xray measurements from the
Extreme Ultraviolet Spectrophotometer (ESP) of the Extreme Ultraviolet
Variability Experiment (EVE) onboard the Solar Dynamics Observatory
(SDO). The ESP zeroth-order channel observes the Sun as a star without
spatial resolution in the wavelength range of 0.1 to 7.0 nm (the energy
range is 0.18 to 12.4 keV). The amplitude spectrum of the oscillations,
calculated from six-day time series, showed a significant increase in
the frequency range of 2 to 4 mHz. This increase was interpreted as a
response of the corona to solar acoustic (p) modes, and some p-mode
frequencies among the strongest peaks were identified. In this work
we show how these increases in the 2 to 4 mHz frequency range vary on
a daily basis and compare this variability for quiet and intermediate
levels of solar activity. We found that the increases do not correlate
with the changes of daily mean irradiance or with the standard deviation
of the irradiance.
Title: Propagation and transformation of MHD waves in sunspots
Authors: Parchevsky, K. V.; Kosovichev, A. G.
Bibcode: 2011sdmi.confE..46P
Altcode:
It has long been suggested that the fast MHD waves (which are analog of
the acoustic waves in non-magnetized regions) can convert into other
types of MHD waves (slow MHD and/or Alfven waves) inside sunspots due
to the interaction with the magnetic field. Transformed waves propagate
in the deeper layers of the sunspot along the magnetic field lines,
removing the energy from the observed wave field. Such mode conversion
(if exists) will contribute to the suppression of the acoustic power
inside the sunspots. Results of numerical simulations of interaction
of MHD waves with magnetized areas show that the mode conversion
occurs near the sunspot axis in regions where the wavefront of the
fast MHD wave crosses the level where the plasma parameter beta is of
order of unity. The transformed wave is primarily transverse. Detailed
simulations show, that the transformed wave exists even in case where
the source is located completely outside of the magnetic region, so
the wave, which enters the model of the sunspot, is pure acoustic. To
compare simulations of MHD waves in sunspots and observations we
need to know at what geometrical depth this comparison has to be
done. We propose a method of the wave amplitude and travel-time
shortening corrections. Our method is based on the combination of
three-dimensional numerical simulations of propagation of MHD waves
with 1D LTE radiation transfer simulations of the Stokes profiles of
the HMI line. For measuring the Doppler shift we use the same set
of 6 narrow-band filters which is used by the HMI instrument. Such
simulations will provide the artificial HMI level 1 data (if necessary,
non-simultaneity of frames for different polarization channels can be
simulated) which can test the whole Time-Distance Pipeline.
Title: A Method for the Calculation of Acoustic Green's Functions
for Use in Computing Frechet Traveltime Sensitivity Kernels
Authors: Schlottmann, R. B.; Kosovichev, A. G.
Bibcode: 2011sdmi.confE..27S
Altcode:
We have modified a semi-analytical approach, originally intended
for the calculation of stellar acoustic normal modes, for use in
the efficient calculation of Green's functions. The primary purpose
of this code is to provide necessary acoustic responses in the
calculation of Frechet traveltime sensitivity kernels. Assuming
a spherically symmetric star in hydrostatic equilibrium and the
Cowling approximation, we perform the usual spherical harmonic
decomposition on the linearized acoustic wave equation, resulting in
two coupled first-order ODEs in the radial displacement and Eulerian
pressure. Following Gabriel & Noels (1976), we break the radial
domain into a finite number of non-uniformly spaced intervals within
which we assume that the coefficients of the equations are constant. For
each interval, we obtain a sum of two analytical solutions with unknown
coefficients. Continuity of the fields yields a system of equations
for the coefficients. However, instead of using this system to obtain
eigenfunctions and eigenfrequencies, we instead solve it assuming a
point source, yielding Green's functions for each frequency and degree,
eliminating the usual need to sum a truncated series over mode order. We
have made a couple of other modifications to the method to improve
efficiency. First, the original system of equations is pentadiagonal,
but these can be reduced analytically to a tridiagonal system, which
is solved approximately 10 times faster. Next, using a technique from
complex analysis, we are able to calculate, in the frequency domain,
Green's functions that are windowed in time. This naturally spreads
out the linewidths of the mode resonance peaks present in the Green's
functions, allowing us to fully capture modes with very small linewidths
without resorting to excessively fine sampling in frequency.
Title: Effects of Turbulence Models on Self-Organization Processes
in Solar Convection
Authors: Wray, A. A.; Mansour, N. N.; Rogachevskii, I.; Kleeorin,
N.; Kitiashvili, I. N.; Kosovichev, A. G.
Bibcode: 2011sdmi.confE...5W
Altcode:
Realistic MHD numerical simulations of subsurface flows and magnetic
structures have become achievable because of the development of fast
supercomputer systems and efficient parallel computer codes. The
dynamics of the subsurface layer is particularly critical for
understanding the self-organization processes of magnetoconvection
on different scales. Realistic simulations of solar convection
in the presence of magnetic fields reveal very interesting
dynamics and reproduce several phenomena observed in solar active
regions. “SolarBox”, a 3-D real-gas radiative MHD code developed at
NASA Ames, was used for our simulations. Because both the Reynolds and
magnetic Reynolds numbers are extremely high, research into subgrid
modeling of MHD in the solar context is essential, and an important
feature of this code is the implementation of various subgrid-scale
LES turbulence models. We present a comparison of two such models:
a Smagorinskii-type subgrid resistivity model and the Turbulent
Effective Lorentz Force model (TELF) and discuss the role of LES
models for studying the process of magnetic flux tube formation and
the turbulent properties of magnetoconvection.
Title: Subphotospheric flows and evolution of active regions
Authors: Kosovichev, A. G.; Zhao, J.
Bibcode: 2011sdmi.confE.102K
Altcode:
The HMI instrument on SDO has provided unprecedented opportunities
for studying the subsurface dynamics of active regions during
their emergence and various stages of the evolution. We analyzed
maps of subsurface flows, obtained by using the HMI time-distance
helioseismology pipeline, in order to investigate links between the
subsurface properties and surface magnetic structures, and also their
relationships to flaring and CME activity for several interesting
regions. The results reveal strong shearing and twisting flows during
high-activity periods. We discuss how properties of the subphotospheric
flows, such as divergence, vorticity and helicity, can characterize
the evolution and activity of magnetic regions.
Title: Mean-field Solar Dynamo Models with a Strong Meridional Flow
at the Bottom of the Convection Zone
Authors: Pipin, V. V.; Kosovichev, A. G.
Bibcode: 2011ApJ...738..104P
Altcode: 2011arXiv1104.1433P
This paper presents a study of kinematic axisymmetric mean-field dynamo
models for the case of meridional circulation with a deep-seated
stagnation point and a strong return flow at the bottom of the
convection zone. This kind of circulation follows from mean-field models
of the angular momentum balance in the solar convection zone. The
dynamo models include turbulent sources of the large-scale poloidal
magnetic field production due to kinetic helicity and a combined effect
due to the Coriolis force and large-scale electric current. In these
models the toroidal magnetic field, which is responsible for sunspot
production, is concentrated at the bottom of the convection zone and is
transported to low-latitude regions by a meridional flow. The meridional
component of the poloidal field is also concentrated at the bottom
of the convection zone, while the radial component is concentrated
in near-polar regions. We show that it is possible for this type
of meridional circulation to construct kinematic dynamo models that
resemble in some aspects the sunspot magnetic activity cycle. However,
in the near-equatorial regions the phase relation between the toroidal
and poloidal components disagrees with observations. We also show
that the period of the magnetic cycle may not always monotonically
decrease with the increase of the meridional flow speed. Thus, for
further progress it is important to determine the structure of the
meridional circulation, which is one of the critical properties,
from helioseismology observations.
Title: Observations of Five-minute Solar Oscillations in the Corona
Using the Extreme Ultraviolet Spectrophotometer (ESP) On Board the
Solar Dynamics Observatory Extreme Ultraviolet Variability Experiment
(SDO/EVE)
Authors: Didkovsky, L.; Judge, D.; Kosovichev, A. G.; Wieman, S.;
Woods, T.
Bibcode: 2011ApJ...738L...7D
Altcode:
We report on the detection of oscillations in the corona in the
frequency range corresponding to five-minute acoustic modes of
the Sun. The oscillations have been observed using soft X-ray
measurements from the Extreme Ultraviolet Spectrophotometer (ESP)
of the Extreme Ultraviolet Variability Experiment on board the Solar
Dynamics Observatory. The ESP zeroth-order channel observes the
Sun as a star without spatial resolution in the wavelength range
of 0.1-7.0 nm (the energy range is 0.18-12.4 keV). The amplitude
spectrum of the oscillations calculated from six-day time series
shows a significant increase in the frequency range of 2-4 mHz. We
interpret this increase as a response of the corona to solar acoustic
(p) modes and attempt to identify p-mode frequencies among the strongest
peaks. Due to strong variability of the amplitudes and frequencies of
the five-minute oscillations in the corona, we study how the spectrum
from two adjacent six-day time series combined together affects the
number of peaks associated with the p-mode frequencies and their
amplitudes. This study shows that five-minute oscillations of the
Sun can be observed in the corona in variations of the soft X-ray
emission. Further investigations of these oscillations may improve
our understanding of the interaction of the oscillation modes with
the solar atmosphere, and the interior-corona coupling, in general.
Title: Comparison of numerical simulations and observations of
helioseismic MHD waves in sunspots
Authors: Parchevsky, K. V.; Zhao, J.; Kosovichev, A. G.; Rempel, M.
Bibcode: 2011IAUS..273..422P
Altcode:
Numerical 3D simulations of MHD waves in magnetized regions
with background flows are very important for the understanding of
propagation and transformation of waves in sunspots. Such simulations
provide artificial data for testing and calibration of helioseismic
techniques used for analysis of data from space missions SOHO/MDI,
SDO/HMI, and HINODE. We compare with helioseismic observations
results of numerical simulations of MHD waves in different models
of sunspots. The simulations of waves excited by a localized source
provide a detailed picture of the interaction of the MHD waves with
the magnetic field and background flows (deformation of the waveform,
wave transformation, amplitude variations and anisotropy). The observed
cross-covariance function represents an effective Green's function of
helioseismic waves. As an initial step, we compare it with simulations
of waves generated by a localized source. More thorough analysis
implies using multiple sources and comparison of the observed and
simulated cross-covariance functions. We plan to do such calculations
in the nearest future. Both, the simulations and observations show
that the wavefront inside the sunspot travels ahead of a reference
``quiet Sun'' wavefront, when the wave enters the sunspot. However,
when the wave passes the sunspot, the time lag between the wavefronts
becomes unnoticeable.
Title: Numerical simulations of magnetic structures
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Wray, A. A.; Mansour,
N. N.
Bibcode: 2011IAUS..273..315K
Altcode:
We use 3D radiative MHD simulations of the upper turbulent convection
layer for investigation of physical mechanisms of formation of
magnetic structures on the Sun. The simulations include all essential
physical processes, and are based of the LES (Large-Eddy Simulations)
approach for describing the sub-grid scale turbulence. The simulation
domain covers the top layer of the convection zone and the lower
atmosphere. The results reveal a process of spontaneous formation of
stable magnetic structures from an initially weak vertical magnetic
field, uniformly distributed in the simulation domain. The process
starts concentration of magnetic patches at the boundaries of granular
cells, which are subsequently merged together into a stable large-scale
structure by converging downdrafts below the surface. The resulting
structure represents a compact concentration of strong magnetic
field, reaching 6 kG in the interior. It has a cluster-like internal
structurization, and is maintained by strong downdrafts extending into
the deep layers.
Title: "Hare and Hounds" Tests of Helioseismic Holography
Authors: Birch, A. C.; Parchevsky, K. V.; Braun, D. C.; Kosovichev,
A. G.
Bibcode: 2011SoPh..272...11B
Altcode: 2011SoPh..tmp..315B; 2011SoPh..tmp..290B; 2011SoPh..tmp..257B;
2011SoPh..tmp..188B
We use the output of numerical wave-propagation simulations as synthetic
data for "hare and hounds" tests of helioseismic holography. In
the simple non-magnetic models examined here, we show that when the
inversion method includes a consistent treatment of the filtering
applied during the data analysis the inversions for the subsurface
sound speed are qualitatively correct.
Title: Detection of Emerging Sunspot Regions in the Solar Interior
Authors: Ilonidis, Stathis; Zhao, Junwei; Kosovichev, Alexander
Bibcode: 2011Sci...333..993I
Altcode:
Sunspots are regions where strong magnetic fields emerge from the solar
interior and where major eruptive events occur. These energetic events
can cause power outages, interrupt telecommunication and navigation
services, and pose hazards to astronauts. We detected subsurface
signatures of emerging sunspot regions before they appeared on the
solar disc. Strong acoustic travel-time anomalies of an order of 12 to
16 seconds were detected as deep as 65,000 kilometers. These anomalies
were associated with magnetic structures that emerged with an average
speed of 0.3 to 0.6 kilometer per second and caused high peaks in the
photospheric magnetic flux rate 1 to 2 days after the detection of the
anomalies. Thus, synoptic imaging of subsurface magnetic activity may
allow anticipation of large sunspot regions before they become visible,
improving space weather forecast.
Title: Advances in Plasma Astrophysics (IAU S274)
Authors: Bonanno, Alfio; de Gouveia Dal Pino, Elisabete; Kosovichev,
Alexander G.
Bibcode: 2011IAUS..274.....B
Altcode:
Preface; 1. Plasma astrophysics in the laboratory; 2. Interstellar,
space and planetary plasmas; 3. Solar and stellar plasma; 4. Plasma
around compact objects; 5. Observational and modelling programs for
plasma astrophysics; 6. Plasmas in galaxies and galaxy clusters;
7. Plasma astrophysics in numerical simulations; Author index;
Object index.
Title: Investigation of a sunspot complex by time-distance
helioseismology
Authors: Kosovichev, A. G.; Duvall, T. L.
Bibcode: 2011IAUS..273..320K
Altcode: 2011arXiv1102.3961K
Sunspot regions often form complexes of activity that may live for
several solar rotations, and represent a major component of the Sun's
magnetic activity. It had been suggested that the close appearance of
active regions in space and time might be related to common subsurface
roots, or ``nests'' of activity. EUV images show that the active regions
are magnetically connected in the corona, but subsurface connections
have not been established. We investigate the subsurface structure and
dynamics of a large complex of activity, NOAA 10987-10989, observed
during the SOHO/MDI Dynamics run in March-April 2008, which was a
part of the Whole Heliospheric Interval (WHI) campaign. The active
regions in this complex appeared in a narrow latitudinal range,
probably representing a subsurface toroidal flux tube. We use the
MDI full-disk Dopplergrams to measure perturbations of travel times
of acoustic waves traveling to various depths by using time-distance
helioseismology, and obtain sound-speed and flow maps by inversion of
the travel times. The subsurface flow maps show an interesting dynamics
of decaying active regions with persistent shearing flows, which may be
important for driving the flaring and CME activity, observed during the
WHI campaign. Our analyses, including the seismic sound-speed inversion
results and the distribution of deep-focus travel-time anomalies, gave
indications of diverging roots of the magnetic structures, as could be
expected from Ω-loop structures. However, no clear connection in the
depth range of 0-48 Mm among the three active regions in this complex
of activity was detected.
Title: Helioseismic Response to the X2.2 Solar Flare of 2011
February 15
Authors: Kosovichev, A. G.
Bibcode: 2011ApJ...734L..15K
Altcode: 2011arXiv1105.0953K
The X2.2-class solar flare of 2011 February 15 produced a powerful
"sunquake" event, representing a helioseismic response to the
flare impact in the solar photosphere, which was observed with the
Helioseismic and Magnetic Imager (HMI) instrument on board the Solar
Dynamics Observatory (SDO). The impulsively excited acoustic waves
formed a compact wave packet traveling through the solar interior and
appearing on the surface as expanding wave ripples. The initial flare
impacts were observed in the form of compact and rapid variations
of the Doppler velocity, line-of-sight magnetic field, and continuum
intensity. These variations formed a typical two-ribbon flare structure,
and are believed to be associated with thermal and hydrodynamic effects
of high-energy particles heating the lower atmosphere. The analysis of
the SDO/HMI and X-ray data from RHESSI shows that the helioseismic waves
were initiated by the photospheric impact in the early impulsive phase,
observed prior to the hard X-ray (50-100 keV) impulse, and were probably
associated with atmospheric heating by relatively low-energy electrons
(~6-50 keV) and heat flux transport. The impact caused a short motion
in the sunspot penumbra prior to the appearance of the helioseismic
wave. It is found that the helioseismic wave front traveling through a
sunspot had a lower amplitude and was significantly delayed relative
to the front traveling outside the spot. These observations open new
perspectives for studying the flare photospheric impacts and for using
the flare-excited waves for sunspot seismology.
Title: Investigations of solar plasma in the interior and corona
from Solar Dynamics Observatory
Authors: Kosovichev, A. G.
Bibcode: 2011IAUS..274..287K
Altcode:
The Sun is a plasma laboratory for astrophysics, which allows us to
investigate many important phenomena in turbulent magnetized plasma
in detail. Solar Dynamics Observatory (SDO) launched in February 2010
provides unique information about plasma processes from the interior
to the corona. The primary processes of magnetic field generation
and formation of magnetic structures are hidden beneath the visible
surface. Helioseismic diagnostics, based on observations and analysis
of solar oscillations and waves, give insights into the physical
processes in the solar interior and mechanisms of solar magnetic
activity. In addition, simultaneous high-resolution multi-wavelength
observations of the solar corona provide opportunity to investigate in
unprecedented detail the coronal dynamics and links to the interior
processes. These capabilities are illustrated by initial results on
the large-scale dynamics of the Sun, the subsurface structure and
dynamics of a sunspot and observations of a X-class solar flare.
Title: Realistic MHD simulations of magnetic self-organization in
solar plasma
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Wray, A. A.; Mansour,
N. N.
Bibcode: 2011IAUS..274..120K
Altcode:
Filamentary structure is a fundamental property of the magnetized solar
plasma. Recent high-resolution observations and numerical simulations
have revealed close links between the filamentary structures and
plasma dynamics in large-scale solar phenomena, such as sunspots and
magnetic network. A new emerging paradigm is that the mechanisms
of the filamentary structuring and large-scale organization are
natural consequences of turbulent magnetoconvection on the Sun. We
present results of 3D radiative MHD large-eddy simulations (LES) of
magnetic structures in the turbulent convective boundary layer of the
Sun. The results show how the initial relatively weak and uniformly
distributed magnetic field forms the filamentary structures, which
under certain conditions gets organized on larger scales, creating
stable long-living magnetic structures. We discuss the physics of
magnetic self-organization in the turbulent solar plasma, and compare
the simulation results with observations.
Title: High-resolution 3D Radiative MHD Simulations Of Turbulent
Convection And Spectro-polarimetric Properties
Authors: Kitiashvili, Irina; Kosovichev, A. G.; Mansour, N. N.;
Stenflo, J. O.; Wray, A. A.
Bibcode: 2011SPD....42.1708K
Altcode: 2011BAAS..43S.1708K
Realistic numerical simulations of solar magnetoconvection play a
key role for our understanding of the basic physical phenomena in
the subsurface convective boundary layer and the atmosphere. For the
accurate modeling of the turbulent processes on the Sun it is important
to perform the simulations with the highest possible resolution. Our
results have revealed significant changes in properties of the
turbulent motions when the resolution is increased. It is particularly
interesting that small-scale vortex motions in the intergranular
lanes become ubiquitous and strong, and play a critical role in the
large-scale organization of the solar dynamics. For the comparison
with observational data it is necessary to investigate relationships
between the physical and spectro-polarimetric properties in various
conditions of the quiet-Sun and magnetic regions, and model the observed
parameters. By using the radiative line formation code, SPINOR/STOPRO,
we have calculated the Stokes profiles and other characteristics
for the spectral line of the Hinode/SOT and SDO/HMI instruments,
and compared the simulation results with the observational data.
Title: Helioseismic Measurements Of Meridional Flows In Artificial
Data From 3d Numerical Simulations Of Wave Propagation In The
Whole Sun
Authors: Hartlep, Thomas; Roth, M.; Doerr, H.; Zhao, J.; Kosovichev,
A. G.
Bibcode: 2011SPD....42.1611H
Altcode: 2011BAAS..43S.1611H
Measuring the structure of the deep solar interior is of considerable
interest for understanding how the solar dynamo functions. In
particular, the structure of the meridional flow and the depth of
its return flow are of significant interest. Detecting such small
flows in the deep interior is a challenging problem. Numerical
simulations can provide means for testing and calibrating measurement
techniques and help increase our confidence in the inferences obtained
from observations. We present results from analyzing artificial
helioseismology data obtained from numerical simulations of helioseismic
wave propagation in the whole 3D solar interior with models of the
meridional circulation present in the background state. Two methods
- a time-distance helioseismology technique and a Fourier-Legendre
decomposition technique - are used in this paper to try to detect and
measure this flow from the oscillations at the solar surface.
Title: The Waldmeier Effect and Asymmetry of Solar Magnetic Cycles
in a Surface-Shear Dynamo Model
Authors: Pipin, Valery; Kosovichev, A.
Bibcode: 2011SPD....42.0202P
Altcode: 2011BAAS..43S.0202P
We present a study of solar dynamo model distributed in the bulk of
the convection zone with toroidal magnetic-field flux concentrated in
a near-surface layer. We explore how this effect may depend on spatial
variations of the turbulent parameters and the differential rotation
near the surface. The model includes the magnetic helicity non-linear
feedback on the dynamo alpha-effect. We compute the magnetic cycle
characteristics predicted by the model, including the cycle skewness
(associated with duration of the growth and decay phases) and the
duration-strength dependence (Waldmeier's effects). We confront the
theoretical expectations with the solar sunspot cycle properties.
Title: A Mechanism of the Solar Acoustic Emission
Authors: Kitiashvili, Irina; Kosovichev, A. G.; Lele, S. K.; Mansour,
N. N.; Wray, A. A.
Bibcode: 2011SPD....42.1701K
Altcode: 2011BAAS..43S.1701K
Understanding the mechanism of acoustic waves excitation in the
turbulent surface layer is very important for the interpretation of
helioseismology data and development of new methods of helioseismic
diagnostics of the solar interior, as well as for understanding of
the role of the acoustic flux in the energy transport. A substantial
progress is being made from the analysis of high-resolution
observational data, particularly from Hinode, Sunrise, and SDO/HMI,
and also from high-resolution realistic numerical simulations. The
simulations take into account all essential turbulent and other
physical properties of the solar plasma, and allow us to look at the
scales that cannot be resolved in observations, and also compare the
data and models. We present new results of 3D radiative hydrodynamics
simulations of the upper convection zone and atmosphere, and show that
one of the possible mechanisms of the acoustic waves generation is a
result of interaction two and more vortex tubes. The process of a vortex
annihilation, which produces acoustic waves, the properties of these
waves and vortices, and comparison with the available observational
data will be discussed.
Title: Subsurface Structure and Dynamics of Active Regions Observed
with SDO/HMI
Authors: Kosovichev, Alexander G.; Zhao, J.
Bibcode: 2011SPD....42.2110K
Altcode: 2011BAAS..43S.2110K
The HMI instrument on SDO has provided unprecedented opportunities
to investigate the subsurface evolution of active regions. We use
maps of subsurface flows and wave-speed perturbations, obtained
using the HMI time-distance helioseismology pipeline, to study links
between the subsurface properties and surface magnetic structures,
and their relationships to the flaring and CME activity for several
interesting active regions. A particular attention is paid to AR1158,
which produced X2.2 flare.
Title: Flare-excited Waves in the Solar Interior and Atmosphere
Authors: Kosovichev, Alexander G.
Bibcode: 2011SPD....42.2107K
Altcode: 2011BAAS..43S.2107K
The X2.2 flare of February 15, 2011, produced powerful waves traveling
in the solar interior and atmosphere, which were observed with the HMI
and AIA instruments on SDO. These data provide a unique opportunity
for high-resolution spatial and spectral analyses of the helioseismic
and atmospheric responses and their relationship to the flare energy
release. In particular, the analysis of the SDO/HMI and X-ray data
from RHESSI shows that the helioseismic waves were initiated by the
photospheric impact in the early impulsive phase, observed prior to
the hard X-ray (50-100 keV) impulse, and were probably associated with
atmospheric heating by relatively low-energy electrons (6-50 keV) and
heat flux transport. The impact caused a short wave-like motion in the
sunspot penumbra prior to the appearance of the helioseismic wave. The
AIA observations revealed for the first time the propagation of this
wave in the upper atmosphere, and the accompanying shock wave. The
multi-instrument observations of the flare-excited waves open new
perspectives for studying the energy release and transport in solar
flares, and also for the magnetic field seismology in active regions.
Title: MHD Wave Transformation and Radiation Transfer Simulations
in Sunspots
Authors: Parchevsky, Konstantin; Kosovichev, A.
Bibcode: 2011SPD....42.1707P
Altcode: 2011BAAS..43S.1707P
We present results of resonant wave transformation in MHD models of
sunspot. Numerical 3D MHD simulations of waves excited by subsurface
point sources and traveling through the sunspot models reveal details
of the MHD mode conversion, which happens in the regions where the
wavefront crosses the level where the plasma parameter β is of the
order of unity. In particular, the wave transformation produces
slow MHD waves traveling predominantly along the magnetic lines
in the central part of sunspot. This process is clearly seen when
plotted as a projection of velocity (and magnetic field) variations
along and perpendicular to the local direction of the magnetic field
lines. Detailed simulations show that the transformed wave appears
even in the case where the source is located completely outside the
magnetic region, so that when the initial wave is pure acoustic. To
investigate the wave amplitude and travel-time corrections due to
the variations of thermodynamic properties and magnetic effects in
sunspots we apply the 1D LTE radiation transfer code SPINOR/STOPRO to
the wave simulation results. This permits us to simulate the profile
of the HMI Fe6173A spectral line, and model the HMI observations of
the line-of-sight velocity and magnetic field, and also the Stokes
profiles. For calculation of the observables we use the same set of 6
narrow-band filters as used in the HMI instrument. Such simulations
provide artificial HMI level-1 data for testing helioseismology
measurements in sunspots and magnetic active regions.
Title: Solar Subsurface Dynamics from the First Year of SDO/HMI
Time-Distance Helioseismology Analysis
Authors: Zhao, Junwei; Bogart, R. S.; Kosovichev, A. G.; Nagashima, K.
Bibcode: 2011SPD....42.2131Z
Altcode: 2011BAAS..43S.2131Z
It has been about one year since SDO/HMI started to take continuous
observations of the Sun. Time-distance pipeline designed for a routine
processing of HMI observations has generated plenty of results. We
will present the solar interior rotational rate and meridional flow
speed and their evolution during the entire year. For local areas,
we study the subsurface flows of supergranules and some selected
active regions. We also apply the time-distance analysis in higher
latitude areas, and study the rotational and meridional speed above
the latitude of 65 degree.
Title: Analysis of SOHO/MDI and TRACE Observations of Sunspot
Torsional Oscillation in AR10421
Authors: Gopasyuk, O. S.; Kosovichev, A. G.
Bibcode: 2011ApJ...729...95G
Altcode: 2011arXiv1102.3953G
Rotation of the leading sunspot of active region NOAA 10421 was
investigated using magnetograms and Dopplergrams from the MDI instrument
of the Solar and Heliospheric Observatory, and white-light images
from the Transition Region and Coronal Explorer (TRACE). The vertical,
radial, and azimuthal axisymmetrical components of both magnetic and
velocity field vectors were reconstructed for the sunspot umbra and
penumbra. All three components of both vectors in the umbra and penumbra
show torsional oscillations with the same rotational period of about
3.8 days. The TRACE white-light data also show that the sunspot umbra
and penumbra are torsionally rotating with the same period. Possible
mechanisms of sunspot torsional motions are discussed.
Title: Signatures of Emerging Subsurface Structures in Acoustic
Power Maps of the Sun
Authors: Hartlep, T.; Kosovichev, A. G.; Zhao, J.; Mansour, N. N.
Bibcode: 2011SoPh..268..321H
Altcode: 2010arXiv1003.4305H
We show that under certain conditions, subsurface structures in
the solar interior can alter the average acoustic power observed at
the photosphere above them. By using numerical simulations of wave
propagation, we show that this effect is large enough for it to be
potentially used for detecting emerging active regions before they
appear on the surface. In our simulations, simplified subsurface
structures are modeled as regions with enhanced or reduced acoustic
wave speed. We investigate the dependence of the acoustic power
above a subsurface region on the sign, depth, and strength of the
wave-speed perturbation. Observations from the Solar and Heliospheric
Observatory/Michelson Doppler Imager (SOHO/MDI) prior and during the
emergence of NOAA active region 10488 are used to test the use of
acoustic power as a potential precursor of the emergence of magnetic
flux.
Title: A brief history of the solar oblateness. A review
Authors: Damiani, C.; Rozelot, J. P.; Lefebvre, S.; Kilcik, A.;
Kosovichev, A. G.
Bibcode: 2011JASTP..73..241D
Altcode:
We hereby present a review on solar oblateness measurements. By
emphasizing historical data, we illustrate how the discordance between
experimental results can lead to substantial improvements in the
building of new technical apparatus as well as to the emergence of new
ideas to develop new theories. We stress out the need to get accurate
data from space to enhance our knowledge of the solar core in order
to develop more precise ephemerids and ultimately build possible new
gravitational theories.
Title: Numerical MHD Simulations of Solar Magnetoconvection and
Oscillations in Inclined Magnetic Field Regions
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Mansour, N. N.; Wray,
A. A.
Bibcode: 2011SoPh..268..283K
Altcode: 2010SoPh..tmp..239K; 2010arXiv1011.5527K
The sunspot penumbra is a transition zone between the strong vertical
magnetic field area (sunspot umbra) and the quiet Sun. The penumbra
has a fine filamentary structure that is characterized by magnetic
field lines inclined toward the surface. Numerical simulations of
solar convection in inclined magnetic field regions have provided an
explanation of the filamentary structure and the Evershed outflow in
the penumbra. In this article, we use radiative MHD simulations to
investigate the influence of the magnetic field inclination on the
power spectrum of vertical velocity oscillations. The results reveal a
strong shift of the resonance mode peaks to higher frequencies in the
case of a highly inclined magnetic field. The frequency shift for the
inclined field is significantly greater than that in vertical-field
regions of similar strength. This is consistent with the behavior of
fast MHD waves.
Title: Toward Waveform Heliotomography: Observing Interactions of
Helioseismic Waves with a Sunspot
Authors: Zhao, Junwei; Kosovichev, Alexander G.; Ilonidis, Stathis
Bibcode: 2011SoPh..268..429Z
Altcode: 2010arXiv1011.3837Z; 2010SoPh..tmp..240Z
We investigate how helioseismic waves that originate from effective
point sources interact with a sunspot. These waves are reconstructed
from observed stochastic wavefields on the Sun by cross-correlating
photospheric Doppler-velocity signals. We select the wave sources at
different locations relative to the sunspot, and investigate the p-
and f-mode waves separately. The results reveal a complicated picture
of waveform perturbations caused by the wave interaction with the
sunspot. In particular, it is found that for waves originating from
outside of the sunspot, p-mode waves travel across the sunspot with a
small amplitude reduction and slightly higher speed, and wave amplitude
and phase get mostly restored to the quiet-Sun values after passing the
sunspot. The f-mode wave experiences some amplitude reduction passing
through the sunspot, and the reduced amplitude is not recovered after
that. The wave-propagation speed does not change before encountering
the sunspot and inside the sunspot, but the wavefront becomes faster
than the reference wave after passing through the sunspot. For
waves originating from inside the sunspot umbra, both f- and p-mode
waves show significant amplitude reductions and faster speed for all
propagation paths. A comparison of positive and negative time lags
of cross-correlation functions shows an apparent asymmetry in the
waveform changes for both the f- and p-mode waves. We suggest that the
waveform variations of the helioseismic waves interacting with a sunspot
found in this article can be used for developing a method of waveform
heliotomography, similar to the waveform tomography of the Earth.
Title: LES of turbulent convection in solar-type stars and formation
of large-scale magnetic structures
Authors: Rogachevskii, I.; Kleeorin, N.; Kitiashvili, I. N.;
Kosovichev, A. G.; Wray, A. A.; Mansour, N. N.
Bibcode: 2011arXiv1102.1206R
Altcode:
In this study we investigate the effects of turbulent convection on
formation of large-scale inhomogeneous magnetic structures by means of
Large-Eddy Simulation (LES) for convection in solar-type stars. The
main idea of this study is the implementation of a new subgrid-scale
model for the effective Lorentz force in a three-dimensional nonlinear
radiative magnetohydrodynamics (MHD) code developed for simulating
the upper solar convection zone and lower atmosphere. To this end we
derived the energy budget equations, which include the effects of the
subgrid-scale turbulence on the Lorentz-force, and implemented the
new subgrid-scale turbulence model (TELF-Model) in a three-dimensional
nonlinear MHD LES code. Using imposed initial vertical and horizontal
uniform magnetic fields in LES with the TELF-Model, we have shown that
the magnetic flux tubes formation is started when the initial mean
magnetic field is larger than a threshold value (about 100 G). This is
in agreement with the theoretical studies by Rogachevskii and Kleeorin
(2007). We have determined the vertical profiles of the velocity and
magnetic fluctuations, total MHD energy and anisotropy of turbulent
magneto-convection, kinetic and current and cross helicities.
Title: Excitation of Acoustic Waves by Vortices in the Quiet Sun
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Mansour, N. N.; Wray,
A. A.
Bibcode: 2011ApJ...727L..50K
Altcode: 2010arXiv1011.3775K
The five-minute oscillations are one of the basic properties of solar
convection. Observations show a mixture of a large number of acoustic
wave fronts propagating from their sources. We investigate the process
of acoustic waves excitation from the point of view of individual
events, by using a realistic three-dimensional radiative hydrodynamic
simulation of the quiet Sun. The results show that the excitation events
are related to the dynamics of vortex tubes (or swirls) in intergranular
lanes of solar convection. These whirlpool-like flows are characterized
by very strong horizontal velocities (7-11 km s-1) and
downflows (≈7 km s-1), and are accompanied by strong
decreases of temperature, density, and pressure at the surface and
0.5-1 Mm below the surface. High-speed whirlpool flows can attract
and capture other vortices. According to our simulation results the
processes of vortex interaction, such as vortex annihilation, can
cause excitation of acoustic waves on the Sun.
Title: The Subsurface-shear-shaped Solar αΩ Dynamo
Authors: Pipin, V. V.; Kosovichev, A. G.
Bibcode: 2011ApJ...727L..45P
Altcode: 2010arXiv1011.4276P
We propose a solar dynamo model distributed in the bulk of the
convection zone with toroidal magnetic-field flux concentrated in a
near-surface layer. We show that if the boundary conditions at the top
of the dynamo region allow the large-scale toroidal magnetic fields to
penetrate close to the surface, then the modeled butterfly diagram for
the toroidal magnetic field in the upper convection zone is formed by
the subsurface rotational shear layer. The model is in agreement with
observed properties of the magnetic solar cycle.
Title: First Sunquake of Solar Cycle 24 Observed by Solar Dynamics
Observatory
Authors: Kosovichev, A. G.
Bibcode: 2011arXiv1102.3954K
Altcode:
The X2.2-class solar flare of February 15, 2011, produced a powerful
`sunquake' event, representing a seismic response to the flare
impact. The impulsively excited seismic waves formed a compact
wavepacket traveling through the solar interior and appeared on the
surface as expanding wave ripples. The Helioseismic and Magnetic Imager
(HMI), instrument on SDO, observes variations of intensity, magnetic
field and plasma velocity (Dopplergrams) on the surface of Sun almost
uninterruptedly with high resolution (0.5 arcsec/pixel) and high cadence
(45 sec). The flare impact on the solar surface was observed in the
form of compact and rapid variations of the HMI observables (Doppler
velocity, line-of-sight magnetic field and continuum intensity). These
variations, caused by the impact of high-energy particles in the
photosphere, formed a typical two-ribbon flare structure. The sunquake
can be easily seen in the raw Dopplergram differences without any
special data processing. The source of this quake was located near the
outer boundary of a very complicated complicated sunspot region, NOAA
1158, in a sunspot penumbra and at the penumbra boundary. This caused
an interesting plasma dynamics in the impact region. I present some
preliminary results of analysis of the near-real-time data from HMI,
and discuss properties of the sunquake and the flare impact sources.
Title: HMI time-distance pipeline: An overview and data products
Authors: Zhao, J.; Couvidat, S.; Bogart, R. S.; Duvall, T. L., Jr.;
Kosovichev, A. G.; Beck, J. G.; Birch, A. C.
Bibcode: 2011JPhCS.271a2063Z
Altcode:
The Helioseismic and Magnetic Imager onboard Solar Dynamics
Observatory provides uninterrupted high-resolution observations of solar
oscillations over the entire disk. This gives a unique opportunity for
mapping subsurface flows and wave-speed structures and investigating
their role in the Sun's dynamics and magnetic activity on various
scales by methods of local helioseismology. A data analysis pipeline
for the time-distance helioseismology analysis has been developed
and implemented at the SDO Joint Science Operation Center (JSOC) at
Stanford. It provides near-real time processing of the helioseismology
data. We provide an overview of this pipeline, including the data flow
procedures, measurement and inversion codes, and our data products.
Title: Modeling and Prediction of Solar Cycles Using Data Assimilation
Methods
Authors: Kitiashvili, Irina N.; Kosovichev, Alexander G.
Bibcode: 2011LNP...832..121K
Altcode:
Variations of solar activity are a result of a complicate dynamo process
in the convection zone. We consider this phenomenon in the context
of sunspot number variations, which have detailed observational data
during the past 23 solar cycles. However, despite the known general
properties of the solar cycles a reliable forecast of the 11-year
sunspot number is still a problem. The main reasons are imperfect
dynamo models and deficiency of the necessary observational data. To
solve this problem we propose to use data assimilation methods. These
methods combine observational data and models for best possible,
efficient and accurate estimates of physical properties that cannot be
observed directly. The methods are capable of providing a forecast of
the system future state. It is demonstrated that the Ensemble Kalman
Filter (EnKF) method can be used to assimilate the sunspot number data
into a non-linear α{-}Upomega mean-field dynamo model, which takes
into account dynamics of turbulent magnetic helicity. We apply this
method for characterization of the solar dynamo properties and for
prediction of the sunspot number.
Title: Advances in Global and Local Helioseismology: An Introductory
Review
Authors: Kosovichev, Alexander G.
Bibcode: 2011LNP...832....3K
Altcode: 2011arXiv1103.1707K
Helioseismology studies the structure and dynamics of the Sun's interior
by observing oscillations on the surface. These studies provide
information about the physical processes that control the evolution
and magnetic activity of the Sun. In recent years, helioseismology has
made substantial progress towards the understanding of the physics of
solar oscillations and the physical processes inside the Sun, thanks to
observational, theoretical and modeling efforts. In addition to global
seismology of the Sun based on measurements of global oscillation modes,
a new field of local helioseismology, which studies oscillation travel
times and local frequency shifts, has been developed. It is capable
of providing 3D images of subsurface structures and flows. The basic
principles, recent advances and perspectives of global and local
helioseismology are reviewed in this article.
Title: Detection of Supergranulation Alignment in Polar Regions of
the Sun by Helioseismology
Authors: Nagashima, Kaori; Zhao, Junwei; Kosovichev, Alexander G.;
Sekii, Takashi
Bibcode: 2011ApJ...726L..17N
Altcode: 2010arXiv1011.1025N
We report on a new phenomenon of "alignment" of supergranulation cells
in the polar regions of the Sun. Recent high-resolution data sets
obtained by the Solar Optical Telescope on board the Hinode satellite
enabled us to investigate supergranular structures in high-latitude
regions of the Sun. We have carried out a local helioseismology
time-distance analysis of the data and detected acoustic travel-time
variations due to the supergranular flows. The supergranulation cells
in both the north and south polar regions show systematic alignment
patterns in the north-south direction. The south-pole data sets obtained
in a month-long Hinode campaign indicate that the supergranulation
alignment property may be quite common in the polar regions. We also
discuss the latitudinal dependence of the supergranulation cell sizes;
the data show that the east-west cell size decreases toward higher
latitudes.
Title: Local helioseismology of sunspot regions: Comparison of
ring-diagram and time-distance results
Authors: Kosovichev, A. G.; Basu, S.; Bogart, R.; Duvall, T. L., Jr.;
Gonzalez-Hernandez, I.; Haber, D.; Hartlep, T.; Howe, R.; Komm, R.;
Kholikov, S.; Parchevsky, K. V.; Tripathy, S.; Zhao, J.
Bibcode: 2011JPhCS.271a2005K
Altcode: 2010arXiv1011.0799K
Local helioseismology provides unique information about the subsurface
structure and dynamics of sunspots and active regions. However,
because of complexity of sunspot regions local helioseismology
diagnostics require careful analysis of systematic uncertainties
and physical interpretation of the inversion results. We present new
results of comparison of the ring-diagram analysis and time-distance
helioseismology for active region NOAA 9787, for which a previous
comparison showed significant differences in the subsurface sound-speed
structure, and discuss systematic uncertainties of the measurements
and inversions. Our results show that both the ring-diagram and
time-distance techniques give qualitatively similar results, revealing
a characteristic two-layer seismic sound-speed structure consistent
with the results for other active regions. However, a quantitative
comparison of the inversion results is not straightforward. It must
take into account differences in the sensitivity, spatial resolution
and the averaging kernels. In particular, because of the acoustic
power suppression, the contribution of the sunspot seismic structure
to the ring-diagram signal can be substantially reduced. We show that
taking into account this effect reduces the difference in the depth
of transition between the negative and positive sound-speed variations
inferred by these methods. Further detailed analysis of the sensitivity,
resolution and averaging properties of the local helioseismology methods
is necessary for consolidation of the inversion results. It seems to
be important that both methods indicate that the seismic structure of
sunspots is rather deep and extends to at least 20 Mm below the surface,
putting constraints on theoretical models of sunspots.
Title: Subsurface structure of the Evershed flows in sunspots
Authors: Kitiashvili, Irina N.; Kosovichev, Alexander G.; Mansour,
Nagi N.; Wray, Alan A.
Bibcode: 2011JPhCS.271a2076K
Altcode:
The radial outflows in sunspot penumbrae, known as the Evershed
effect, are of significant interest for understanding the dynamics
of sunspots. Local helioseismology has not been able to determine
the depth of these flows nor their relationship to mass circulation
in sunspots. Recent radiative MHD simulations have provided a
convincing explanation of the Evershed flow as a natural consequence
of magnetoconvection in the strongly inclined magnetic field region of
the penumbra. The simulations reproduce many observational features of
penumbra dynamics, including the filamentary structure, the high-speed
non-stationary "Evershed clouds", and the "sea-serpent" behavior
of magnetic field lines. We present the subsurface structure of the
Evershed effect, obtained from numerical simulations, and determine the
depth of the radial outflows for various magnetic field strengths and
inclinations. The simulations predict that Evershed flows are rather
shallow and concentrated in the top 0.5 - 1 Mm layer of the convection
zone. This prediction can be tested by local helioseismology methods.
Title: Helioseismology Study of Subsurface Dynamics in the Polar
Regions of the Sun
Authors: Nagashima, K.; Zhao, J.; Kosovichev, A. G.; Sekii, T.
Bibcode: 2010AGUFM.S32A..03N
Altcode:
We report on our time-distance helioseismology study of the
subsurface dynamics in the polar regions of the Sun. It is generally
difficult to observe the polar regions in details because of severe
foreshortening. The high-resolution data obtained by Solar Optical
Telescope (SOT) onboard the Hinode satellite, however, enabled
us to investigate dynamics in the regions with up to 80 degrees
in latitude. The measurements are obtained by calculating the
cross-covariance function of the random solar oscillations observed
as fluctuations of intensity of the Ca II H line, and by fitting a
Gabor-wavelet function for estimating the phase and group travel times
of solar acoustic waves. We obtain the maps of subsurface velocity field
by inverting the differences of the phase travel times, calculated for
the cross-correlations with positive and negative lag times, using a
ray-path approximation. Among the subsurface dynamical processes in the
polar region, we focus on supergranulation in this study. Supergranules
are thought to be one of convective cells in the convective envelope
of the Sun. Typical temporal and spatial scales of supergranular cells
are 1 day and 30 Mm, respectively. The supergranulation is considered
to play important roles in the magnetic flux transport and formation
of the magnetic network. However, we still do not have sufficient
knowledge of their origin and properties. In our study, we have observed
a curious alignment of the supergranular cells in the polar regions
approximately in the North-South direction. The alignment was seen
in both northern and southern polar regions. We discuss properties
and temporal evolution of the supergranular structures in the region,
as well as the possibilities of measuring the differential rotation
and meridional flows in the polar regions, which are critical for the
solar dynamo theories.
Title: Initial Analysis of the Solar Dynamics Observatory Radiation
Environment
Authors: Vafai, A. D.; Close, S.; Kosovichev, A. G.; Stern, R. A.
Bibcode: 2010AGUFMSH23C1875V
Altcode:
The purpose of this poster is to explain the method used to extract data
from images sent by the Solar Dynamics Observatory (SDO) to investigate
patterns of cosmic rays and electron fluxes in space. For this study we
analyze cosmic ray and X-ray hits in the corners of the AIA images. The
X-rays are produced by high-energy electrons in the outer radiation
belts interacting with the shielding/instrument/spacecraft. In the past
twenty years, several spacecraft in geosynchronous and geostationary
orbits have suffered mechanical and systems damage that are believed
to be caused by energetic protons and electrons. Using Dark Image data
from the GOES-13 spacecraft, the number of X-ray hits per day show a
variation correlated with the GOES-12 high energy electron fluxes. We
demonstrate how the SDO/AIA images are used to study this behavior. We
also provide an overview on methods used to distinguish cosmic rays
hits from X-ray detections within an image.
Title: Validating Helioseismic Imaging Techniques using 3D Global-Sun
Simulations of Helioseismic Wave Propagation
Authors: Hartlep, T.; Zhao, J.; Kosovichev, A. G.; Mansour, N. N.
Bibcode: 2010AGUFM.S32A..05H
Altcode:
Significant advancements in our understanding of the interior
structure and dynamics of the Sun have been made by studying solar
oscillation. Waves, randomly excited by the vigorous convective flows
near the solar surface, propagate through the interior where they
are affected by inhomogeneities such as flows and local temperature
variations. Local helioseismology tries to infer the properties of
these inhomogeneities by analyzing the observed waves field near
the solar photosphere. In general, such inferences are based on
more or less simplified models of wave propagation in the Sun. For
correct interpretation of helioseismic measurements and for testing
and validating inversion techniques, numerical simulation of wave
propagation play a crucial role. We present result from testing
various local helioseismology techniques using numerical simulation
of wave propagation in global-Sun models. Our simulation code solves
the time-dependent linearized wave propagation problem in a full
spherical model and takes into account mass flows as well as sound
speed variations. We present results from testing time-distance
helioseismology far-side imaging and imaging of the solar tachocline,
results from with active regions and subsurface regions, differential
rotation, and meridional flows. Datasets are available for download
at the included URL.
Title: Interaction of MHD Waves with Sunspots
Authors: Parchevsky, K.; Zhao, J.; Kosovichev, A. G.; Rempel, M.
Bibcode: 2010AGUFM.S32A..07P
Altcode:
Understanding of MHD wave propagation, transformation and scattering
by sunspots and their interaction with the non-uniform background
magnetic field and flows is very important for improving helioseismic
inversion procedures. Such simulations also provide artificial data
for testitng and calibration techniques used for analysis of data from
space missions SOHO/MDI, SDO/HMI, and HINODE. We developed 3D linear MHD
code for numerical simulation of excitation and propagation of MHD waves
in non-uniform medium in presence of the background magnetic field and
flows. We present simulations of MHD wave propagation in magnetostatic
and dynamic models of sunspots. We consider separately two cases when
the waves are excited by point sources, located at different distances
from the spot, and by stochastic noise source. The results are compared
with the waveforms of the cross-correlation function extracted from
the observational data. We discuss the differences between the models
and observations in terms of the amplitude variations and travel-time
shifts. Comparison of the simulations with helioseismic observations
allows us to test the sunspot and helioseismic models, and suggest
improvements. The numerically simulated helioseismic data are publicly
accessible for the helioseismic community for testing and verification
of various ambient noise imaging techniques of helioseismology
(time-distance, holography, and ring diagrams).
Title: Initial Results from SDO/HMI Time-Distance Helioseismology
Data Analysis Pipeline
Authors: Zhao, J.; Bogart, R. S.; Couvidat, S. P.; Duvall, T. L.;
Birch, A. C.; Parchevsky, K.; Kosovichev, A. G.; Beck, J. G.
Bibcode: 2010AGUFMSH14A..08Z
Altcode:
The Helioseismic and Magnetic Imager on Solar Dynamics Observatory
provides uninterrupted high-resolution observations of solar
oscillations over the entire disk. Time-distance helioseismology
data analysis pipeline was developed to perform a near real-time
analysis of these observations, and provide full-disk subsurface flow
fields and wave-speed perturbation maps every 8 hours. These routine
productions give us substantial information of the solar interior,
and are very useful to study the solar interior dynamics, connections
between subsurface dynamics and photospheric activities, and solar
large-scale and global-scale flows and structures. We present our
initial results in these respects.
Title: Investigation of Formation and Subsurface Dynamics of Active
Regions by Local Helioseismology from SDO
Authors: Kosovichev, A. G.; Duvall, T. L.; Zhao, J.
Bibcode: 2010AGUFMSH11A1604K
Altcode:
Despite a long history of observations of sunspots and active
regions the mechanisms of their formation and stability are still a
puzzle. These mechanisms are controlled by a complex interaction of
magnetic fields and turbulent convection below the solar surface. The
helioseismology observations on SOHO/MDI have provided snapshots of the
subsurface dynamics during formation and evolution of several active
regions and sunspots. They showed substantial changes in the structure
and flow patterns at various stages of the evolution and flaring
activity. However, the MDI data were too fragmented for systematic
studies. The Helioseismic and Magnetic Imager (HMI) on SDO gives us a
unique opportunity for detailed high-resolution investigations of the
subsurface structures and mass flows associated with the formation and
life cycle of active regions. We present the results of the initial
analysis of magnetic active regions by time-distance helioseismology
of the HMI Doppler-shift data. These include some interesting events,
previously not investigated by helioseismology, such as splitting of a
sunspot into two separate spot and formation of penumbra. We discuss
also the capabilities of SDO for time-distance helioseismology,
current uncertainties, and potentials based on the initial experience.
Title: Realistic MHD Simulations of Formation of Sunspot-like
Structures and Comparison with Observations
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Mansour, N. N.; Wray,
A. A.
Bibcode: 2010AGUFMSH31A1782K
Altcode:
The process of formation of magnetic structures such as sunspot and
pores in the turbulent convection zone is still enigma. However,
the recent progress in numerical radiative MHD simulations provides
clues about the possible mechanism of magnetic field accumulation in
spontaneously formed stable structures. Implementation of sub-grid
turbulent models in our "SolarBox" code, gives us the possibility
to model more accurately turbulent properties, and reproduce the
dynamics of the magnetized plasma. The code takes into account
non-ideal (tabular) EOS, effects of ionization, chemical composition,
radiation, turbulence and magnetic field. Our simulation results show
an important role of vortices, which create local cavity of pressure
and are associated with strong converging flows under the surface,
during the initial stage of the spontaneous structure formation. The
resulting structure represents a compact self-organized concentration
of strong magnetic field, reaching ~6 kG in the interior, and ~1.5 kG
on the surface. It has a cluster-like internal structurization, and
is maintained by strong downdrafts extending into the deep layers. We
discuss the role of turbulent MHD dynamics in this mechanism, and
compare the simulation results with observations of the sunspot
formation process during a magnetic flux emergence, from the Solar
Dynamics Observatory and Hinode.
Title: Cross-helicity turbulence model: Application to MHD phenomena
from solar convection zone to heliosphere
Authors: Yokoi, N.; Kitiashvili, I. N.; Kosovichev, A. G.
Bibcode: 2010AGUFMSH31A1793Y
Altcode:
Cross helicity (velocity-magnetic field correlation) is expected
to play a key role in several geo/astrophysical processes including
dynamo action, suppression of turbulent transport, etc. We discuss the
relevance of the cross-helicity effects with the aid of the turbulence
model. A turbulence model with the cross-helicity effects incorporated
may be called the “cross-helicity turbulence model”. This model is
applied to several MHD phenomena ranging from the formations of magnetic
fields and plasma motions in the solar convection zone to the solar-wind
evolution in the heliosphere. Generation of turbulence quantities
depends on the inhomogeneity of large-scale fields, and turbulence
in turn determines the configuration of the mean fields through the
turbulent transport. Such nonlinear interactions between the mean- and
fluctuation-fields are explored with the aid of numerical simulations
with cross-helicity turbulence model. Through the comparisons to the
observation, validity of the turbulence model is examined. Examinations
include (i) A large-eddy simulation of the sunspot flow reveals how and
how much cross helicity is generated there; (ii) A eddy-viscosity-type
turbulence model shows how the turbulence quantities evolves under
the influence of the large-scale velocity and magnetic-field shears.
Title: The future of helioseismology
Authors: Kosovichev, Alexander G.
Bibcode: 2010HiA....15..352K
Altcode:
Helioseismology has provided us with the unique knowledge of the
interior structure and dynamics of the Sun, and the variations with the
solar cycle. However, the basic mechanisms of solar magnetic activity,
formation of sunspots and active regions are still unknown. Determining
the physical properties of the solar dynamo, detecting emerging active
regions and observing the subsurface dynamics of sunspots are among
the most important and challenging problems. The current status and
perspectives of helioseismology are briefly discussed.
Title: Numerical simulation of propagation of the MHD waves in
sunspots
Authors: Parchevsky, K.; Kosovichev, A.; Khomenko, E.; Olshevsky,
V.; Collados, M.
Bibcode: 2010HiA....15..354P
Altcode:
We present results of numerical 3D simulation of propagation of MHD
waves in sunspots. We used two self consistent magnetohydrostatic
background models of sunspots. There are two main differences
between these models: (i) the topology of the magnetic field and
(ii) dependence of the horizontal profile of the sound speed on
depth. The model with convex shape of the magnetic field lines near
the photosphere has non-zero horizorntal perturbations of the sound
speed up to the depth of 7.5 Mm (deep model). In the model with concave
shape of the magnetic field lines near the photosphere Δ c/c is close
to zero everywhere below 2 Mm (shallow model). Strong Alfven wave is
generated at the wave source location in the deep model. This wave is
almost unnoticeable in the shallow model. Using filtering technique
we separated magnetoacoustic and magnetogravity waves. It is shown,
that inside the sunspot magnetoacoustic and magnetogravity waves
are not spatially separated unlike the case of the horizontally
uniform background model. The sunspot causes anisotropy of the
amplitude distribution along the wavefront and changes the shape
of the wavefront. The amplitude of the waves is reduced inside the
sunspot. This effect is stronger for the magnetogravity waves than for
magnetoacoustic waves. The shape of the wavefront of the magnetogravity
waves is distorted stronger as well. The deep model causes bigger
anisotropy for both mgnetoacoustic and magneto gravity waves than the
shallow model.
Title: A Precise Asteroseismic Age and Radius for the Evolved Sun-like
Star KIC 11026764
Authors: Metcalfe, T. S.; Monteiro, M. J. P. F. G.; Thompson, M. J.;
Molenda-Żakowicz, J.; Appourchaux, T.; Chaplin, W. J.; Doǧan, G.;
Eggenberger, P.; Bedding, T. R.; Bruntt, H.; Creevey, O. L.; Quirion,
P. -O.; Stello, D.; Bonanno, A.; Silva Aguirre, V.; Basu, S.; Esch,
L.; Gai, N.; Di Mauro, M. P.; Kosovichev, A. G.; Kitiashvili, I. N.;
Suárez, J. C.; Moya, A.; Piau, L.; García, R. A.; Marques, J. P.;
Frasca, A.; Biazzo, K.; Sousa, S. G.; Dreizler, S.; Bazot, M.; Karoff,
C.; Frandsen, S.; Wilson, P. A.; Brown, T. M.; Christensen-Dalsgaard,
J.; Gilliland, R. L.; Kjeldsen, H.; Campante, T. L.; Fletcher, S. T.;
Handberg, R.; Régulo, C.; Salabert, D.; Schou, J.; Verner, G. A.;
Ballot, J.; Broomhall, A. -M.; Elsworth, Y.; Hekker, S.; Huber, D.;
Mathur, S.; New, R.; Roxburgh, I. W.; Sato, K. H.; White, T. R.;
Borucki, W. J.; Koch, D. G.; Jenkins, J. M.
Bibcode: 2010ApJ...723.1583M
Altcode: 2010arXiv1010.4329M
The primary science goal of the Kepler Mission is to provide
a census of exoplanets in the solar neighborhood, including the
identification and characterization of habitable Earth-like planets. The
asteroseismic capabilities of the mission are being used to determine
precise radii and ages for the target stars from their solar-like
oscillations. Chaplin et al. published observations of three bright
G-type stars, which were monitored during the first 33.5 days of science
operations. One of these stars, the subgiant KIC 11026764, exhibits a
characteristic pattern of oscillation frequencies suggesting that it
has evolved significantly. We have derived asteroseismic estimates of
the properties of KIC 11026764 from Kepler photometry combined with
ground-based spectroscopic data. We present the results of detailed
modeling for this star, employing a variety of independent codes and
analyses that attempt to match the asteroseismic and spectroscopic
constraints simultaneously. We determine both the radius and the age
of KIC 11026764 with a precision near 1%, and an accuracy near 2%
for the radius and 15% for the age. Continued observations of this
star promise to reveal additional oscillation frequencies that will
further improve the determination of its fundamental properties.
Title: Realistic MHD numerical simulations of solar convection and
oscillations in inclined magnetic field regions
Authors: Kitiashvili, Irina N.; Kosovichev, Alexander G.; Wray,
Alan A.; Mansour, Nagi N.
Bibcode: 2010HiA....15..348K
Altcode:
It is known that physical properties of solar turbulent convection and
oscillations strongly depend on magnetic field. In particular, recent
observations from SOHO/MDI revealed significant changes of the wave
properties in inclined magnetic field regions of sunspots, which affect
helioseismic inferences. We use realistic 3D radiative MHD numerical
simulations to investigate solar convection and oscillations and their
relationship in the presence of inclined magnetic field. In the case of
highly inclined and strong 1-1.5 kG field the solar convection develops
filamentary structure and high-speed flows (Fig. 1a), which provide an
explanation to the Evershed effect in sunspot penumbra (Kitiashvili,
et al. 2009).
Title: Modeling of the subgrid-scale term of the filtered magnetic
field transport equation
Authors: Balarac, G.; Kosovichev, A. G.; Brugière, O.; Wray, A. A.;
Mansour, N. N.
Bibcode: 2010arXiv1010.5759B
Altcode:
Accurate subgrid-scale turbulence models are needed to perform realistic
numerical magnetohydrodynamic (MHD) simulations of the subsurface flows
of the Sun. To perform large-eddy simulations (LES) of turbulent MHD
flows, three unknown terms have to be modeled. As a first step, this
work proposes to use a priori tests to measure the accuracy of various
models proposed to predict the SGS term appearing in the transport
equation of the filtered magnetic field. It is proposed to evaluate the
SGS model accuracy in term of "structural" and "functional" performance,
i.e. the model capacity to locally approximate the unknown term and
to reproduce its energetic action, respectively. From our tests, it
appears that a mixed model based on the scale-similarity model has
better performance.
Title: Mechanism of Spontaneous Formation of Stable Magnetic
Structures on the Sun
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Wray, A. A.; Mansour,
N. N.
Bibcode: 2010ApJ...719..307K
Altcode: 2010arXiv1004.2288K
One of the puzzling features of solar magnetism is formation of
long-living compact magnetic structures, such as sunspots and
pores, in the highly turbulent upper layer of the solar convective
zone. We use realistic radiative three-dimensional MHD simulations
to investigate the interaction between magnetic field and turbulent
convection. In the simulations, a weak vertical uniform magnetic
field is imposed in a region of fully developed granular convection,
and the total magnetic flux through the top and bottom boundaries is
kept constant. The simulation results reveal a process of spontaneous
formation of stable magnetic structures, which may be a key to
understanding the magnetic self-organization on the Sun and formation
of pores and sunspots. This process consists of two basic steps: (1)
formation of small-scale filamentary magnetic structures associated with
concentrations of vorticity and whirlpool-type motions, and (2) merging
of these structures due to the vortex attraction, caused by converging
downdrafts around magnetic concentration below the surface. In the
resulting large-scale structure maintained by the converging plasma
motions, the magnetic field strength reaches ~1.5 kG at the surface
and ~6 kG in the interior, and the surface structure resembles solar
pores. The magnetic structure remains stable for the whole simulation
run of several hours with no sign of decay.
Title: Explanation of the Sea-serpent Magnetic Structure of Sunspot
Penumbrae
Authors: Kitiashvili, I. N.; Bellot Rubio, L. R.; Kosovichev, A. G.;
Mansour, N. N.; Sainz Dalda, A.; Wray, A. A.
Bibcode: 2010ApJ...716L.181K
Altcode: 2010arXiv1003.0049K
Recent spectro-polarimetric observations of a sunspot showed the
formation of bipolar magnetic patches in the mid-penumbra and their
propagation toward the outer penumbral boundary. The observations
were interpreted as being caused by sea-serpent magnetic fields near
the solar surface. In this Letter, we develop a three-dimensional
radiative MHD numerical model to explain the sea-serpent structure
and the wave-like behavior of the penumbral magnetic field lines. The
simulations reproduce the observed behavior, suggesting that the
sea-serpent phenomenon is a consequence of magnetoconvection in
a strongly inclined magnetic field. It involves several physical
processes: filamentary structurization, high-speed overturning
convective motions in strong, almost horizontal magnetic fields with
partially frozen field lines, and traveling convective waves. The
results demonstrate a correlation of the bipolar magnetic patches with
high-speed Evershed downflows in the penumbra. This is the first time
that a three-dimensional numerical model of the penumbra results in
downward-directed magnetic fields, an essential ingredient of sunspot
penumbrae that has eluded explanation until now.
Title: Diverging and Converging Flows around Sunspot Structures in
Rotating and Non-Rotating Axisymmetric MHD Simulations
Authors: Hartlep, T.; Busse, F. H.; Hulburt, N. E.; Kosovichev, A. G.
Bibcode: 2010arXiv1006.4156H
Altcode:
We present results on modeling solar pores and sunspots using 2D
axisymmetric magneto-hydrodynamic (MHD) simulations. These models are
helpful for understanding the mechanisms of magnetic field concentration
in sunspots, and the large-scale flow patterns associated with
them. The simulations provide consistent, self-maintained, although
not fully realistic, models of concentrated magnetic field near the
solar surface. In this paper, we explore under which conditions the
associated flows are converging or diverging near the surface. We
find that in most cases in which a stable, pore-like concentration of
magnetic field forms, a configuration with converging over diverging
flow is established.
Title: Sea-Serpent Magnetic Structure of Sunspot Penumbrae:
Observations and MHD Simulations
Authors: Kitiashvili, Irina; Bellot Rubio, L. R.; Kosovichev, A. G.;
Mansour, N. N.; Sainz Dalda, A.; Wray, A. A.
Bibcode: 2010AAS...21631706K
Altcode: 2010BAAS...41..899K
Recent high-resolution spectro-polarimetric observations of a sunspot
detected formation of bipolar magnetic patches in the mid penumbra and
propagation of these patches toward the outer penumbral boundary. The
observations have been interpreted as an evidence of sea-serpent field
lines near the solar surface. Using a radiative 3D MHD code, we model
the behavior of solar magnetoconvection in strongly inclined magnetic
field of penumbra. The numerical simulation results reproduce the
moving bipolar magnetic elements observed in high-resolution SOHO/MDI
and Hinode/SOT data and also their physical properties, supporting the
sea-serpent model. The simulations explain the sea-serpent structure
and dynamics of the penumbral field as a consequence of turbulent
magnetoconvection in a highly inclined, strong magnetic field,
which forms filamentary structures and has properties of traveling
convective wave. The model also shows that the appearance of the
sea-serpent magnetic field lines is closely related to high-speed
patches ("Evershed clouds") of the penumbra radial outflow.
Title: Observations of Emerging Active Regions and Sunspot Formation
from SDO/HMI
Authors: Kitiashvili, Irina; Kosovichev, A. G.; Mansour, N. N.; Wray,
A. A.
Bibcode: 2010AAS...21640233K
Altcode:
Continuous high-resolution data of magnetic fields, Doppler velocity
and intensity from the Helioseismic and Magnetic Imager (HMI) on
SDO provide an excellent opportunity to investigate the process of
formation of sunspots and active regions and compare with theoretical
models. We analyze the HMI observations of an emerging active
region and formation of sunspots. The results show that the sunspot
formation involves accumulation of small-scale magnetic elements
into a large-scale magnetic structure and substantial changes of the
properties of convection in the region of flux emergence. We discuss
the HMI capabilities for studying these processes, and compare the
observations with results of numerical MHD simulations.
Title: Initial Time-Distance Helioseismology Results from SDO/HMI. I.
Authors: Zhao, Junwei; Couvidat, S.; Bogart, R.; Parchevsky, K. V.;
Duvall, T. L., Jr.; Kosovichev, A. G.; Beck, J. G.; Birch, A. C.
Bibcode: 2010AAS...21640234Z
Altcode:
The Helioseismic and Magnetic Imager on Solar Dynamics Observatory
provides uninterrupted high-resolution observations of solar
oscillations over the entire disk. This gives a unique opportunity for
mapping subsurface flows and wave-speed structures and investigating
their role in the Sun's dynamics and magnetic activity on various
scales by methods of local helioseismology. A data analysis pipeline
for the time-distance helioseismology analysis has been developed
and implemented at the SDO Joint Science Operation Center (JSOC) at
Stanford. It provides near-real time processing of the helioseismology
data. We present the basic characteristics and capabilities of the
pipeline, initial time-distance measurement results, and compare these
with the simultaneous SOHO/MDI measurements.
Title: Initial Time-distance Helioseismology Results from SDO/HMI. II.
Authors: Zhao, Junwei; Couvidat, S.; Bogart, R.; Parchevsky, K. V.;
Duvall, T. L., Jr.; Kosovichev, A. G.; Beck, J. G.; Birch, A. C.
Bibcode: 2010AAS...21640235Z
Altcode:
Investigations of the interaction between large-scale subsurface
flows and magnetic fields are very important for understanding and
predicting the processes of solar dynamo and evolution of active
regions. The time-distance helioseismology data analysis pipeline
(presented in our poster I) is designed to provide global-Sun maps
of subsurface flows from the SDO/HMI observations every 8 hours. We
present an initial subsurface flow map, and compare this with the
magnetic field distribution also obtained from the HMI instrument.
Title: Supergranulation in the Polar Regions Observed by Hinode/SOT
Authors: Nagashima, Kaori; Zhao, J.; Kosovichev, A.; Sekii, T.
Bibcode: 2010AAS...21640002N
Altcode: 2010BAAS...41..855N
We report on our investigation of the supergranular structure in the
polar regions of the Sun by local helioseismology. Supergranules
represent large-scale convective cells: the horizontal spatial
scale is about 30 Mm and the lifetime is about 1 day. They play
important role in the magnetic flux transport and formation of the
magnetic network. Recent helioseismological studies have found
that the depth of the cells is much smaller than the horizontal
scale, and that the supergranulation pattern exhibits a wave-like
behavior. However, we still do not have sufficient knowledge of
the origin and properties of the supergranulation. In this work, we
have carried out a new time-distance helioseismology analysis using
high-resolution datasets of the polar regions of the Sun obtained
by the Solar Optical Telescope (SOT) onboard the Hinode satellite
during the periods of the high inclination of the solar axis to the
ecliptic. Because of the foreshortening such measurements are not
currently possible with any other helioseismology instrument. We have
measured the travel-time shifts of acoustic waves traveling to various
depth below the photosphere, and obtain the subphotospheric horizontal
flow maps by inversion. We have detected the supergranular cells in
the polar regions and studied their properties. For comparison we did a
similar analysis for low-latitude regions at the East limb of the Sun,
and at the disk center. Comparing with the cells in the lower-latitude
regions, we have observed a curious alignment of the cells in the polar
regions, approximately in the North-South direction. This `alignment'
has been found in both the North and the South polar regions. In
the presentation, we discuss the alignment phenomenon as well as the
physical properties of the supergranular cells in the polar regions.
Title: Division II: Sun and Heliosphere
Authors: Melrose, Donald B.; Martinez Pillet, Valentin; Webb, David
F.; Bougeret, Jean-Louis; Klimchuk, James A.; Kosovichev, Alexander;
van Driel-Gesztelyi, Lidia; von Steiger, Rudolf
Bibcode: 2010IAUTB..27..146M
Altcode:
This report is on activities of the Division at the General Assembly
in Rio de Janeiro. Summaries of scientific activities over the past
triennium have been published in Transactions A, see Melrose et
al. (2008), Klimchuk et al. (2008), Martinez Pillet et al. (2008) and
Bougeret et al. (2008). The business meeting of the three Commissions
were incorporated into the business meeting of the Division. This
report is based in part on minutes of the business meeting, provided
by the Secretary of the Division, Lidia van Driel-Gesztelyi, and it
also includes reports provided by the Presidents of the Commissions
(C10, C12, C49) and of the Working Groups (WGs) in the Division.
Title: Diverging And Converging Flows Around Sunspot Structures In
Axisymmetric Mhd Simulations
Authors: Hartlep, Thomas; Busse, F. H.; Kosovichev, A. G.; Hurlburt,
N. E.
Bibcode: 2010AAS...21640005H
Altcode: 2010BAAS...41..855H
We present results on modeling solar pores and sunspots using 2D
axisymmetric magneto-hydrodynamic (MHD) simulations. These models
are helpful for understanding the mechanisms of magnetic field
concentration in sunspots, and the large-scale flow patterns associated
with them. The simulations provide consistent, self-maintained, even
if not fully realistic, models of concentrated magnetic field near
the solar surface. We explore under which conditions the associated
flows are converging or diverging in nature near the surface.
Title: Radiative Hydrodynamic Simulations of Turbulent Convection
and Oscillations from Solar-Type to A-Type Stars
Authors: Kitiashvili, Irina; Kosovichev, A. G.; Saio, H.; Shibahashi,
H.; Wray, A. A.; Mansour, N. N.
Bibcode: 2010AAS...21640012K
Altcode: 2010BAAS...41..856K
We use 3D numerical radiative hydrodynamic simulations to study
convective and oscillation properties of main sequence stars from
the solar-type stars to more massive stars. The solar-type pulsators
are characterized by acoustic oscillation modes excited by turbulent
granular convection in the upper convective boundary layer. As the
stellar mass increases the convection zone shrinks, the scale and
intensity of the turbulent motions increases, providing more energy for
excitation of acoustic modes. When the stellar mass reaches about 1.6
solar masses the upper convection zone consists of two very thin layers
corresponding to H and He ionization, and in addition to the acoustic
modes the stars show strong internal gravity modes The thin convection
zone is often considered insignificant for the stellar dynamics and
variability. However, the 3D radiative hydrodynamics simulations reveal
supersonic granular-type convection of the scale significantly larger
than the solar granulation, and strong overshooting plumes penetrating
into the stable radiative zone. These plumes may contribute to the
excitation of oscillation in A-type stars.
Title: Time-Distance Helioseismology Analysis of a Numerically
Simulated Sunspot
Authors: Ilonidis, Efstathios; Zhao, J.; Kosovichev, A.
Bibcode: 2010AAS...21640007I
Altcode: 2010BAAS...41..855I
In this work a numerically simulated sunspot embedded in a realistic
model of solar magnetoconvection is analyzed using time-distance
technique of local helioseismology. The purpose of this study
is to improve our understanding of travel-time shifts measured in
sunspots. We use the oscillation data obtained from the simulations for
measurements of p-modes travel-time shifts by applying the time-distance
helioseismology technique. The measurements are carried out for several
distances permitted by the depth of the simulation box, with and
without use of phase-speed filters. The phase-speed filtering procedure
improves the signal-to-noise ratio of the travel-time measurements for
short distances, but it may also introduce a bias in the travel-time
shifts. We discuss the interior properties of the sunspot inferred
from our measurements. The data for this work is kindly provided by
Matthias Rempel (HAO/NCAR).
Title: Investigation of Subsurface Connections in Complexes of
Activity by Local Helioseismology
Authors: Kosovichev, Alexander G.; Duvall, T. L., Jr.
Bibcode: 2010AAS...21631903K
Altcode: 2010BAAS...41..909K
Solar active regions often emerge close to each other forming complexes
of activity. The complexes may live for several solar rotations,
and represent a major component of the Sun's magnetic activity. It
had been suggested that the close appearance of active regions in
space and time might be related to common subsurface roots, "nests" of
activity. The EUV images show that the active regions are magnetically
connected in the corona, but subsurface connections have not been
established. We investigate the subsurface structure and dynamics of
two large complexes of activity, observed during the SOHO/MDI Dynamics
campaigns: AR 10484-10488 in October-November 2003, and AR 10987-10989
in March-April 2008 (this complex is a part of the Whole Heliospheric
Interval campaign). The former was organized across the equator in a
range of longitudes, while the later appeared in a narrow latitudinal
range, probably representing a subsurface toroidal flux tube. We use
the MDI full-disk Dopplergrams to measure perturbations of travel times
of acoustic waves traveling to various depths by applying the surface-
and deep-focusing time-distance helioseismology techniques, and obtain
the sound-speed and flow maps by inversion. We compare the evolution
of the large-scale subsurface sound-speed structures and dynamics of
these complexes, and discuss the evidence of subsurface connections.
Title: Tests of Helioseismic Holography Sound-Speed Inversions Using
Synthetic Data
Authors: Birch, Aaron; Braun, D.; Crouch, A.; Parchevsky, K.;
Kosovichev, A.
Bibcode: 2010AAS...21640006B
Altcode: 2010BAAS...41..855B
Helioseismic holography is an important method for measuring physical
conditions in the solar interior. Synthetic data is a powerful tool
for validating the methods of local helioseismology. Here we show
some example inversions of surface-focusing holography measurements of
synthetic data sets. We show that Born-approximation based inversions
of phase-speed and ridge-filtered measurements yield models that
are consistent with the true sound-speed structure in some simple
test cases. This work is supported by NASA contracts NNH09CE41C and
NNG07EI51C.
Title: Limits on Energy, Momentum and Excitation Mechanism of
Sunquakes
Authors: Kosovichev, Alexander G.
Bibcode: 2010AAS...21640004K
Altcode: 2010BAAS...41..855K
Investigations of the helioseismic response to solar flares
("sunquakes") are of significant interest for understanding the
mechanisms of the flare energy release and the dynamics of the solar
atmosphere and interior. The seismic response is observed in the form
of expanding ripples on the surface in Doppler-shift data, and in
images of an acoustic signal integrated over local ares. Recently,
it was suggested that solar flares can also excite global low-degree
acoustic modes of the Sun. I present results of a general theory of
the seismic response to the energy release in solar flares putting
strong limits on the energy and momentum, which can be deposited in the
solar oscillation modes. These limits show that it is unlikely that the
signals observed in the SOHO/VIRGO and GOLF data during solar flares
are caused by sunquakes. However, the energy and momentum released in
flaring stars may be sufficient for generating detectable signals.
Title: Overview of First-Result SDO Posters
Authors: Kosovichev, Alexander G.; SDO Team
Bibcode: 2010AAS...21630806K
Altcode:
Solar Dynamics Observatory launched on February 11, 2010, is designed to
study the internal dynamics, magnetism, coronal structures and spectral
irradiance of the Sun with high spatial and temporal resolutions
uninterruptedly during the rising phase of the activity cycle and the
solar maximum. This talk presents an overview of the late-breaking
poster presentations of the initial SDO results.
Title: Ray-Path and Born Kernels for SDO Time-Distance Helioseismology
Pipeline
Authors: Parchevsky, Konstantin; Birch, A.; Kosovichev, A.; Couvidat,
S.; Duvall, T., Jr.; Zhao, J.
Bibcode: 2010AAS...21640009P
Altcode: 2010BAAS...41..856P
We present a detailed procedure for calculating 3D sound speed and
flow kernels in the ray-path and Born approximations for inversion
procedures in the SDO/HMI Time-Distance Helioseismology pipeline. The
total integral from the kernel is a convenient parameter which can
be used for comparison of contribution of different factors (ray-path
and wave approximation, spherical and Cartesian geometry). Comparison
of the total integral from sound speed kernels in ray-path and Born
approximation with full disk MDI observations show good agreement. For
distances shorter than 25 Mm the total integrals from Born kernels
are systematically lower than integrals from ray-path kernels that
can be explained by contribution of wave effects. Taking into account
sphericity in ray-path kernels causes a reducing the total integral
of 2%.
Title: Numerical Simulation of Propagation and Transformation of
the MHD Waves in Sunspots
Authors: Parchevsky, Konstantin; Zhao, J.; Kosovichev, A.
Bibcode: 2010AAS...21621106P
Altcode:
Direct numerical simulation of propagation of MHD waves in stratified
medium in regions with non-uniform magnetic field is very important
for understanding of scattering and transformation of waves by
sunspots. We present numerical simulations of wave propagation
through the sunspot in 3D. We compare results propagation in two
different magnitostatic models of sunspots refferred to as "deep" and
"shallow" models. The "deep" model has convex shape of magnetic field
lines near the photosphere and non-zero horizorntal perturbations of
the sound speed up to the bottom of the model. The "shallow" model
has concave shape of the magnetic field lines near the photosphere
and horizontally uniform sound speed below 2 Mm. Waves reduce their
amplitude when they reach the center of the sunspot and estore the
amplitude when pass the center. For the "deep" model this effect is
bigger than for the "shallow" model. The wave amplitude depends on
the distance of the source from the sunspot center. For the "shallow"
model and source distance of 9 Mm from the sunspot center the wave
amplitude at some moment (when the wavefront passes the sunspot center)
becomes bigger inside the sunspot than outside. For the source distance
of 12 Mm the wave amplitude remains smaller inside the sunspot than
outside for all moments of time. Using filtering technique we
separated magnetoacoustic and magnetogravity waves. Simulations show
that the sunspot changes the shape of the wave front and amplitude
of the f-modes significantly stronger than the p-modes. It is shown,
that inside the sunspot magnetoacoustic and magnetogravity waves are
not spatially separated unlike the case of the horizontally uniform
background model. We compared simulation results with the wave signals
(Green's functions) extracted from the SOHO/MDI data for AR9787.
Title: Validating Time-Distance Subsurface Sound-Speed Structure
Inversions
Authors: Zhao, Junwei; Parchevsky, K. V.; Hartlep, T.; Kosovichev,
A. G.
Bibcode: 2010AAS...21631905Z
Altcode: 2010BAAS...41..910Z
Inversions for subsurface sound-speed perturbations in the Sun using
time-distance helioseismology are important for studying the interior
structures of sunspots and supergranules. However, it is essential
to assess how well these inversion techniques perform. By analyzing
numerical simulations of acoustic wave fields, which are obtained on
both local and global scales employing various subsurface sunspot
models, we invert for the subsurface sound-speed structures. These
inverted structures are then compared with the models prescribed in
the simulations. We find that our inversions generally agree well with
the models.
Title: The Asteroseismic Potential of Kepler: First Results for
Solar-Type Stars
Authors: Chaplin, W. J.; Appourchaux, T.; Elsworth, Y.; García,
R. A.; Houdek, G.; Karoff, C.; Metcalfe, T. S.; Molenda-Żakowicz,
J.; Monteiro, M. J. P. F. G.; Thompson, M. J.; Brown, T. M.;
Christensen-Dalsgaard, J.; Gilliland, R. L.; Kjeldsen, H.; Borucki,
W. J.; Koch, D.; Jenkins, J. M.; Ballot, J.; Basu, S.; Bazot, M.;
Bedding, T. R.; Benomar, O.; Bonanno, A.; Brandão, I. M.; Bruntt,
H.; Campante, T. L.; Creevey, O. L.; Di Mauro, M. P.; Doǧan,
G.; Dreizler, S.; Eggenberger, P.; Esch, L.; Fletcher, S. T.;
Frandsen, S.; Gai, N.; Gaulme, P.; Handberg, R.; Hekker, S.; Howe,
R.; Huber, D.; Korzennik, S. G.; Lebrun, J. C.; Leccia, S.; Martic,
M.; Mathur, S.; Mosser, B.; New, R.; Quirion, P. -O.; Régulo, C.;
Roxburgh, I. W.; Salabert, D.; Schou, J.; Sousa, S. G.; Stello, D.;
Verner, G. A.; Arentoft, T.; Barban, C.; Belkacem, K.; Benatti, S.;
Biazzo, K.; Boumier, P.; Bradley, P. A.; Broomhall, A. -M.; Buzasi,
D. L.; Claudi, R. U.; Cunha, M. S.; D'Antona, F.; Deheuvels, S.;
Derekas, A.; García Hernández, A.; Giampapa, M. S.; Goupil, M. J.;
Gruberbauer, M.; Guzik, J. A.; Hale, S. J.; Ireland, M. J.; Kiss,
L. L.; Kitiashvili, I. N.; Kolenberg, K.; Korhonen, H.; Kosovichev,
A. G.; Kupka, F.; Lebreton, Y.; Leroy, B.; Ludwig, H. -G.; Mathis, S.;
Michel, E.; Miglio, A.; Montalbán, J.; Moya, A.; Noels, A.; Noyes,
R. W.; Pallé, P. L.; Piau, L.; Preston, H. L.; Roca Cortés, T.;
Roth, M.; Sato, K. H.; Schmitt, J.; Serenelli, A. M.; Silva Aguirre,
V.; Stevens, I. R.; Suárez, J. C.; Suran, M. D.; Trampedach, R.;
Turck-Chièze, S.; Uytterhoeven, K.; Ventura, R.; Wilson, P. A.
Bibcode: 2010ApJ...713L.169C
Altcode: 2010arXiv1001.0506C
We present preliminary asteroseismic results from Kepler on three G-type
stars. The observations, made at one-minute cadence during the first
33.5 days of science operations, reveal high signal-to-noise solar-like
oscillation spectra in all three stars: about 20 modes of oscillation
may be clearly distinguished in each star. We discuss the appearance of
the oscillation spectra, use the frequencies and frequency separations
to provide first results on the radii, masses, and ages of the stars,
and comment in the light of these results on prospects for inference
on other solar-type stars that Kepler will observe.
Title: Solar and Stellar Variability: Impact on Earth and Planets
Authors: Kosovichev, Alexander G.; Andrei, Alexandre H.; Rozelot,
Jean-Pierre
Bibcode: 2010IAUS..264.....K
Altcode:
Preface; Organizing committee; Conference participants; 1. Introduction:
the Sun and stars as the primary energy input in planetary atmospheres;
2. Observations of solar and stellar variability; 3. Solar and stellar
cycles and variability on century timescale; 4. Magnetic activity
and dynamo mechanisms; 5. Physical mechanisms of solar and stellar
variability; 6. Effects on space weather and climate; 7. Effects of
magnetic activity on planet formation and evolution; 8. Impact of
solar and stellar variability on planetary atmospheres and climate;
9. Current and future space missions and ground-based observing
programs; 10. Summary and conclusions; Author index; Subject index.
Title: The quest for the solar g modes
Authors: Appourchaux, T.; Belkacem, K.; Broomhall, A. -M.; Chaplin,
W. J.; Gough, D. O.; Houdek, G.; Provost, J.; Baudin, F.; Boumier,
P.; Elsworth, Y.; García, R. A.; Andersen, B. N.; Finsterle, W.;
Fröhlich, C.; Gabriel, A.; Grec, G.; Jiménez, A.; Kosovichev, A.;
Sekii, T.; Toutain, T.; Turck-Chièze, S.
Bibcode: 2010A&ARv..18..197A
Altcode: 2010A&ARv.tmp....1A; 2009arXiv0910.0848A
Solar gravity modes (or g modes)—oscillations of the solar interior
on which buoyancy acts as the restoring force—have the potential
to provide unprecedented inference on the structure and dynamics of
the solar core, inference that is not possible with the well-observed
acoustic modes (or p modes). The relative high amplitude of the g-mode
eigenfunctions in the core and the evanesence of the modes in the
convection zone make the modes particularly sensitive to the physical
and dynamical conditions in the core. Owing to the existence of the
convection zone, the g modes have very low amplitudes at photospheric
levels, which makes the modes extremely hard to detect. In this article,
we review the current state of play regarding attempts to detect g
modes. We review the theory of g modes, including theoretical estimation
of the g-mode frequencies, amplitudes and damping rates. Then we go
on to discuss the techniques that have been used to try to detect g
modes. We review results in the literature, and finish by looking to
the future, and the potential advances that can be made—from both
data and data-analysis perspectives—to give unambiguous detections
of individual g modes. The review ends by concluding that, at the time
of writing, there is indeed a consensus amongst the authors that there
is currently no undisputed detection of solar g modes.
Title: Numerical Simulation of Excitation and Propagation of
Helioseismic MHD Waves in Magnetostatic Models of Sunspots
Authors: Parchevsky, K.; Kosovichev, A.; Khomenko, E.; Olshevsky,
V.; Collados, M.
Bibcode: 2010arXiv1002.1117P
Altcode:
We present comparison of numerical simulations of propagation of
MHD waves,excited by subphotospheric perturbations, in two different
("deep" and "shallow") magnetostatic models of the sunspots. The "deep"
sunspot model distorts both the shape of the wavefront and its amplitude
stronger than the "shallow" model. For both sunspot models, the surface
gravity waves (f-mode) are affected by the sunspots stronger than
the acoustic p-modes. The wave amplitude inside the sunspot depends
on the photospheric strength of the magnetic field and the distance
of the source from the sunspot axis. For the source located at 9 Mm
from the center of the sunspot, the wave amplitude increases when
the wavefront passes through the central part of the sunspot. For
the source distance of 12 Mm, the wave amplitude inside the sunspot
is always smaller than outside. For the same source distance from
the sunspot center but for the models with different strength of the
magnetic field, the wave amplitude inside the sunspot increases with
the strength of the magnetic field. The simulations show that unlike
the case of the uniform inclined background magnetic field, the p-
and f-mode waves are not spatially separated inside the sunspot where
the magnetic field is strongly non-uniform. These properties have to
be taken into account for interpretation of observations of MHD waves
traveling through sunspot regions.
Title: Prediction of solar activity cycles by assimilating sunspot
data into a dynamo model
Authors: Kitiashvili, Irina N.; Kosovichev, Alexander G.
Bibcode: 2010IAUS..264..202K
Altcode:
Solar activity is a determining factor for space climate of the
Solar system. Thus, predicting the magnetic activity of the Sun is
very important. However, our incomplete knowledge about the dynamo
processes of generation and transport of magnetic fields inside
Sun does not allow us to make an accurate forecast. For predicting
the solar cycle properties use the Ensemble Kalman Filter (EnKF)
to assimilate the sunspot data into a simple dynamo model. This
method takes into account uncertainties of both the dynamo model and
the observed sunspot number series. The method has been tested by
calculating predictions of the past cycles using the observed annual
sunspot numbers only until the start of these cycles, and showed a
reasonable agreement between the predicted and actual data. After this,
we have calculated a prediction for the upcoming solar cycle 24, and
found that it will be approximately 30% weaker than the previous one,
confirming some previous expectations. In addition, we have investigated
the properties of the dynamo model during the solar minima, and their
relationship to the strength of the following solar cycles. The results
show that prior the weak cycles, 20 and 23, and the upcoming cycle,
24, the vector-potential of the poloidal component of magnetic field
and the magnetic helicity substantial decrease. The decrease of the
poloidal field corresponds to the well-known correlation between the
polar magnetic field strength at the minimum and the sunspot number at
the maximum. However, the correlation between the magnetic helicity and
the future cycle strength is new, and should be further investigated.
Title: Note on Travel Time Shifts Due to Amplitude Modulation in
Time-Distance Helioseismology Measurements
Authors: Nigam, R.; Kosovichev, A. G.
Bibcode: 2010ApJ...708.1475N
Altcode: 2009arXiv0911.4295N
Correct interpretation of acoustic travel times measured by
time-distance helioseismology is essential to get an accurate
understanding of the solar properties that are inferred from them. It
has long been observed that sunspots suppress p-mode amplitude, but its
implications on travel times have not been fully investigated so far. It
has been found in test measurements using a "masking" procedure, in
which the solar Doppler signal in a localized quiet region of the Sun
is artificially suppressed by a spatial function, and using numerical
simulations that the amplitude modulations in combination with the
phase-speed filtering may cause systematic shifts of acoustic travel
times. To understand the properties of this procedure, we derive
an analytical expression for the cross-covariance of a signal that
has been modulated locally by a spatial function that has azimuthal
symmetry and then filtered by a phase-speed filter typically used
in time-distance helioseismology. Comparing this expression to the
Gabor wavelet fitting formula without this effect, we find that there
is a shift in the travel times that is introduced by the amplitude
modulation. The analytical model presented in this paper can be useful
also for interpretation of travel time measurements for the non-uniform
distribution of oscillation amplitude due to observational effects.
Title: High-Resolution Helioseismic Imaging of Subsurface Structures
and Flows of a Solar Active Region Observed by Hinode
Authors: Zhao, Junwei; Kosovichev, Alexander G.; Sekii, Takashi
Bibcode: 2010ApJ...708..304Z
Altcode: 2009arXiv0911.1161Z
We analyze a solar active region observed by the Hinode Ca II H line
using the time-distance helioseismology technique, and infer wave-speed
perturbation structures and flow fields beneath the active region
with a high spatial resolution. The general subsurface wave-speed
structure is similar to the previous results obtained from Solar and
Heliospheric Observatory/Michelson Doppler Imager observations. The
general subsurface flow structure is also similar, and the downward
flows beneath the sunspot and the mass circulations around the sunspot
are clearly resolved. Below the sunspot, some organized divergent flow
cells are observed, and these structures may indicate the existence of
mesoscale convective motions. Near the light bridge inside the sunspot,
hotter plasma is found beneath, and flows divergent from this area
are observed. The Hinode data also allow us to investigate potential
uncertainties caused by the use of phase-speed filter for short travel
distances. Comparing the measurements with and without the phase-speed
filtering, we find out that inside the sunspot, mean acoustic travel
times are in basic agreement, but the values are underestimated by a
factor of 20%-40% inside the sunspot umbra for measurements with the
filtering. The initial acoustic tomography results from Hinode show
a great potential of using high-resolution observations for probing
the internal structure and dynamics of sunspots.
Title: Recent Progress and Future Directions for Helioseismology
Authors: Kosovichev, A.; Zhao, J.; Sekii, T.; Nagashima, K.;
Mitra-Kraev, U.
Bibcode: 2009ASPC..415..399K
Altcode:
Hinode/SOT observations provide unique data for high-resolution
helioseismology. These data have allowed us for the first
time to resolve the subsurface convective boundary layer, obtain
high-resolution images of structures and mass flows beneath a sunspot,
detect flare-generated MHD waves in the sunspot umbra, carry out
multi-wavelength studies of solar oscillations, and obtain unique
helioseismic data for probing the subsurface dynamics in near-polar
regions. Future directions in helioseismology will be focused on
understanding the mechanism of solar dynamo, diagnostics of emerging
magnetic flux, formation and evolution of sunspot regions and their
flaring activity. Of particular interest are investigations of solar
convection dynamics, differential rotation and meridional flows
in the near polar regions. These tasks require development of new
helioseismology methods for probing conditions in strong magnetic
field regions and improving temporal and spatial resolutions. The new
developments in helioseismology will be supported by realistic MHD
simulations and based on massive data analysis from Hinode and Solar
Dynamics Observatory.
Title: Travel-Time Analyses of an Emerging-Flux Region
Authors: Nagashima, K.; Sekii, T.; Kosovichev, A. G.; Zhao, J.;
Tarbell, T. D.
Bibcode: 2009ASPC..415..417N
Altcode:
Travel-time analyses of a newly-formed plage region are presented. The
dataset has been obtained from the 12-hr Hinode observation of an
emerging-flux region (to be NOAA AR 10975) close to the disc center
on 23 November 2007. The SOT provides data in Ca II H line and in
Fe I 557.6nm line; we use both chromospheric intensity oscillation
data and photospheric Dopplergrams for travel-time measurement by
a cross-correlation method. In the plage region, we have detected
a travel-time anomaly in the chromospheric data, but not in the
photospheric data. This can be interpreted as a signature of downflows
in the chromosphere. This result illustrates how time-distance
techniques can be used to study chromospheric flows.
Title: Signatures of Emerging Subsurface Structures in Acoustic
Power Maps
Authors: Hartlep, T.; Kosovichev, A. G.; Zhao, J.; Mansour, N. N.
Bibcode: 2009ASPC..416..147H
Altcode:
We show that under suitable conditions subsurface structures can
alter the average acoustic power observed at the photosphere. By using
numerical simulations of wave propagation, we investigate whether this
effect can be used to detect emerging active regions before they appear
on the surface. In the simulations, subsurface structures are modeled
as regions with enhanced or reduced acoustic wave speed. We show how
the acoustic power above a subsurface region depends on the sign, depth
and strength of the wave speed perturbation. For comparison, we analyze
observations from SoHO/MDI of the emergence of active region NOAA~10488.
Title: Transport of Supergranules and their Vertical Coherence
Authors: Švanda, M.; Kosovichev, A. G.; Klvaňa, M.; Sobotka, M.;
Duvall, T. L., Jr.
Bibcode: 2009ASPC..416..547S
Altcode:
In recent papers, we have introduced a method for measuring the
photospheric flow field that is based on the tracking of supergranular
structures. Here, in combination with helioseismic data, we are
able to estimate the depth in the solar convection envelope to
which the detected large-scale flow field is coherent. We show that
the upper 10 Mm in the convection zone depicts similar features in
horizontal velocity. Our interpretation of this observation is that
the supergranulation is a coherent structure 10 Mm deep and is subject
to large-scale transport by the underlying velocity field.
Title: Time-Distance Helioseismic Imaging of a Numerically Simulated
Solar Tachocline
Authors: Zhao, J.; Hartlep, T.; Kosovichev, A. G.; Mansour, N. N.
Bibcode: 2009ASPC..416...25Z
Altcode:
The solar tachocline, located near the bottom of the convection zone,
is a very important region for solar dynamics and the solar dynamo. We
develop a time-distance helioseismology technique, including both
surface- and deep-focusing measurements, together with inversions,
to derive the interior image of the sound speed perturbation at the
tachocline with a latitudinal dependence. We test the technique on
numerically simulated global wavefields and find that the technique is
able to recover the major features that are preset in the numerical
model, though a bit more widespread into the deeper interior. This
measurement and inversion technique will be applied to MDI observations
to derive the structures of solar tachocline.
Title: Excitation, Propagation and Conversion of Helioseismic MHD
Waves in Strong Field Regions
Authors: Parchevsky, K. V.; Kosovichev, A. G.
Bibcode: 2009ASPC..416...61P
Altcode:
We present the results of 3D numerical simulations of MHD wave
excitation, propagation, and conversion in various configurations of the
background magnetic field, including realistic models of sunspots. The
background field causes anisotropy of the wave amplitude along the
wave front and distortion of the wave front itself. This effect is
stronger for f- than for p-modes. Comparison of calculated travel time
variations caused by the magnetic field with observations obtained from
MDI Doppler velocity data shows that only about 25% of the observed
mean travel time variation around sunspots can be explained by the
direct interaction of MHD waves with the inclined magnetic field
(even for strong fields of 1400-1900 G). We show that for a height of
300 km and such strong fields the phase of the simulated travel time
variations coincides with the observed phase.
Title: New Results of High-Resolution Helioseismology from Hinode
Authors: Kosovichev, A.; Zhao, J.; Sekii, T.; Nagashima, K.;
Mitra-Kraev, U.
Bibcode: 2009ASPC..416...41K
Altcode:
The Solar Optical Telescope of Hinode provides unique multi-wavelength
high-resolution data for local helioseismic diagnostics of the
sub-surface structure and dynamics of the Sun. The helioseismology data
from Hinode have allowed us for the first time to observe oscillations
of very high angular degree and high frequencies, and substantially
improve the spatial resolution of time-distance helioseismology in
near-surface layers of the Sun, compared to the previous SOHO/MDI
data. The Hinode data have also provided important insight on the
nature of sunspot oscillations, and the correlated component of
stochastic excitation. Initial attempts have been made to investigate
the dynamics of the polar regions, critical for dynamo modeling, but
previously unaccessible for helioseismology. In addition, a new type
of flare-excited MHD oscillations was detected from Hinode observations
of the solar flare of December 13, 2006.
Title: Subsurface Structures and Flow Fields of an Active Region
Observed by Hinode
Authors: Zhao, J.; Kosovichev, A. G.; Sekii, T.
Bibcode: 2009ASPC..415..411Z
Altcode:
We analyze a solar active region observed by Hinode Ca II H line using
the time-distance helioseismology technique, and derive the subsurface
structure and flow fields of this active region. The basic subsurface
wave speed structure is essentially the same as the previous results
obtained from MDI observations. The subsurface flow structure is
also similar to the previous results, but the downward flows are more
resolved. Additionally, vertical mass circulations outside the sunspot
are more clearly detected, although no mass conservation constraints
are imposed in the inversion procedure. Near a light bridge area,
hotter plasma is found beneath, and flows divergent from this area
are observed. Inside the sunspot umbra and penumbra, some organized
divergent flow cells of an intermediate size between granulation and
supergranulation are also observed. These initial results demonstrate
the potential and importance of high-resolution helioseismology of
sunspots.
Title: Realistic MHD Simulations of Solar Convection and Oscillations
in Magnetic Regions: Mode Excitation and Effects of Acoustic Halos
Authors: Jacoutot, L.; Kosovichev, A.; Wray, A.; Mansour, N.
Bibcode: 2009ASPC..416...67J
Altcode:
We have used a 3D, compressible, non-linear radiative
magnetohydrodynamics code developed at the NASA Ames Research Center
to model solar convection and oscillations in magnetic regions. This
code takes into account several physical phenomena: compressible fluid
flow in a highly stratified medium, sub-grid scale turbulence models,
radiative energy transfer, and a real-gas equation of state. We have
studied the influence of the magnetic field of various strength
on the convective cells and on the excitation mechanisms of the
acoustic oscillations by calculating spectral properties of the
convective motions and oscillations. The results reveal substantial
changes of the granulation structure with increased magnetic field,
and a frequency-dependent reduction in the oscillation power in a
good agreement with solar observations. The simulations provide a
solution to the long-standing problem of enhanced high-frequency
acoustic emission at the boundaries of active region ("acoustic
halos"), suggesting that this phenomenon is caused by the changes of
the spatial-temporal spectrum of the turbulent convection in magnetic
field, resulting in turbulent motions of smaller scales and higher
frequencies than in quiet Sun regions. These simulation results may
have also important implications for understanding high-frequency
oscillations of magnetic stars.
Title: Modeling of sunspot structures using simulations of
axisymmetric MHD convection
Authors: Hartlep, T.; Hurlburt, N. E.; Busse, F. H.; Kosovichev, A. G.
Bibcode: 2009AGUFMSH23B1538H
Altcode:
We present our efforts on modeling solar pores and sunspots in 2D
and 2.5D axisymmetric magneto-hydrodynamic (MHD) simulations of
the upper layers of the Sun's convection zone. One goals of this
work is to produce consistent, self-maintained, even if not fully
realistic, models of concentrated, strong magnetic field in subsurface
layers of the Sun. These models are important for understanding the
mechanisms of magnetic field concentration in sunspots, and associated
large-scale flow patterns. Also, these can be used as background
models in acoustic wave propagation simulations for testing local
helioseismology techniques and their performance and problems in
magnetic regions. This study explores the parameters under which
magnetic field can spontaneously concentrate, and the kind of flow
patterns that form in these cases (for instance if there are inflows
or outflow at the surface).
Title: A Functional Analytic Approach to Fréchet Traveltime Kernels
Authors: Schlottmann, R.; Kosovichev, A. G.
Bibcode: 2009AGUFMSH13A1509S
Altcode:
Fréchet kernels, an emerging tool in helioseismic data analysis,
describe the first-order (linear) relationship between medium
properties affecting helioseismic wave propagation and measured wave
traveltimes. Although many treatments of these kernels exist in the
literature, we believe a need exists to provide a consistent, systematic
method of deriving them. We present an approach using some basic tools
and concepts of functional analysis, such as functional derivatives
and functional Taylor series. Although no fundamentally new results are
obtained from this approach, it clarifies the approximations used and
provides a nice organizing principle for the theory. In addition to the
theoretical apparatus, we also provide general formulas for any desired
medium property that can affect helioseismic wave propagation. We also
supply specific formulas for some properties, such as wave speed and
density, and calculate some sample kernels.
Title: Implementation of Data Assimilation Methods for Dynamo Models
to Predict Solar Activity
Authors: Kitiashvili, I.; Kosovichev, A.
Bibcode: 2009ASPC..416..511K
Altcode:
Cyclic variations of solar activity are a result of a complicated
dynamo process in the convection zone. Despite the regular cyclic
variations of solar activity, the chaotic variations of sunspot number
from cycle to cycle are difficult to predict. The main reasons are the
imperfect dynamo models and deficiency of the necessary observational
data. Data assimilation methods iterate observational data and models
for possible efficient and accurate estimations of physical properties,
which cannot be observed directly. We apply the Ensemble Kalman Filter
method for assimilation of the sunspot data into a non-linear mean-field
dynamo model, which takes into variations of magnetic helicity and
parameters of the solar convection zone from helioseismology. We
present the results of application of this data assimilation method
for representation of the solar cycles and prediction of variations
of the sunspot number, and discuss potentials of data assimilation
methods for solar dynamo modeling.
Title: NUMERICAL SIMULATION OF PROPAGATION AND SCATTERING OF THE
MHD WAVES IN SUNSPOTS
Authors: Parchevsky, K.; Kosovichev, A. G.; Khomenko, E.; Collados, M.
Bibcode: 2009AGUFMSH23B1535P
Altcode:
We present comparison of numerical simulation results of MHD wave
propagation in two different magnitostatic models of sunspots
refferred to as "deep" and "shallow" models. The "deep" model has
convex shape of magnetic field lines near the photosphere and non-zero
horizorntal perturbations of the sound speed up to the bottom of the
model (7.5 Mm). The "shallow" model has concave shape of the magnetic
field lines near the photosphere and horizontally uniform sound speed
below 2 Mm. Common feature of MHD waves behaviour in these two models
is that for weak magnetic field (less than 1kG at the photosphere)
waves reduce their amplitude when they reach the center of the sunspot
and restore the amplitude when pass the center. For the "deep" model
this effect is bigger than for the "shallow" model. The wave amplitude
inside sunspots depends on the strength of the magnetic field. For the
"shallow" model with photospheric magnetic field of 2.2 kG the wave
amplitude inside the sunspot becomes bigger than outside (opposite to
the weak magnetic field). The wave amplitude depends on the distance
of the source from the sunspot center. For the "shallow" model and
source distance of 9 Mm from the sunspot center the wave amplitude at
some moment (when the wavefront passes the sunspot center) becomes
bigger inside the sunspot than outside. For the source distance
of 12 Mm the wave amplitude remains smaller inside the sunspot
than outside for all moments of time. Using filtering technique we
separated magnetoacoustic and magnetogravity waves. Simulations show
that the sunspot changes the shape of the wave front and amplitude
of the f-modes significantly stronger than the p-modes. It is shown,
that inside the sunspot magnetoacoustic and magnetogravity waves are
not spatially separated unlike the case of the horizontally uniform
background model. Strong Alfven wave is generated at the wave source
location in the "deep" model. This wave exists in the "shallow" model
as well, but with much smaller amplitude.
Title: Time-Distance Helioseismology Data Analysis Pipeline for
SDO/HMI
Authors: Zhao, J.; Couvidat, S. P.; Parchevsky, K.; Duvall, T. L.;
Beck, J. G.; Birch, A. C.; Kosovichev, A. G.
Bibcode: 2009AGUFMSH13A1507Z
Altcode:
Solar Dynamics Observatory (SDO) will be launched shortly, and
high-resolution helioseismic data from Heliosesimic and Magnetic
Imager (HMI) will soon be available. We have developed a data
analysis procedure based on the time-distance helioseismology method,
to automatically process 2 TB/day of solar oscillation data from the
HMI instrument. The pipeline will provide the following data to the
solar physics community: nearly whole disk maps of acoustic travel
times, subsurface flow fields and sound-speed perturbation maps
every eight hours. The data products include also real-time updated
synoptic maps for subsurface flows and sound-speed perturbations. For
the helioseismology community, the pipeline will provide intermediate
data products such as cross-covariances of solar oscillations. This
poster explains how we process the observational data, how we perform
the travel-time measurements, how we derive the sensitivity kernels
for inversions, and how we perform and test the inversion results.
Title: The Origin of High-Speed Evershed Flows in Sunspots
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Wray, A. A.; Mansour,
N. N.
Bibcode: 2009AGUFMSH23B1534K
Altcode:
Radial outflow of magnetized plasma in sunspot penumbrae, the
Evershed effect, has a long history of observations and modeling,
since its discovery in 1909. There are several different approaches
for explaining this phenomenon, but these models cannot describe
various observational facts. We have carried out 3D radiative MHD
simulations of solar convection in the presence of magnetic field
of different strength and inclination. The simulation results show
that when the magnetic field is strong (1-2 kG) and highly inclined
towards the surface (by 80-85 degrees) the granular convective cells
transform into filamentary, flux-tube like structures with strong
horizontal velocities. In addition, the convective process takes the
form of traveling waves with convective cells moving in the direction
of the field inclination. The combination of these two effects, the
filamentary structure and the traveling convective waves, result in
high-speed, 4-6 km/s, plasma streams, identified in observations as
"Evershed clouds". In the case of a very strong magnetic field the
streams form coherent structures across the field. The simulation
results reproduce many observed features of the Evershed effect,
and allow us to establish links with the previous models.
Title: Realistic Numerical Modeling of Solar Magnetoconvection
and Oscillations
Authors: Kitiashvili, I.; Jacoutot, L.; Kosovichev, A.
Bibcode: 2009ASPC..415...83K
Altcode: 2009arXiv0901.4369K
We have developed 3D, compressible, non-linear radiative MHD simulations
to study the influence of magnetic fields of various strengths and
geometries on the turbulent convective cells and on the excitation
mechanisms of the acoustic oscillations. The results reveal substantial
changes of the granulation structure with increased magnetic field,
and a frequency-dependent reduction in the oscillation power. These
simulation results reproduce the enhanced high-frequency acoustic
emission observed at the boundaries of active region (``acoustic halo''
phenomenon). In the presence of inclined magnetic field the solar
convection develops filamentary structure with flows concentrated
along magnetic filaments, and also exhibits behavior of running
magnetoconvective waves, resembling recent observations of the sunspot
penumbra dynamics from Hinode/SOT.
Title: Comparing the Hinode and SOHO/MDI Data to the Simulated Large
Scale Solar Convection
Authors: Georgobiani, D.; Zhao, J.; Kosovichev, A.; Benson, D.; Stein,
R. F.; Nordlund, Å.
Bibcode: 2009ASPC..415..421G
Altcode:
Large-scale simulations of solar turbulent convection produce realistic
data and provide a unique opportunity to study solar oscillations
and test various techniques commonly used for the analysis of solar
observations. We applied helioseismic methods to the sets of simulated
as well as observed data and find remarkable similarities. Power
spectra, k-ν diagrams, time-distance diagrams exhibit similar details,
although sometimes subtle differences are present.
Title: Solar Oscillations
Authors: Kosovichev, A. G.
Bibcode: 2009AIPC.1170..547K
Altcode: 2010arXiv1001.5283K
In recent years solar oscillations have been studied in great detail,
both observationally and theoretically; so, perhaps, the Sun currently
is the best understood pulsating star. The observational studies include
long, almost uninterrupted series of oscillation data from the SOHO
spacecraft and ground-based networks, GONG and BiSON, and more recently,
extremely high-resolution observations from the Hinode mission. These
observational data cover the whole oscillation spectrum, and have been
extensively used for helioseismology studies, providing frequencies
and travel times for diagnostics of the internal stratification,
differential rotation, zonal and meridional flows, subsurface convection
and sunspots. Together with realistic numerical simulations, they lead
to better understanding of the excitation mechanism and interactions
of the oscillations with turbulence and magnetic fields. However,
many problems remain unsolved. In particular, the precision of the
helioseismology measurements is still insufficient for detecting
the dynamo zone and deep routes of sunspots. Our knowledge of the
oscillation physics in strong magnetic field regions is inadequate for
interpretation of MHD waves in sunspots and for sunspot seismology. A
new significant progress in studying the solar oscillations is expected
from the Solar Dynamics Observatory scheduled for launch in 2009.
Title: Imaging the Solar Tachocline by Time-Distance Helioseismology
Authors: Zhao, Junwei; Hartlep, Thomas; Kosovichev, A. G.; Mansour,
N. N.
Bibcode: 2009ApJ...702.1150Z
Altcode: 2009arXiv0907.2118Z
The solar tachocline at the bottom of the convection zone is an
important region for the dynamics of the Sun and the solar dynamo. In
this region, the sound speed inferred by global helioseismology
exhibits a bump of approximately 0.4% relative to the standard solar
model. Global helioseismology does not provide any information on
possible latitudinal variations or asymmetries between the northern and
southern hemisphere. Here, we develop a time-distance helioseismology
technique, including surface- and deep-focusing measurement schemes and
a combination of both, for two-dimensional tomographic imaging of the
solar tachocline that infers radial and latitudinal variations in the
sound speed. We test the technique using artificial solar oscillation
data obtained from numerical simulations. The technique successfully
recovers major features of the simplified tachocline models. The
technique is then applied to SOHO/MDI medium-ell data and provides
for the first time a full two-dimensional sound-speed perturbation
image of the solar tachocline. The one-dimensional radial profile
obtained by latitudinal averaging of the image is in good agreement
with the previous global helioseismology result. It is found that the
amplitude of the sound-speed perturbation at the tachocline varies
with latitude, but it is not clear whether this is in part or fully an
effect of instrumental distortion. Our initial results demonstrate that
time-distance helioseismology can be used to probe the deep interior
structure of the Sun, including the solar tachocline.
Title: Numerical Modeling of Solar Convection and Oscillations in
Magnetic Regions
Authors: Kitiashvili, I. N.; Jacoutot, L.; Kosovichev, A. G.; Wray,
A. A.; Mansour, N. N.
Bibcode: 2009AIPC.1170..569K
Altcode:
Solar observations show that the spectra of turbulent convection
and oscillations significantly change in magnetic regions, resulting
in interesting phenomena, such as high-frequency ``acoustic halos''
around active regions. In addition, recent observations from SOHO/MDI
revealed significant changes of the wave properties in inclined magnetic
field regions of sunspots, which affect helioseismic inferences. We
use realistic 3D radiative MHD numerical simulations to investigate
properties of solar convection and excitation and propagation of
oscillations in magnetic regions. A new feature of these simulations is
implementation of a dynamic sub-grid turbulence model, which allows more
accurate description of turbulent dissipation and wave excitation. We
present the simulation results for a wide range of the field strength
and inclination in the top 6 Mm layer of the convection zone. The
results show interesting and unexpected effects in the dynamics and
large-scale organization of the magnetoconvection (including traveling
waves and shearing flows), and also changes in the excitation properties
and spectrum of oscillations, suggesting an explanation of the acoustic
``halos'' observed above the acoustic cut-off frequency.
Title: Traveling Waves of Magnetoconvection and the Origin of the
Evershed Effect in Sunspots
Authors: Kitiashvili, I. N.; Kosovichev, A. G.; Wray, A. A.; Mansour,
N. N.
Bibcode: 2009ApJ...700L.178K
Altcode: 2009arXiv0904.3599K
Discovered in 1909, the Evershed effect represents strong mass outflows
in sunspot penumbra, where the magnetic field of sunspots is filamentary
and almost horizontal. These flows play an important role in sunspots
and have been studied in detail using large ground-based and space
telescopes, but the basic understanding of its mechanism is still
missing. We present results of realistic numerical simulations of
the Sun's subsurface dynamics, and argue that the key mechanism of
this effect is in nonlinear magnetoconvection that has properties
of traveling waves in the presence of a strong, highly inclined
magnetic field. The simulations reproduce many observed features of
the Evershed effect, including the high-speed "Evershed clouds," the
filamentary structure of the flows, and the nonstationary quasiperiodic
behavior. The results provide a synergy of previous theoretical models
and lead to an interesting prediction of a large-scale organization
of the outflows.
Title: Cosmic Magnetic Fields (IAU S259)
Authors: Strassmeier, Klaus G.; Kosovichev, Alexander G.; Beckman,
John E.
Bibcode: 2009IAUS..259.....S
Altcode:
Preface K. G. Strassmeier, A. G. Kosovichev and J. E. Beckman;
Organising committee; Conference photograph; Conference participants;
Session 1. Interstellar magnetic fields, star-forming regions and
the Death Valley Takahiro Kudoh and Elisabeta de Gouveia Dal Pino;
Session 2. Multi-scale magnetic fields of the Sun; their generation
in the interior, and magnetic energy release Nigel O. Weiss;
Session 3. Planetary magnetic fields and the formation and evolution
of planetary systems and planets; exoplanets Karl-Heinz Glassmeier;
Session 4. Stellar magnetic fields: cool and hot stars Swetlana Hubrig;
Session 5. From stars to galaxies and the intergalactic space Dimitry
Sokoloff and Bryan Gaensler; Session 6. Advances in methods and
instrumentation for measuring magnetic fields across all wavelengths
and targets Tom Landecker and Klaus G. Strassmeier; Author index;
Object index; Subject index.
Title: Cosmic Magnetic Fields (IAU S259)
Authors: Strassmeier, Klaus G.; Kosovichev, Alexander G.; Beckman,
John E.
Bibcode: 2009cmf..book.....S
Altcode:
Preface K. G. Strassmeier, A. G. Kosovichev and J. E. Beckman;
Organising committee; Conference photograph; Conference participants;
Session 1. Interstellar magnetic fields, star-forming regions and
the Death Valley Takahiro Kudoh and Elisabeta de Gouveia Dal Pino;
Session 2. Multi-scale magnetic fields of the Sun; their generation
in the interior, and magnetic energy release Nigel O. Weiss;
Session 3. Planetary magnetic fields and the formation and evolution
of planetary systems and planets; exoplanets Karl-Heinz Glassmeier;
Session 4. Stellar magnetic fields: cool and hot stars Swetlana Hubrig;
Session 5. From stars to galaxies and the intergalactic space Dimitry
Sokoloff and Bryan Gaensler; Session 6. Advances in methods and
instrumentation for measuring magnetic fields across all wavelengths
and targets Tom Landecker and Klaus G. Strassmeier; Author index;
Object index; Subject index.
Title: Testing the Sunspot Subsurface ("Coffee-Cup") Structure
Authors: Kosovichev, Alexander G.; Zhao, J.; Parchevsky, K. V.;
Hartlep, T.
Bibcode: 2009SPD....40.0703K
Altcode:
Helioseismic inferences of the subsurface structure of sunspots
have been in the focus of local helioseismology investigations and
discussions during the past decade. Initially obtained from the SOHO/MDI
high-resolution data using a simple ray-path perturbation theory for
inversions of acoustic travel times the results draw significant
criticism and concerns, but, in general, have been confirmed by
subsequent measurements and inversions. Nevertheless, because of the
complexity of the wave excitation and propagation in the subsurface
layers there are still potential systematic uncertainties that must
be resolved. Recently developed 3D simulations of MHD waves provide an
important tool for the verification and testing of local helioseismology
results. However, the use of the simulations is not as straightforward
as originally thought. In particular, we present results of testing of
the time-distance helioseismology measurement and inversion procedures
for sound-speed perturbations modeling the sunspot subsurface structure,
and discuss the limitations and uncertainties of the simulations and
helioseismic inferences.
Title: Simulated Large Scale Solar Convection Versus Observations:
A Multiwavelength Approach
Authors: Georgobiani, Dali; Zhao, J.; Kosovichev, A. G.; Benson, D.;
Stein, R. F.; Nordlund, A.
Bibcode: 2009SPD....40.0301G
Altcode:
The realistic 3D radiative-hydrodynamic simulations of the upper layers
of solar convection provide a perfect opportunity to validate various
techniques, widely used in solar physics and helioseismology. Our
aim is to perform multiwavelength analysis of large scale flows. We
analyze the simulated intensity and velocities at certain heights
in the solar atmosphere, and compare our results with the outcome
of the similar analysis of the SOHO/MDI and Hinode observations. To
fine tune the comparison, we use the instrumental response functions
to weigh the simulated parameters at different heights to emulate
the observational lines. We find the remarkable similarity between
the simulated and observed power spectra, their spatial parts, and
time-distance diagrams. This demonstrates one more time that the
simulations can be efficiently used to perform and validate local
helioseismology techniques, and to study solar flows and structures
beneath the surface, inaccessible for direct observations.
Title: Observing Interactions of Helioseismic Waves with Sunspots
Authors: Zhao, Junwei; Kosovichev, A. G.
Bibcode: 2009SPD....40.0302Z
Altcode:
Recently, the interactions between solar magnetic field and acoustic
waves have been widely investigated using both numerical simulations
and theoretical analysis,including effects of MHD wave transformations
and absorption. However, a picture of how solar waves interact
with a sunspot at the photospherical level has not been obtained
observationally. In this presentation, we apply a time-distance
correlation analysis to high-resolution MDI observations to investigate
how the solar acoustic and surface gravity waves interact with
sunspots. The analysis is carried out separately for the p- and f-mode
waves. It is found that when acoustic sources are located outside
the sunspot, the p-mode waves propagate through the sunspot with a
faster speed and a reduced amplitude. The speed and amplitude become
similar to the quiet Sun values after the waves pass through the active
region. If the acoustic source is located inside the sunspot, the waves
propagate with a reduced amplitude, and initially have a reduced speed
but then accelerate. The f-mode waves often go through the sunspot with
a reduced amplitude and a faster speed, but their amplitude and speed
do not recover after passing the sunspot area. The results are nicely
illustrated in movies showing the waves interacting with a sunspot.
Title: Acoustic Power and Travel Time Signatures of Emerging
Subsurface Structures
Authors: Kosovichev, A. G.; Hartlep, Thomas; Zhao, J.; Mansour, N. N.
Bibcode: 2009SPD....40.0709K
Altcode:
It is generally believed that larger magnetic structures rising from
below the surface by self induction and convection can lead to the
formation of active regions and sunspots on the solar surface. For
space weather forecasting, one would like to detect such subsurface
structures before they become visible on the surface. In this work
we investigate signatures of such emerging subsurface structures. Numerical simulations of solar acoustic wave propagation performed
by us show that under suitable conditions subsurface structures that
modify the propagation speed of the waves can result, depending on
the sign of the perturbation, in a reduction or enhancement of the
acoustic power observed at the photosphere above them. Results from our
simulations and observational evidence is presented. In principle,
measurements of travel times may also be able to detect signatures of
such emerging subsurface structures. The limiting factor is here the
short measurement time required, since active region usually emerge
rather quickly, and the associated noise level. We will present results
from analyzing artificial data obtained from our simulations as well
as MDI observations.
Title: Large-scale horizontal flows in the solar photosphere IV. On
the vertical structure of large-scale horizontal flows
Authors: Švanda, M.; Klvaňa, M.; Sobotka, M.; Kosovichev, A. G.;
Duvall, T. L.
Bibcode: 2009NewA...14..429S
Altcode: 2008arXiv0812.1971S
In the recent papers, we introduced a method utilised to measure
the flow field. The method is based on the tracking of supergranular
structures. We did not precisely know, whether its results represent
the flow field in the photosphere or in some subphotospheric
layers. In this paper, in combination with helioseismic data, we
are able to estimate the depths in the solar convection envelope,
where the detected large-scale flow field is well represented by
the surface measurements. We got a clear answer to question what
kind of structures we track in full-disc Dopplergrams. It seems that
in the quiet Sun regions the supergranular structures are tracked,
while in the regions with the magnetic field the structures of the
magnetic field are dominant. This observation seems obvious, because
the nature of Doppler structures is different in the magnetic regions
and in the quiet Sun. We show that the large-scale flow detected by
our method represents the motion of plasma in layers down to ∼10
Mm. The supergranules may therefore be treated as the objects carried
by the underlying large-scale velocity field.
Title: Realistic 3D MHD Simulations of the Evershed Effect
Authors: Kitiashvili, Irina; Kosovichev, A. G.; Wray, A. A.; Mansour,
N. N.
Bibcode: 2009SPD....40.0906K
Altcode:
Effect of the horizontal radial outflow in a sunspot penumbra (called
"Evershed effect") has a 100-year history of investigations, but its
physical nature is not clear yet. The Evershed flows begin at bright
penumbral grains and propagate outward along penumbra filaments
with the mean velocity of 1 - 2 km/s. High-resolution observations
reveal that the Evershed flows are non-stationary, and that the
strongest, 4 - 5 km/s, flows appear in quasi-periodic patches,
"Evershed clouds". To study the nature of the Evershed effect we
simulate behavior of convective motions in the presence strong
inclined magnetic field. We use a 3D radiative non-linear MHD code,
which describes realistic physical properties: compressible fluid
flow in a highly stratified and magnetized plasma, 3D multi-group
radiative energy transfer, a real-gas equation of state, and sub-grid
scale turbulence models. We present a set of numerical experiments,
which include the upper solar convection zone and lower atmosphere for
different magnetic field strength (600 - 2000 Gauss) and inclination
(0 - 90 degrees). The results show the development of filamentary
magnetic structures and systematic flows in the direction of field
inclination, strongly resembling the Evershed effect in penumbra. In
particular, the simulations reproduce the high-speed "Evershed clouds",
relationships between the flow velocity and the field strength and
inclination, and other observational characteristics. We discuss the
simulation results in the context of previously models, such as the
embedded flux tube model, the magnetic gap model and the overturning
magnetoconvection model, and argue that the physical mechanism of
the Evershed effect is in a non-linear interaction between the narrow
overturning convective motions and traveling magnetoconvection waves,
formed in highly inclined strong magnetic field regions.
Title: Subsurface Flows in Solar Active Regions and Polar Areas from
Hinode Observations
Authors: Zhao, Junwei; Kosovichev, A. G.; Sekii, T.
Bibcode: 2009SPD....40.0706Z
Altcode:
High-resolution observations of Ca II H line intensity made by Hinode
have provided us an opportunity to study subsurface flow fields of
solar active regions with an unprecedented high spatial resolution,
and a possibility to study subsurface meridional flows in the solar
polar areas. For a large active region, we have found a strong
downdraft below the sunspot, a mass circulation around the spot, and
an evidence of magnetoconvection beneath the strong field area. For
the polar regions, we have been able to derive a poleward meridional
flow of an order of 5 m/s for the first time using the time-distance
helioseismology technique.
Title: Scattering of MHD Waves by Sunspots
Authors: Parchevsky, Konstantin; Kosovichev, A. G.
Bibcode: 2009SPD....40.0704P
Altcode:
Investigation of the interaction of MHD waves with sunspots is very
important for correct interpretation of local helioseismology data and
sunspot seismology. We compare results of 3D numerical simulations of
propagation of the MHD waves in subphotospheric and photospheric layers
of sunspots. Two self-consistent magnetohydrostatic background models
of sunspots based on a solution of Pizzo and a self-similar solution of
Low were chosen as the background models. We consider the models with
different magnetic field and sound-speed depth structures, extended into
the deep interior and shallow. The MHD waves form two distinct classes:
magnetoacoustic and magnetogravity modes. There are similarities and
differences in wave behavior in the sunspot models. It is shown, that
inside the sunspot magnetoacoustic and magnetogravity waves are not
spatially separated unlike the case of the horizontally uniform magnetic
field. The sunspot causes anisotropy of the amplitude distribution along
the wavefront and changes the shape of the wavefront. The amplitude of
the waves is reduced inside the sunspot. This effect is stronger for
the magnetogravity waves than for the magnetoacoustic waves. The shape
of the wavefront of the magnetogravity waves is distorted stronger
as well. For the deep sunspot model the anisotropy is stronger for
both magnetoacoustic and magnetogravity waves than for the shallow
model. These effects cause changes in wave travel times. A strong
Alfven wave is generated at the wave source location in the deep
sunspot model. This wave is almost unnoticeable in the shallow model.
Title: Prediction of solar magnetic cycles by a data assimilation
method
Authors: Kitiashvili, Irina N.; Kosovichev, Alexander G.
Bibcode: 2009IAUS..259..235K
Altcode:
We consider solar magnetic activity in the context of sunspot number
variations, as a result of a non-linear oscillatory dynamo process. The
apparent chaotic behavior of the 11-year sunspot cycles and undefined
errors of observations create uncertainties for predicting the
strength and duration of the cycles. Uncertainties in dynamo model
parameters create additional difficulties for the forecasting. Modern
data assimilation methods allow us to assimilate the observational
data into the models for possible efficient and accurate estimations
of the physical properties, which cannot be observed directly, such as
the internal magnetic fields and helicity. We apply the Ensemble Kalman
Filter method to a low-order non-linear dynamo model, which takes into
account variations of the turbulent magnetic helicity and reproduces
basic characteristics of the solar cycles. We investigate the predictive
capabilities of this approach, and present test results for prediction
of the previous cycles and a forecast of the next solar cycle 24.
Title: Magnetic fields and dynamics of the Sun's interior
Authors: Kosovichev, Alexander G.
Bibcode: 2009IAUS..259..147K
Altcode:
Advances in helioseismology provide new knowledge about the origin of
solar magnetic activity. The key questions addressed by helioseismology
are: what is the physical mechanism of the solar dynamo, how deep inside
the Sun are the magnetic fields generated, how are they transported to
the surface and form sunspots? Direct helioseismic signatures of the
internal magnetic fields are weak and difficult to detect. Therefore,
most of the information comes from observations of dynamical effects
caused by the magnetic fields. I review results of recent helioseismic
observations of the magnetohydrodynamics of the solar interior on
various scales, including global dynamics associated with the dynamo
processes, and formation of sunspots and active regions.
Title: Photospheric and Subphotospheric Dynamics of Emerging
Magnetic Flux
Authors: Kosovichev, A. G.
Bibcode: 2009SSRv..144..175K
Altcode: 2009arXiv0901.0035K
Magnetic fields emerging from the Sun’s interior carry information
about physical processes of magnetic field generation and transport
in the convection zone. Soon after appearance on the solar surface the
magnetic flux gets concentrated in sunspot regions and causes numerous
active phenomena on the Sun. This paper discusses some properties
of the emerging magnetic flux observed on the solar surface and
in the interior. A statistical analysis of variations of the tilt
angle of bipolar magnetic regions during the emergence shows that
the systematic tilt with respect to the equator (the Joy’s law) is
most likely established below the surface. However, no evidence of the
dependence of the tilt angle on the amount of emerging magnetic flux,
predicted by the rising magnetic flux rope theories, is found. Analysis
of surface plasma flows in a large emerging active region reveals strong
localized upflows and downflows at the initial phase of emergence but
finds no evidence for large-scale flows indicating future appearance
a large-scale magnetic structure. Local helioseismology provides
important tools for mapping perturbations of the wave speed and mass
flows below the surface. Initial results from SOHO/MDI and GONG reveal
strong diverging flows during the flux emergence, and also localized
converging flows around stable sunspots. The wave speed images obtained
during the process of formation of a large active region, NOAA 10488,
indicate that the magnetic flux gets concentrated in strong field
structures just below the surface. Further studies of magnetic flux
emergence require systematic helioseismic observations from the ground
and space, and realistic MHD simulations of the subsurface dynamics.
Title: Solution to the discrepancy between the seismic and
photospheric solar radius
Authors: Haberreiter, M.; Kosovichev, A. G.; Schmutz, W.
Bibcode: 2009EGUGA..11.3961H
Altcode:
Two methods are usually used to observationally determine the solar
radius: One is the observation of the intensity profile at the limb,
the other one uses f-mode frequencies to derive a 'seismic' solar
radius which is then corrected to optical depth unity. The two methods
are inconsistent and lead to a difference in the solar radius of
approx. 0.3 Mm. Based on radiative transfer calculations we show that
this discrepancy can be explained by the difference between the height
at disk center where tau500=1 and the inflection point of the intensity
profile on the limb. We calculate the intensity profile of the limb
for the MDI continuum and the continuum at 5000 A for two atmosphere
structures and compare the position of the inflection points with the
radius at optical depth unity. The calculated difference between the
'seismic' radius and the inflection point is 0.347 Mm with respect to
optical depth unity and 0.333 Mm with respect to the Rossland mean
opacity. We conclude that the standard solar radius in evolutionary
models has to be lowered by 0.333 Mm and is 695.66 Mm. This correction
reconciles inflection point measurements and the seismic radius within
the uncertainty. This finding is very important for the analysis of
the solar diameter measurements with the SODISM instrument on PICARD.
Title: Helioseismic Signature of Chromospheric Downflows in Acoustic
Travel-Time Measurements From Hinode
Authors: Nagashima, Kaori; Sekii, Takashi; Kosovichev, Alexander G.;
Zhao, Junwei; Tarbell, Theodore D.
Bibcode: 2009ApJ...694L.115N
Altcode: 2009arXiv0903.1323N
We report on a signature of chromospheric downflows in two emerging
flux regions detected by time-distance helioseismology analysis. We
use both chromospheric intensity oscillation data in the Ca II H line
and photospheric Dopplergrams in the Fe I 557.6 nm line obtained
by Hinode/SOT for our analyses. By cross-correlating the Ca II
oscillation signals, we have detected a travel-time anomaly in the
plage regions; outward travel times are shorter than inward travel
times by 0.5-1 minute. However, such an anomaly is absent in the Fe
I data. These results can be interpreted as evidence of downflows in
the lower chromosphere. The downflow speed is estimated to be below
10 km s-1. This result demonstrates a new possibility of
studying chromospheric flows by time-distance analysis.
Title: Numerical Simulation of Excitation and Propagation of
Helioseismic MHD Waves: Effects of Inclined Magnetic Field
Authors: Parchevsky, K. V.; Kosovichev, A. G.
Bibcode: 2009ApJ...694..573P
Altcode: 2008arXiv0806.2897P
Investigation of propagation, conversion, and scattering of MHD
waves in the Sun is very important for understanding the mechanisms of
observed oscillations and waves in sunspots and active regions. We have
developed a three-dimensional linear MHD numerical model to investigate
the influence of the magnetic field on excitation and properties of
the MHD waves. The results show that surface gravity waves (f-modes)
are affected by the background magnetic field more than acoustic-type
waves (p-modes). Comparison of our simulations with the time-distance
helioseismology results from Solar and Heliospheric Observatory/MDI
shows that the amplitude of travel time variations with azimuth around
sunspots caused by the inclined magnetic field does not exceed 25%
of the observed amplitude even for strong fields of 1400-1900 G. This
can be an indication that other effects (e.g., background flows and
nonuniform distribution of the magnetic field) can contribute to the
observed azimuthal travel time variations. The azimuthal travel time
variations caused by the wave interaction with the magnetic field are
similar for simulated and observed travel times for strong fields of
1400-1900 G if Doppler velocities are taken at the height of 300 km
above the photosphere where the plasma parameter β Lt 1. For the
photospheric level the travel times are systematically smaller by
approximately 0.12 minutes than for the height of 300 km above the
photosphere for all studied ranges of the magnetic field strength and
inclination angles. Numerical MHD wave modeling and new data from the
HMI instrument of the Solar Dynamics Observatory will substantially
advance our knowledge of the wave interaction with strong magnetic
fields on the Sun and improve the local helioseismology diagnostics.
Title: POLAR investigation of the Sun—POLARIS
Authors: Appourchaux, T.; Liewer, P.; Watt, M.; Alexander, D.;
Andretta, V.; Auchère, F.; D'Arrigo, P.; Ayon, J.; Corbard, T.;
Fineschi, S.; Finsterle, W.; Floyd, L.; Garbe, G.; Gizon, L.; Hassler,
D.; Harra, L.; Kosovichev, A.; Leibacher, J.; Leipold, M.; Murphy,
N.; Maksimovic, M.; Martinez-Pillet, V.; Matthews, B. S. A.; Mewaldt,
R.; Moses, D.; Newmark, J.; Régnier, S.; Schmutz, W.; Socker, D.;
Spadaro, D.; Stuttard, M.; Trosseille, C.; Ulrich, R.; Velli, M.;
Vourlidas, A.; Wimmer-Schweingruber, C. R.; Zurbuchen, T.
Bibcode: 2009ExA....23.1079A
Altcode: 2008ExA...tmp...40A; 2008arXiv0805.4389A
The POLAR Investigation of the Sun (POLARIS) mission uses a combination
of a gravity assist and solar sail propulsion to place a spacecraft
in a 0.48 AU circular orbit around the Sun with an inclination of 75°
with respect to solar equator. This challenging orbit is made possible
by the challenging development of solar sail propulsion. This first
extended view of the high-latitude regions of the Sun will enable
crucial observations not possible from the ecliptic viewpoint or from
Solar Orbiter. While Solar Orbiter would give the first glimpse of
the high latitude magnetic field and flows to probe the solar dynamo,
it does not have sufficient viewing of the polar regions to achieve
POLARIS’s primary objective: determining the relation between the
magnetism and dynamics of the Sun’s polar regions and the solar cycle.
Title: Theoretical Modeling of Propagation of Magnetoacoustic Waves
in Magnetic Regions Below Sunspots
Authors: Khomenko, E.; Kosovichev, A.; Collados, M.; Parchevsky, K.;
Olshevsky, V.
Bibcode: 2009ApJ...694..411K
Altcode: 2008arXiv0809.0278K
We use two-dimensional numerical simulations and eikonal approximation
to study properties of magnetohydrodynamic (MHD) waves traveling below
the solar surface through the magnetic structure of sunspots. We
consider a series of magnetostatic models of sunspots of different
magnetic field strengths, from 10 Mm below the photosphere to the
low chromosphere. The purpose of these studies is to quantify the
effect of the magnetic field on local helioseismology measurements
by modeling waves excited by subphotospheric sources. Time-distance
propagation diagrams and wave travel times are calculated for models
of various field strengths and compared to the nonmagnetic case. The
results clearly indicate that the observed time-distance helioseismology
signals in sunspot regions correspond to fast MHD waves. The slow MHD
waves form a distinctly different pattern in the time-distance diagram,
which has not been detected in observations. The numerical results are
in good agreement with the solution in the short-wavelength (eikonal)
approximation, providing its validation. The frequency dependence of
the travel times is in good qualitative agreement with observations.
Title: Nonlinear dynamical modeling of solar cycles using dynamo
formulation with turbulent magnetic helicity
Authors: Kitiashvili, I. N.; Kosovichev, A. G.
Bibcode: 2009GApFD.103...53K
Altcode: 2008arXiv0807.3192K
Variations of the sunspot number are important indicators of the
solar activity cycles. The sunspot formation is a result of a dynamo
process inside the Sun, which is far from being understood. We use
simple dynamical models of the dynamo process to simulate the magnetic
field evolution and investigate general properties of the sunspot
number variations during the solar cycles. We have found that the
classical Parker's model with a standard kinetic helicity quenching
cannot represent the typical profiles of the solar-cycle variations
of the sunspot number, and also does not give chaotic solutions. For
modeling of the solar cycle properties we use a nonlinear dynamo model
of Kleeorin and Ruzmaikin (1982), which takes into account dynamics of
the turbulent magnetic helicity. We have obtained a series of periodic
and chaotic solutions for different layers of the convective zone. The
solutions qualitatively reproduce some basic observational features of
the solar cycle properties, in particular, the relationship between
the growth time and the cycle amplitude. Also, on the longer time
scale the dynamo model with the magnetic helicity has intermittent
solutions, which may be important for modeling long-term variations
of the solar cycles.
Title: Division II: Sun and Heliosphere
Authors: Melrose, Donald B.; Martínez Pillet, Valentin; Webb, David
F.; van Driel-Gesztelyi, Lidia; Bougeret, Jean-Louis; Klimchuk,
James A.; Kosovichev, Alexander; von Steiger, Rudolf
Bibcode: 2009IAUTA..27...73M
Altcode:
Division II of the IAU provides a forum for astronomers and
astrophysicists studying a wide range of phenomena related to the
structure, radiation and activity of the Sun, and its interaction with
the Earth and the rest of the solar system. Division II encompasses
three Commissions, 10, 12 and 49, and four Working Groups.
Title: Photospheric and Subphotospheric Dynamics of Emerging
Magnetic Flux
Authors: Kosovichev, A. G.
Bibcode: 2009odsm.book..175K
Altcode:
Magnetic fields emerging from the Sun's interior carry information
about physical processes of magnetic field generation and transport
in the convection zone. Soon after appearance on the solar surface the
magnetic flux gets concentrated in sunspot regions and causes numerous
active phenomena on the Sun. This paper discusses some properties of
the emerging magnetic flux observed on the solar surface and in the
interior. A statistical analysis of variations of the tilt angle
of bipolar magnetic regions during the emergence shows that the
systematic tilt with respect to the equator (the Joy's law) is most
likely established below the surface. However, no evidence of the
dependence of the tilt angle on the amount of emerging magnetic flux,
predicted by the rising magnetic flux rope theories, is found. Analysis
of surface plasma flows in a large emerging active region reveals strong
localized upflows and downflows at the initial phase of emergence but
finds no evidence for large-scale flows indicating future appearance
a large-scale magnetic structure. Local helioseismology provides
important tools for mapping perturbations of the wave speed and mass
flows below the surface. Initial results from SOHO/MDI and GONG reveal
strong diverging flows during the flux emergence, and also localized
converging flows around stable sunspots. The wave speed images obtained
during the process of formation of a large active region, NOAA 10488,
indicate that the magnetic flux gets concentrated in strong field
structures just below the surface. Further studies of magnetic flux
emergence require systematic helioseismic observations from the ground
and space, and realistic MHD simulations of the subsurface dynamics.
Title: Solar Dynamo and Magnetic Self-Organization
Authors: Kosovichev, A. G.; Arlt, R.; Bonanno, A.; Brandenburg,
A.; Brun, A. S.; Busse, F.; Dikpati, M.; Hill, F.; Gilman, P. A.;
Nordlund, A.; Ruediger, G.; Stein, R. F.; Sekii, T.; Stenflo, J. O.;
Ulrich, R. K.; Zhao, J.
Bibcode: 2009astro2010S.160K
Altcode:
No abstract at ADS
Title: Signatures of emerging subsurface structures in the sun
Authors: Hartlep, T.; Kosovichev, A. G.; Zhao, J.; Mansour, N. N.
Bibcode: 2009arXiv0901.4585H
Altcode:
The complex dynamics that lead to the emergence of active regions on the
sun are poorly understood. One possibility is that magnetic structures
(flux tubes, etc.) rise from below the surface by self induction and
convection that lead to the formation of active regions and sunspots
on the solar surface. For space weather forecasting, one would like
to detect the subsurface structures before they reach the surface. The
goal of this study is to investigate whether sound speed perturbations
associated with subsurface structures could affect the acoustic power
observed at the solar surface above them. Possible mechanisms for
this effect are wave reflection, scattering or diffraction. By using
numerical simulations of wave propagation in the solar interior, we
investigate whether observations of the acoustic power can be used to
detect emerging active regions before they appear on the surface. In
the simulations, subsurface structures are modeled as regions with
enhanced or reduced acoustic wavespeed. We show how the acoustic power
above a subsurface region depends on the sign, depth and strength of
the wavespeed perturbation. For comparison, we analyze observations
from SOHO/MDI of the emergence of solar active region NOAA 10488.
Title: Commission 12: Solar Radiation and Structure
Authors: Martínez Pillet, Valentin; Kosovichev, Alexander;
Mariska, John T.; Bogdan, Thomas J.; Asplund, Martin; Cauzzi, Gianna;
Christensen-Dalsgaard, Jørgen; Cram, Lawrence E.; Gan, Weiqun; Gizon,
Laurent; Heinzl, Petr; Rovira, Marta G.; Venkatakrishnan, P.
Bibcode: 2009IAUTA..27..104M
Altcode:
Commission 12 encompasses investigations on the internal structure
and dynamics of the Sun, mostly accessible through the techniques of
local and global helioseismology, the quiet solar atmosphere, solar
radiation and its variability, and the nature of relatively stable
magnetic structures like sunspots, faculae and the magnetic network. A
revision of the progress made in these fields is presented. For some
specific topics, the review has counted with the help of experts
outside the Commission Organizing Committee that are leading and/or
have recently presented relevant works in the respective fields. In
this cases the contributor's name is given in parenthesis.
Title: Validating Time-Distance Far-Side Imaging of Solar Active
Regions through Numerical Simulations
Authors: Hartlep, Thomas; Zhao, Junwei; Mansour, Nagi N.; Kosovichev,
Alexander G.
Bibcode: 2008ApJ...689.1373H
Altcode: 2008arXiv0805.0472H
Far-side imaging using time-distance helioseismology methods is
assessed using numerically generated artificial data. The data are
generated using direct numerical simulations of acoustic oscillations
in a spherical solar model. Localized variations of the sound speed in
the surface and subsurface layers are used to model the perturbations
associated with sunspots and active regions. The accuracy of acoustic
travel-time far-side maps is shown to depend on the size and location of
active regions. Potential artifacts in the far-side imaging procedure,
such as those caused by the presence of active regions on the solar
near side, are also investigated.
Title: Tilt of Emerging Bipolar Magnetic Regions on the Sun
Authors: Kosovichev, A. G.; Stenflo, J. O.
Bibcode: 2008ApJ...688L.115K
Altcode:
Magnetic fields emerging from the Sun's interior carry information about
the physical processes of magnetic field generation and transport in the
convection zone. A statistical analysis of variations of the tilt angle
of bipolar magnetic regions during the emergence, observed from SOHO
MDI, shows that the systematic tilt with respect to the equator (Joy's
law) is established by the middle of the emergence period. This suggests
that the tilt is most likely generated below the surface. However,
the data do not show evidence of a dependence of the tilt angle on
the amount of flux or a relaxation of the bipolar orientation toward
the east-west direction, in contrast to the predictions of the rising
magnetic flux rope theories.
Title: Using Data Assimilation Methods for Modeling and Predicting
Solar Activity Cycles
Authors: Kitiashvili, I. N.; Kosovichev, A. G.
Bibcode: 2008AGUFMSH13A1506K
Altcode:
Modern data assimilation methods allow us to adapt a model to
observations by estimating the true state of a system and taking into
account uncertainties in the data and the model. The Ensemble Kalman
Filter (EnKF) method provides an effective data assimilation for
models of nonlinear dynamics. It is based on analysis of an ensemble
of model solutions. We implement the EnKF method for modeling the
11-year sunspot number variations. Using this approach we propose a
new physics-based method for predicting for the strength of the solar
sunspot cycles. For the initial modeling of the sunspot number we use
a dynamo model of Kleeorin and Ruzmaikin dynamo model in a low-mode
approximation. The model includes the Parker's dynamo equations
and an equation for conservation of the magnetic helicity. Also, we
accept Bracewell's suggestion to relate the toroidal magnetic field,
B, to the sunspot number, W,in the form of a three-halfs law: W ~
B3/2. We investigate non-linear solutions of the dynamo model and find
periodic and chaotic solutions for the convection zone parameters, which
represent basic properties of the solar cycles, such as the mean profile
of solar cycle and the relationship between the cycle amplitude and the
growth and decay times. By applying the EnKF method to the non-linear
periodic solutions we reproduce the annual variations of the sunspot
number and investigate the predictive capabilities. For testing we
calculate forecasts for the 10 previous cycles and find a reasonable
agreement with the observations. The calculations of the forecast of the
upcoming solar cycle 24 indicate that this cycle will be weaker than
the previous one, with the maximum sunspot number of about 80. This
investigation shows that data assimilation methods may be useful for
evaluating solar dynamo models and for forecasting solar activity.
Title: High-Resolution Helioseismology from Hinode
Authors: Kosovichev, A. G.; Zhao, J.; Sekii, T.; Nagashima, K.;
Mitra-Kraev, U.
Bibcode: 2008AGUFMSH41B1627K
Altcode:
The Solar Optical Telescope (SOT) on the Hinode space mission provides
unique multi-wavelength high-resolution data for local helioseismic
diagnostics of the sub-photospheric structure and dynamics of the
Sun. The helioseismology data from Hinode have allowed us for the
first time to observe oscillations of very high angular degree and
high frequencies. The Hinde data provide a potential for substantial
improvement of the spatial resolution of time-distance helioseismology
in near-surface layers of the Sun, compared to the previous SOHO/MDI
data. The Hinode data have also provided important insight in the nature
of sunspot oscillations. Simultaneous observations of solar oscillations
in two different spectral interval have allowed us to investigate the
mode physics and the correlated component of stochastic excitation. In
addition, a new type of flare-excited MHD oscillations was detected
from Hinode observations of the solar flare of December 13, 2006.
Title: Helioseismic Measurement of Subsurface Flows at Solar High
Latitude
Authors: Zhao, J.; Kosovichev, A. G.; Sekii, T.
Bibcode: 2008AGUFMSH44A..03Z
Altcode:
The solar polar magnetic field is of great interest as it is where
solar magnetic field reversal starts. Surface and subsurface plasma
flows are very important in understanding the field reversal because
magnetic field flux is transported to solar high latitude from lower
latitude according to flux transport theory. Local helioseismology
has been able to derive subsurface flow fields, rotation rates,
and meridional flows up to 30 Mm in depth. The results obtained by
time-distance helioseismology during Solar Cycle 23 from SOHO/MDI have
also revealed significant changes of the speed and the longitudinal
structure of the flows. We used these measurements to compare with the
magnetic flux transport determined from the magnetic field synoptic
data. Furthermore, by use of MDI dynamic campaign observations and
a recent high resolution observation of solar South Pole by Hinode,
we explore the possibility to detect subsurface flow fields in solar
high latitude.
Title: Application of Data Assimilation Method for Predicting
Solar Cycles
Authors: Kitiashvili, I.; Kosovichev, A. G.
Bibcode: 2008ApJ...688L..49K
Altcode: 2008arXiv0807.3284K
Despite the known general properties of the solar cycles, a reliable
forecast of the 11 yr sunspot number variations is still a problem. The
difficulties are caused by the apparent chaotic behavior of the
sunspot numbers from cycle to cycle and by the influence of various
turbulent dynamo processes, which we are far from understanding. For
predicting the solar cycle properties we make an initial attempt to use
the Ensemble Kalman Filter (EnKF), a data assimilation method, which
takes into account uncertainties of a dynamo model and measurements, and
allows us to estimate future observational data. We present the results
of forecasting of the solar cycles obtained by the EnKF method in
application to a low-mode nonlinear dynamical system modeling the solar
α Ω -dynamo process with variable magnetic helicity. Calculations
of the predictions for the previous sunspot cycles show a reasonable
agreement with the actual data. This forecast model predicts that the
next sunspot cycle will be significantly weaker (by ~30%) than the
previous cycle, continuing the trend of low solar activity.
Title: Realistic Numerical Simulations of Solar Convection and
Oscillations in Magnetic Regions
Authors: Jacoutot, L.; Kosovichev, A. G.; Wray, A.; Mansour, N. N.
Bibcode: 2008ApJ...684L..51J
Altcode:
We have used 3D, compressible, nonlinear radiative magnetohydrodynamics
simulation to study the influence of magnetic fields of various
strengths on convective cells and on the excitation mechanisms of
acoustic oscillations by calculating the spectral properties of the
convective motions and oscillations. The results reveal substantial
changes of the granulation structure with increased magnetic field and
a frequency-dependent reduction in the oscillation power in a good
agreement with solar observations. These simulations suggest that
the enhanced high-frequency acoustic emission at the boundaries of
active regions ("acoustic halo" phenomenon) is caused by changes of
the spatial-temporal spectrum of turbulent convection in a magnetic
field, resulting in turbulent motions of smaller scales and higher
frequencies than in quiet-Sun regions.
Title: Numerical Simulation of Excitation of Solar Oscillation Modes
for Different Turbulent Models
Authors: Jacoutot, L.; Kosovichev, A. G.; Wray, A. A.; Mansour, N. N.
Bibcode: 2008ApJ...682.1386J
Altcode: 2007arXiv0710.2317J
The goal of this research is to investigate how well various turbulence
models can describe the physical properties of the upper convective
boundary layer of the Sun. Accurate modeling of these turbulent
motions is necessary for understanding the excitation mechanisms of
solar oscillation modes. We have carried out realistic numerical
simulations using a hyperviscosity approach and various physical
Large-Eddy Simulation (LES) models (Smagorinsky and dynamic models)
to investigate how the differences in turbulence modeling affect
the damping and excitation of the oscillations and their spectral
properties and to compare with observations. We have first calculated
the oscillation power spectra of radial and nonradial modes supported
by the computational box with the different turbulence models, followed
by calculation of the work integral input to the modes to estimate
the influence of the turbulence model on the depth and strength of
the oscillation sources. We have compared these results with previous
studies and with the observed properties of solar oscillations. We
find that the dynamic turbulence model provides the best agreement
with the helioseismic observations.
Title: A solar mean field dynamo benchmark
Authors: Jouve, L.; Brun, A. S.; Arlt, R.; Brandenburg, A.; Dikpati,
M.; Bonanno, A.; Käpylä, P. J.; Moss, D.; Rempel, M.; Gilman, P.;
Korpi, M. J.; Kosovichev, A. G.
Bibcode: 2008A&A...483..949J
Altcode:
Context: The solar magnetic activity and cycle are linked to an
internal dynamo. Numerical simulations are an efficient and accurate
tool to investigate such intricate dynamical processes.
Aims:
We present the results of an international numerical benchmark
study based on two-dimensional axisymmetric mean field solar dynamo
models in spherical geometry. The purpose of this work is to provide
reference cases that can be analyzed in detail and that can help in
further development and validation of numerical codes that solve such
kinematic problems.
Methods: The results of eight numerical
codes solving the induction equation in the framework of mean field
theory are compared for three increasingly computationally intensive
models of the solar dynamo: an αΩ dynamo with constant magnetic
diffusivity, an αΩ dynamo with magnetic diffusivity sharply varying
with depth and an example of a flux-transport Babcock-Leighton dynamo
which includes a non-local source term and one large single cell of
meridional circulation per hemisphere. All cases include a realistic
profile of differential rotation and thus a sharp tachocline.
Results: The most important finding of this study is that all codes
agree quantitatively to within less than a percent for the αΩ dynamo
cases and within a few percent for the flux-transport case. Both
the critical dynamo numbers for the onset of dynamo action and the
corresponding cycle periods are reasonably well recovered by all
codes. Detailed comparisons of butterfly diagrams and specific cuts of
both toroidal and poloidal fields at given latitude and radius confirm
the good quantitative agreement.
Conclusions: We believe that
such a benchmark study will be a very useful tool since it provides
detailed standard cases for comparison and reference.
Title: Effects of Solar Active Regions on Meridional Flows
Authors: Švanda, Michal; Kosovichev, Alexander G.; Zhao, Junwei
Bibcode: 2008ApJ...680L.161S
Altcode: 2008arXiv0805.1789S
The aim of this Letter is to extend our previous study of the
solar-cycle variations of meridional flows and to investigate their
latitudinal and longitudinal structure in the subphotospheric layer,
especially their variations in magnetic regions. Helioseismology
observations indicate that mass flows around active regions are
dominated by inflows into those regions. On average, those local
flows are more important around the leading magnetic polarities of
active regions than around the following polarities and depend on
the evolutionary stage of particular active regions. We present a
statistical study based on MDI/SOHO observations of 1996-2002 and show
that this effect explains a significant part of the cyclic change
of meridional flows in near-equatorial regions, but not at higher
latitudes. A different mechanism driving solar-cycle variations of
the meridional flow probably operates.
Title: Influence of Nonuniform Distribution of Acoustic Wavefield
Strength on Time-Distance Helioseismology Measurements
Authors: Parchevsky, Konstantin V.; Zhao, Junwei; Kosovichev,
Alexander G.
Bibcode: 2008ApJ...678.1498P
Altcode: 2008arXiv0802.3866P
By analyzing numerically simulated solar oscillation data we study
the influence of nonuniform distribution of acoustic wave amplitude,
acoustic source strength, and perturbations of the sound speed on
the shifts of acoustic travel times measured by the time-distance
helioseismology method. It is found that for short distances, the
contribution to the mean travel-time shift caused by nonuniform
distribution of acoustic sources in sunspots may be comparable to
(but smaller than) the contribution from the sound-speed perturbation
in sunspots, and that it has the opposite sign to the sound-speed
effect. This effect may cause some underestimation of the negative
sound-speed perturbations in sunspots just below the surface, which
was found in previous time-distance helioseismology inferences. This
effect cannot be corrected by artificially increasing the amplitude of
oscillations in sunspots. For large time-distance annuli, the nonuniform
distribution of wavefields does not have significant effects on the mean
travel times, and thus the sound-speed inversion results. The measured
travel-time differences, which are used to determine the mass flows
beneath sunspots, can also be systematically shifted by this effect,
but only by an insignificant magnitude.
Title: Study of the magnetoconvection and oscillations in the upper
convection zone by means of realistic numerical simulations
Authors: Jacoutot, L.; Kosovichev, A.; Mansour, N.; Wray, A.
Bibcode: 2008AGUSMSP21A..06J
Altcode:
Recent high-resolution data from the Hinode space mission have
shown that magnetic fields on the Sun are concentrated in small
high-intensity magnetic flux tubes in the intergranular lanes. Our
objective is to investigate how magnetic field affects the dynamics
of granular convection and excitation of solar oscillations by means
of realistic numerical simulations. We have used a 3D, compressible,
non-linear radiative magnetohydrodynamics code developed at the NASA
Ames Research Center. This code takes into account several physical
phenomena: compressible fluid flow in a highly stratified medium,
sub-grid scale turbulence models, radiative energy transfer between the
fluid elements, and a real-gas equation of state. We have studied the
influence of the magnetic field of various strength on the convective
cells and on the excitation mechanisms of the acoustic oscillations
by calculating spectral properties of the convective motions and
oscillations. The results reveal substantial changes of the granulation
structure with increased magnetic field, and a frequency-dependent
reduction in the oscillation power. The simulation results are compared
with Hinode observations.
Title: Solar convection and oscillations in magnetic regions
Authors: Jacoutot, L.; Kosovichev, A. G.; Wray, A.; Mansour, N. N.
Bibcode: 2008arXiv0805.3741J
Altcode:
The goal of this research is to investigate how magnetic field
affects the dynamics of granular convection and excitation of solar
oscillations by means of realistic numerical simulations. We have
used a 3D, compressible, non-linear radiative magnetohydrodynamics
code developed at the NASA Ames Research Center. This code takes
into account several physical phenomena: compressible fluid flow in a
highly stratified medium, sub-grid scale turbulence models, radiative
energy transfer between the fluid elements, and a real-gas equation of
state. We have studied the influence of the magnetic field of various
strength on the convective cells and on the excitation mechanisms of
the acoustic oscillations by calculating spectral properties of the
convective motions and oscillations. The results reveal substantial
changes of the granulation structure with increased magnetic field,
and a frequency-dependent reduction in the oscillation power in a good
agreement with solar observations. These simulations suggest that the
enhanced high-frequency acoustic emission at the boundaries of active
region ("acoustic halo" phenomenon) is caused by the changes of the
spatial-temporal spectrum of the turbulent convection in magnetic field,
resulting in turbulent motions of smaller scales and higher frequencies
than in quiet Sun regions.
Title: Initial Helioseismology Results from Hinode
Authors: Kosovichev, A.; Zhao, J.; Sekii, T.; Nagashima, K.
Bibcode: 2008AGUSMSP21A..01K
Altcode:
Solar Optical Telescope of Hinode provides unique multi-wavelength
high-resolution data for local helioseismic diagnostics of the
sub-surface structure and dynamics of the Sun. The helioseismology data
from Hinode have allowed us for the first time to observe oscillations
of very high angular degree and high frequencies, and substantially
improve the spatial resolution of time-distance helioseismology in
near-surface layers of the Sun, compared to the previous SOHO/MDI
data. The Hinode data have also provided important insight in the
nature of sunspot oscillations. Initial attempts have been made to
investigate the dynamics of the polar regions, previously unaccessible
for helioseismology. We present the first results and discuss the
potential and perspective of the Hinode helioseismology program.
Title: Development of Time-Distance Helioseismology Data Analysis
Pipeline for SDO/HMI
Authors: Duvall, T. L.; Zhao, J.; Couvidat, S.; Parchevsky, K. V.;
Beck, J.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 2008AGUSMSP51B..15D
Altcode:
The Helioseismic and Magnetic Imager of SDO will provide uninterrupted
4kx4k-pixel Doppler-shift images of the Sun with ~45 sec cadence. These
data will have a unique potential for advancing local helioseismic
diagnostics of the Sun's interior structure and dynamics. They
will help to understanding the basics mechanisms of solar activity
and developing predictive capabilities for the NASA's Living with
a Star Program. Because of the tremendous amount of data the HMI
team is developing a data analysis pipeline, which will provide maps
of subsurface flows and sound-speed distributions inferred from the
Doppler data by the time-distance technique. We discuss the development
plan, methods and algorithms, and present the status of the pipeline,
testing results and examples of the data products.
Title: First Detection of MHD Oscillations Excited by Solar Flare
in Sunspot Umbra
Authors: Kosovichev, A.; Sekii, T.
Bibcode: 2008AGUSMSP21B..07K
Altcode:
We report on detection of a new type of flare-excited oscillations
from Hinode observations of the solar flare of December 13, 2006. The
oscillations observed in Ca II H images appeared in the sunspot
umbra immediately after the impulsive phase of the flare. They had
the amplitude 2-4 times larger than the pre-flare oscillations in
the umbra. Also, their frequency seemed to be higher. There is an
evidence that during the first 30-40 min the oscillations represent
waves traveling through the umbra in the direction away from the flare
ribbon with a speed of 50-100 km/s. Then, the oscillation become more
irregular with some occasional wave packets. The lifetime of these
oscillations is probable about 8 hours. The estimated speed indicates
that the waves are of an MHD type, and if their speed is of the order
of magnitude of the Alfven speed then they should propagate rather low
in the sunspot chromosphere. Sunspot oscillations have been studied
intensively for many years but the Hinode observations are the first
that show enhanced oscillations in the umbra, associated with a solar
flare. Further investigations of these oscillations are of great
interest for understanding the processes in solar flares and sunspots.
Title: Modeling of Helioseismic MHD Waves in Presence of Inclined
Magnetic Field.
Authors: Parchevsky, K. V.; Kosovichev, A. G.
Bibcode: 2008AGUSMSH54A..02P
Altcode:
Studying of propagation, conversion, and scattering of MHD waves in
the Sun is very important for developing robust local helioseismology
techniques and diagnostics of subsurface magnetic fields. We use
3D numerical simulations to study propagation of MHD waves from
wave sources of different types (vertical force and pressure) in
presence of uniform and non-uniform inclined magnetic field, and for
an equilibrium sunspot model. Numerical simulations show that presence
of the background magnetic field significantly alters properties of
the surface gravity f-modes, but has substantially smaller effect on
p-modes. When the wave source is located in a strong field region it
generates Alfven waves. The Alfven waves do not appear if the wave
source is located in the region without magnetic field, and waves
propagate from that region into the region with magnetic field without
significant transformation. The wave travel times obtained from cross
covariance of the p-mode line-of-sight velocities at the observation
point and the source point show variation of about 1 min along the wave
front. Due to asymmetry, average of travel times along the wave front
is not zero and show variations of about a few seconds. Comparison of
the simulations with the time-distance helioseismology results from
SOHO/MDI shows that the observed variations of acoustic travel times
obtained from the Doppler velocity measurements are not caused by the
inclined magnetic field effects contrary to previous suggestions.
Title: Application of Data Assimilation Methods to Non-Linear Dynamo
Models of Solar Cycle
Authors: Kitiashvili, I.; Kosovichev, A.
Bibcode: 2008AGUSMSP23A..02K
Altcode:
Solar dynamo is a very complicated non-linear oscillatory MHD process,
which is far from understanding. It produces 11-year sunspot cycles,
which show chaotic behavior and are hard to predict. It has been
suggested that the basic oscillatory behavior of the solar dynamo can
be described in terms of simple non-linear dynamical systems. The data
assimilation approach developed in meteorology and Earth science makes
possible efficient and accurate estimations of physical properties,
which cannot be observed directly. The applications of data assimilation
to non-linear dynamo models for modeling and predicting the solar
cycle are discussed in this presentation.
Title: Time-Distance Helioseismic Image of Solar Tachocline Area
Authors: Zhao, J.; Hartlep, T.; Kosovichev, A. G.; Mansour, N. N.
Bibcode: 2008AGUSMSP21A..02Z
Altcode:
Solar tachocline is an area located at the bottom of solar convection
zone. It is also a location where sound speed exhibits a bump
relative to the standard solar model, and where solar rotation has a
large shear. Tachocline is generally believed as the location where
solar dynamo operates, therefore, it is important and interesting
to image its structure, as well as its structure evolution with
solar cycle. Using both surface- and deep-focusing time-distance
helioseismology techniques, we have designed measurement schemes and
developed inversion codes to derive sound speed perturbations at the
tachocline area. Those codes are tested on numerically simulated data,
which simulates the global acoustic wavefields with an artificial
tachocline model. After getting some satisfactory results from
simulation data, we apply the technique on SOHO/MDI medium-l data,
and study the acoustic structure of solar interior, as well as its
evolution with the solar cycle.
Title: Properties of high-degree oscillation modes of the Sun observed
with Hinode/SOT
Authors: Mitra-Kraev, U.; Kosovichev, A. G.; Sekii, T.
Bibcode: 2008A&A...481L...1M
Altcode: 2007arXiv0711.2210M
Aims:With the Solar Optical Telescope on Hinode, we investigate
the basic properties of high-degree solar oscillations observed at
two levels in the solar atmosphere, in the G-band (formed in the
photosphere) and in the Ca II H line (chromospheric emission).
Methods: We analyzed the data by calculating the individual power
spectra as well as the cross-spectral properties, i.e., coherence and
phase shift. The observational properties are compared with a simple
theoretical model, which includes the effects of correlated noise.
Results: The results reveal significant frequency shifts between the
Ca II H and G-band spectra, in particular above the acoustic cut-off
frequency for pseudo-modes. The cross-spectrum phase shows peaks
associated with the acoustic oscillation (p-mode) lines, and begins to
increase with frequency around the acoustic cut-off. However, we find
no phase shift for the (surface gravity wave) f-mode. The observed
properties for the p-modes are qualitatively reproduced in a simple
model with a correlated background if the correlated noise level in
the Ca II H data is higher than in the G-band data. These results
suggest that multi-wavelength observations of solar oscillations,
in combination with the traditional intensity-velocity observations,
may help to determine the level of the correlated background noise
and to determine the type of wave excitation sources on the Sun.
Title: Solving the Discrepancy between the Seismic and Photospheric
Solar Radius
Authors: Haberreiter, M.; Schmutz, W.; Kosovichev, A. G.
Bibcode: 2008ApJ...675L..53H
Altcode:
Two methods are used to observationally determine the solar radius:
One is the observation of the intensity profile at the limb; the other
one uses f-mode frequencies to derive a "seismic" solar radius which is
then corrected to optical depth unity. The two methods are inconsistent
and lead to a difference in the solar radius of ~0.3 Mm. Because of the
geometrical extension of the solar photosphere and the increased path
lengths of tangential rays the Sun appears to be larger to an observer
who measures the extent of the solar disk. Based on radiative transfer
calculations we show that this discrepancy can be explained by the
difference between the height at disk center where τ5000 =
1 (τRoss = 2/3) and the inflection point of the intensity
profile on the limb. We calculate the intensity profile of the limb
for the MDI continuum and the continuum at 5000 Å for two atmosphere
structures and compare the position of the inflection points with
the radius at τ5000 = 1 (τRoss = 2/3). The
calculated difference between the seismic radius and the inflection
point is 0.347 +/- 0.006 Mm with respect to τ5000 = 1,
and 0.333 +/- 0.008 Mm with respect to τRoss = 2/3. We
conclude that the standard solar radius in evolutionary models has to
be lowered by 0.333 +/- 0.008 Mm and is 695.66 Mm. Furthermore, this
correction reconciles inflection point measurements and the seismic
radii within the uncertainties.
Title: Modeling Non-Uniform Distribution of Acoustic Sources and
Wave Leakage in Sunspots
Authors: Parchevsky, K. V.; Kosovichev, A. G.
Bibcode: 2008ASPC..383..289P
Altcode:
Observations show suppression of the amplitude of 5-min oscillations
in sunspots. We developed a 3D numerical simulation code to model
wave excitation and propagation in the upper convection zone and the
atmosphere of the Sun. We model how suppression of acoustic sources
affects the oscillation amplitude in sunspot regions. The calculations
show that this suppression (due to strong magnetic field) significantly
reduces the oscillation amplitude to a level comparable with the
observed amplitude deficit. The precise value of the amplitude ratio
outside and inside sunspots depends on the rate of wave leakage and
damping in the lower atmosphere. We present the results of detailed
investigation of this effect, including modeling of wave damping at
various heights in the atmosphere and the frequency dependence of the
amplitude ratio. These results show the importance of the wave energy
leakage through the atmospheric layers of sunspots.
Title: Probing solar and stellar interior dynamics and dynamo
Authors: Kosovichev, Alexander G.
Bibcode: 2008AdSpR..41..830K
Altcode:
Solar and stellar activity is a result of complex interaction between
magnetic field, turbulent convection and differential rotation in
a star’s interior. Magnetic field is believed to be generated by
a dynamo process in the convection zone. It emerges on the surface
forming sunspots and starspots. Localization of the magnetic spots and
their evolution with the activity cycle is determined by large-scale
interior flows. Thus, the internal dynamics of the Sun and other
stars hold the key to understanding the dynamo mechanism and activity
cycles. Recently, significant progress has been made for modeling
magnetohydrodynamics of the stellar interiors and probing the internal
rotation and large-scale dynamics of the Sun by helioseismology. Also,
asteroseismology is beginning to probe interiors of distant stars. I
review key achievements and challenges in our quest to understand the
basic mechanisms of solar and stellar activity.
Title: Local Helioseismology and Magnetic Flux Emergence
Authors: Kosovichev, A. G.; Duvall, T. L., Jr.
Bibcode: 2008ASPC..383...59K
Altcode:
Investigations of emerging magnetic flux are important for understanding
the basic properties of solar magnetism (such as the depth of the solar
dynamo processes and ``nests'' of solar activity, formation of sunspots
and active regions, organization of solar activity on various spatial
and temporal scales), and also for forecasting solar activity and space
weather. Local helioseismology is capable of detecting emerging magnetic
flux in the solar interior, and determining variations of the sound
speed and large-scale flows caused by the emerging flux. The initial
results obtained by time-distance helioseismology for large emerging
active regions reveal unexpected properties of the flux emergence and
challenge the current theories and models. In this paper, we present
results for AR 10488, which was observed from SOHO/MDI in October
2003. In particular, it is found that the magnetic flux propagates
very rapidly in the upper convective zone. The active regions are a
result of multiple flux emergence events, occurring in the region of
the Sun during a period at least several days long. The emergence
is accompanied by strong localized shearing outflows. However, no
large-scale diverging flow pattern or significant upflows are detected
prior to the emergence. The initial analysis shows that it is necessary
to develop special local helioseismology methodology and theoretical
models for studying fast dynamical processes associated with magnetic
flux emergence.
Title: Solar Polar Imager: Observing Solar Activity from a New
Perspective
Authors: Liewer, P. C.; Ayon, J.; Alexander, D.; Kosovichev, A.;
Mewaldt, R. A.; Socker, D. G.; Vourlidas, A.
Bibcode: 2008nssv.book....1L
Altcode:
No abstract at ADS
Title: Realistic Numerical Simulations of Solar Magneto-Convection
and Oscillations
Authors: Jacoutot, L.; Kosovichev, A. G.; Mansour, N. N.; Wray, A.
Bibcode: 2007AGUFMSH23A1167J
Altcode:
The objective of this research is to study how magnetic field affects
the structure and dynamics of solar convection and the sources that
drive the waves in the Sun. We use a 3D, compressible, non-linear
radiative magnetohydrodynamics code developed by Dr. A. Wray for
simulating the upper solar photosphere and lower atmosphere. This
code takes into account several physical phenomena: compressible
fluid flow in a highly stratified medium, radiative energy transfer
between the fluid elements, magnetic phenomena, and a real-gas equation
of state. Magnetic fields play a crucial role in the structure and
heating of the outer atmosphere. Our objectives are to understand the
processes related to magnetic fields in this zone. We investigate the
interaction between convection, magnetic fields, and oscillations.
Title: Imaging Solar Farside and Tachocline Using SOHO/MDI Data and
Numerical Simulations
Authors: Zhao, J.; Hartlep, T.; Kosovichev, A. G.; Mansour, N. N.
Bibcode: 2007AGUFMSH32A0791Z
Altcode:
To be able to see large solar active regions when they are located
on the farside of the Sun before rotating into the earth side is of
great importance for space weather forecast. By analyzing acoustic wave
signals that are reflected back to the near-side from the farside after
four and five skips, we are able to map the real-time farside active
regions with good signal to noise ratio by use of SOHO/MDI medium-l
observations. This technique is validated by employing numerical data
that simulate the solar global acoustic wavefields. Solar tachocline
is generally believed as the location where solar dynamo operates. We
have developed a time-distance helioseismology code to measure and
invert sound speed perturbations at the tachocline area, and also
tested the code by use of numerical simulation data. The technique is
then used on SOHO/MDI medium-l observations to map the evolution of
the tachocline from 1996 through 2007.
Title: Validation of Far-side Imaging of Solar Active Regions through
Numerical Simulations
Authors: Hartlep, T.; Zhao, J.; Mansour, N. N.; Kosovichev, A. G.
Bibcode: 2007AGUFMSH32A0790H
Altcode:
Helioseismology provides important tools for understanding the solar
interior as well as for space weather forecast. Using observation
data from instruments such as MDI (Michelson Doppler Imager) aboard
the SOHO (Solar and Heliospheric Observatory) spacecraft, helioseismic
inferences have tremendously advanced our knowledge and understanding
of the interior structure and dynamics of the Sun. In general, the
methods used for analyzing observations are based on simplified models
of wave propagation such as ray or Born approximation, but have not been
validated by more sophisticated models or numerical simulations. Here,
we evaluate one technique, far-side imaging of solar active region by
time-distance helioseismology, by using artificial oscillation data
derived from numerical simulations of wave propagation in the Sun. The
simulations are performed for the full spherical Sun. Active regions
are modeled by locally modifying the speed of sound. We evaluate the
performance of the far-side imaging technique by varying the size and
location of the artificial active region, and analyse the appearance of
ghost images and artifacts. This research is supported by NASA's Living
with a Star program. The support ot the NASA Postdoctoral Program
administered by Oak Ridge Associated Universities is gratefully
acknowledged. Simulations have been performed on the Columbia
Supercomputer at NASA Ames Research Center.
Title: Numerical Simulations of MHD Waves in the Sun
Authors: Parchevsky, K. V.; Kosovichev, A. G.
Bibcode: 2007AGUFMSH31A0220P
Altcode:
Investigation of propagation, conversion, and scattering of MHD
waves in the Sun is very important for understanding the mechanisms
of transformation of the acoustic waves into different types of MHD
waves. Such studying is also an essential part of developing robust
local helioseismology techniques and diagnostics of subsurface magnetic
fields. In this studying we investigate excitation, propagation, and
conversion of different kinds of MHD waves in presence of the background
inclined magnetic field. Waves are generated by the localized force
and pressure sources with different frequencies placed at different
depths. We have developed a complete linear 3D MHD numerical model
to investigate influence of the magnetic field on wave properties and
helioseismology measurements in realistic solar conditions. The results
show that the magnetic field effects can substantially change the
properties of the surface gravity waves (f-mode), but their influence
on the acoustic-type waves (p-modes) is rather moderate. We find that
magnetic field can lead to a collimation of the wave front. We also
observe formation of Alfven waves in our simulations. The numerical
modeling and new data from the HMI instrument on SDO will substantially
advance our knowledge of the wave interaction with magnetic fields on
the Sun and improve the local helioseismology diagnostics.
Title: Initial Observations of Sunspot Oscillations Excited by
Solar Flare
Authors: Kosovichev, A. G.; Sekii, T.
Bibcode: 2007ApJ...670L.147K
Altcode: 2007arXiv0710.1808K
Observations of a large solar flare on 2006 December 13 using Solar
Optical Telescope (SOT) on board the Hinode spacecraft revealed
high-frequency oscillations excited by the flare in the sunspot
chromosphere. These oscillations are observed in the region of strong
magnetic field of the sunspot umbra and may provide a new diagnostic
tool for probing the structure of sunspots and understanding physical
processes in solar flares.
Title: Helioseismic Observations of Active Regions Below the Solar
Surface from SOHO/MDI
Authors: Kosovichev, A. G.; Duvall, T. L.
Bibcode: 2007AGUFMSH23A1168K
Altcode:
We apply the time-distance helioseismology technique to obtain 3D
tomographic images of sound-speed variations and mass flow velocity maps
below the visible surface of the Sun, for emerging and evolving magnetic
active regions. In particular, using uninterrupted helioseismology
observations from the MDI instrument on the SOHO spacecraft we
investigate the development of the large complex of activity NOAA
10484-10488, which produced a series of giant proton flares in October,
2003. The flow maps reveal new interesting properties, such as strong
divergent and shearing flows associated with the magnetic flux emergence
and flaring activity. Using the sound-speed image we attempt to find
the common roots and links of these remarkable active regions.
Title: Initial Helioseismic Observations by Hinode/SOT
Authors: Sekii, Takashi; Kosovichev, Alexander G.; Zhao, Junwei;
Tsuneta, Saku; Shibahashi, Hiromoto; Berger, Thomas E.; Ichimoto,
Kiyoshi; Katsukawa, Yukio; Lites, Bruce; Nagata, Shin'ichi; Shimizu,
Toshifumi; Shine, Richard A.; Suematsu, Yoshinori; Tarbell, Theodore
D.; Title, Alan M.
Bibcode: 2007PASJ...59S.637S
Altcode: 2007arXiv0709.1806S
Results from initial helioseismic observations by the Solar Optical
Telescope on-board Hinode are reported. It has been demonstrated
that intensity oscillation data from the Broadband Filter Imager
can be used for various helioseismic analyses. The k - ω power
spectra, as well as the corresponding time-distance cross-correlation
function, which promise high-resolution time-distance analysis below
the 6-Mm travelling distance, were obtained for G-band and CaII-H
data. Subsurface supergranular patterns were observed from our first
time-distance analysis. The results show that the solar oscillation
spectrum is extended to much higher frequencies and wavenumbers, and
the time-distance diagram is extended to much shorter travel distances
and times than were observed before, thus revealing great potential
for high-resolution helioseismic observations from Hinode.
Title: Observations of Sunspot Oscillations in G Band and CaII H
Line with Solar Optical Telescope on Hinode
Authors: Nagashima, Kaori; Sekii, Takashi; Kosovichev, Alexander G.;
Shibahashi, Hiromoto; Tsuneta, Saku; Ichimoto, Kiyoshi; Katsukawa,
Yukio; Lites, Bruce; Nagata, Shin'ichi; Shimizu, Toshifumi; Shine,
Richard A.; Suematsu, Yoshinori; Tarbell, Theodore D.; Title, Alan M.
Bibcode: 2007PASJ...59S.631N
Altcode: 2007arXiv0709.0569N
Exploiting high-resolution observations made by the Solar Optical
Telescope on board Hinode, we investigate the spatial distribution
of the power spectral density of the oscillatory signal in and around
the active region NOAA 10935. The G-band data show that in the umbra
the oscillatory power is suppressed in all frequency ranges. On
the other hand, in CaII H intensity maps oscillations in the umbra,
so-called umbral flashes, are clearly seen with the power peaking around
5.5mHz. The CaII H power distribution shows the enhanced elements with
the spatial scale of the umbral flashes over most of the umbra, but
there is a region with suppressed power at the center of the umbra. The
origin and property of this node-like feature remain unexplained.
Title: The Cause of Photospheric and Helioseismic Responses to Solar
Flares: High-Energy Electrons or Protons?
Authors: Kosovichev, A. G.
Bibcode: 2007ApJ...670L..65K
Altcode: 2007arXiv0710.0757K
Analysis of the hydrodynamic and helioseismic effects in the photosphere
during the solar flare of 2002 July 23, observed by MDI/SOHO, and
high-energy images from RHESSI show that these effects are closely
associated with sources of the hard X-ray emission but that no such
effects existed in the centroid region of the flare's gamma-ray
emission. These results demonstrate that, contrary to expectations,
these hydrodynamic and helioseismic responses (``sunquakes'') are
more likely to be caused by accelerated electrons than by high-energy
protons. A series of multiple impulses of high-energy electrons form
a hydrodynamic source that is moving in the photosphere at supersonic
speed. This moving source plays a critical role in the formation of
the anisotropic wave front of sunquakes.
Title: Speed of Meridional Flows and Magnetic Flux Transport on
the Sun
Authors: Švanda, Michal; Kosovichev, Alexander G.; Zhao, Junwei
Bibcode: 2007ApJ...670L..69S
Altcode: 2007arXiv0710.0590S
We use the magnetic butterfly diagram to determine the speed of the
magnetic flux transport on the solar surface toward the poles. The
manifestation of the flux transport is clearly visible as elongated
structures extended from the sunspot belt to the polar regions. The
slopes of these structures are measured and interpreted as meridional
magnetic flux transport speed. Comparison with the time-distance
helioseismology measurements of the mean speed of the meridional flows
at a depth of 3.5-12 Mm shows a generally good agreement, but the
speeds of the flux transport and the meridional flow are significantly
different in areas occupied by the magnetic field. The local circulation
flows around active regions, especially the strong equatorward flows
on the equatorial side of active regions, affect the mean velocity
profile derived by helioseismology but do not influence the magnetic
flux transport. The results show that the mean longitudinally averaged
meridional flow measurements by helioseismology may not be used directly
in solar dynamo models for describing the magnetic flux transport, and
that it is necessary to take into account the longitudinal structure
of these flows.
Title: Observations of Helioseismic Response to Flare energy-release
Events
Authors: Kosovichev, A. G.
Bibcode: 2007ASPC..369..325K
Altcode:
The energy release in solar flares may cause not only plasma eruptions
in the corona and heliosphere, but also may lead to excitation of
acoustic wave packets propagating inside the Sun. These waves are
observed on the solar surface as ripples propagating from the places
where high-energy particles hit the lower chromosphere. Observations of
such seismic event (called "sunquakes") provide important information
about properties of the high-energy particles accelerated during the
energy release events, their interaction with the plasma of the low
atmosphere, hydrodynamic and MHD processes in solar flares, and also
open perspectives for developing new helioseismic diagnostics. In
this paper, I review some recent observational results and discuss an
observing program for Solar-B to study such events.
Title: Effect of Suppressed Excitation on the Amplitude Distribution
of 5 Minute Oscillations in Sunspots
Authors: Parchevsky, K. V.; Kosovichev, A. G.
Bibcode: 2007ApJ...666L..53P
Altcode: 2007arXiv0707.2586P
Five minute oscillations on the Sun (acoustic and surface gravity waves)
are excited by subsurface turbulent convection. However, in sunspots
the excitation is suppressed because a strong magnetic field inhibits
convection. We use three-dimensional simulations to investigate how
the suppression of excitation sources affects the distribution of
the oscillation power in sunspot regions. The amplitude of random
acoustic sources was reduced in circular-shaped regions to simulate
the suppression in sunspots. The simulation results show that the
amplitude of the oscillations can be approximately 2-4 times lower in
the sunspot regions in comparison to the quiet Sun, just because of
the suppressed sources. Using SOHO MDI data we measured the amplitude
ratio for the same frequency bands outside and inside sunspots and
found that this ratio is approximately 3-4. Hence, the absence of
excitation sources inside sunspots makes a significant contribution
(about 50% or higher) to the observed amplitude ratio and must be
taken into account in sunspot seismology.
Title: Three-dimensional Numerical Simulations of the Acoustic Wave
Field in the Upper Convection Zone of the Sun
Authors: Parchevsky, K. V.; Kosovichev, A. G.
Bibcode: 2007ApJ...666..547P
Altcode: 2006astro.ph.12364P
Results of numerical three-dimensional (3D) simulations of propagation
of acoustic waves inside the Sun are presented. A linear 3D code
which utilizes the realistic OPAL equation of state was developed. A
modified convectively stable standard solar model with a smoothly joined
chromosphere was used as a background model. A high-order dispersion
relation-preserving numerical scheme was used. The top nonreflecting
boundary condition established in the chromosphere absorbs waves with
frequencies greater than the acoustic cutoff frequency which pass
through the chromosphere, simulating a realistic situation. We simulate
acousto-gravity wave fields on the Sun, generated by localized randomly
distributed sources in a subphotospheric layer. Three applications for
solar wave physics are presented: changes in oscillation properties due
to the mechanism of wave damping, effects of nonuniform distribution
of sources, and effects of nonuniform localized perturbations on
wave properties. In particular, we studied two models of wave damping
with leakage and with an explicit friction-type damping term in the
photospheric layers and chromosphere. In both cases we were able to
reproduce observed characteristics of the acoustic spectrum (line
widths and amplitude distribution). We found that the suppression of
acoustic sources, e.g., in sunspots, may significantly contribute to
the observed power deficit. The lower sound speed in sunspot areas
may cause an increase of the wave amplitude, but this effect is less
important for the acoustic power distribution than the suppression of
the acoustic sources.
Title: Magnetic Effect on Wavelike Properties of Solar
Supergranulation
Authors: Green, C. A.; Kosovichev, A. G.
Bibcode: 2007ApJ...665L..75G
Altcode:
Supergranular convective cells on the Sun rotate faster than surface
plasma or any other feature. Recent time-distance helioseismology
results suggest that supergranulation has properties of traveling
waves. We have suggested that these properties may be due to the steep
gradient of the subsurface shear layer. We used a linear model to
calculate the phase speeds of the unstable convective modes. These
phase speeds are greater than the speed of the surface plasma;
however, they are significantly lower than the observed speed of the
supergranular pattern. This suggests to us that the subsurface shear
layer is a plausible explanation for the wavelike behavior. We now
investigate the contribution of the magnetic field to the wavelike
behavior produced by the shear gradient, and we show that a horizontal
(toroidal) magnetic field in the subsurface layer will result in an
increase in the phase speed of the traveling convective modes. This
prediction can be checked by an analysis of the observational data.
Title: Joint Discussion 8 Solar and stellar activity cycles
Authors: Kosovichev, Alexander G.; Strassmeier, Klaus G.
Bibcode: 2007HiA....14..271K
Altcode:
The solar magnetic field and its associated atmospheric activity
exhibits periodic variations on a number of time scales. The 11-year
sunspot cycle and its underlying 22-year magnetic cycle are, besides
the 5-minute oscillation, the most widely known. Amplitudes and
periods range from a few parts per million (ppm) and 2 3 minutes for
p-modes in sunspots, a few 10 ppm and 10 minutes for the granulation
turn around, a few 100 ppm and weeks for the lifetime of plages and
faculae, 1000 ppm and 27 days for the rotational signal from spots, to
the long-term cycles of 90 yr (Gleissberg cycle), 200 - 300 yr (Wolf,
Spörer, Maunder minima), 2,400 yr from 14C tree-ring data,
and possibly in excess of 100,000 yr.
Title: Effects of Magnetism and Turbulent Viscosity on the Wave-like
Behaviour of Supergranulation
Authors: Green, Cristina; Kosovichev, A. G.
Bibcode: 2007AAS...210.4605G
Altcode: 2007BAAS...39..161G
Supergranular convective cells on the Sun rotate faster than surface
plasma or any other feature. Recent time-distance helioseismology
results suggest that supergranulation also has properties of travelling
waves. We had suggested that these properties may be due to the steep
gradient of the subsurface shear layer, and had calculated the phase
speeds of the unstable convective modes, using a linear model. These
phase speeds were greater than the speed of the surface plasma;
however, they were significantly lower than the observed speed of the
supergranular pattern. We now consider the contributions of viscosity
and magnetic field. Adding either of these to the previous linear
model can produce phase speeds matching the observations. We consider
different viscosity and magnetic field profiles, and investigate if
the observed phase speeds can be reproduced with realistic viscosity
or magnetic fields.
Title: Investigation of Emerging Active Regions by Time-Distance
Helioseismology
Authors: Kosovichev, Alexander G.; Duvall, T. L., Jr.
Bibcode: 2007AAS...210.4602K
Altcode: 2007BAAS...39R.160K
Prediction of emerging active regions and their evolution is one of the
central problems of local helioseismology. Previous investigations
showed that magnetic flux emerges very rapidly, so that it very
difficult to obtain tomographic images of the associated sound-speed
perturbations and flows in the interior before the first magnetic
field elements appear on the surface. We continue investigating this
problems by reducing the time intervals between the sound-speed
images and flow maps to 2 hours. The helioseismology results are
compared with the photospheric magnetograms and white-light images. In
particular, we present results of the investigation of the emergence
and development of large active region NOAA 10488 observed during the
SOHO/MDI Dynamics campaign in October 2003, compare the results with
helioseismic observations of other emerging active regions, and discuss
the potential of local helioseismology for forecasting emergence and
evolution of active regions.
Title: Validating Time-Distance Helioseismology by Use of Realistic
Simulations of Solar Convection
Authors: Zhao, Junwei; Georgobiani, D.; Kosovichev, A. G.; Benson,
D.; Stein, R. F.; Nordlund, A.
Bibcode: 2007AAS...210.2203Z
Altcode: 2007BAAS...39..124Z
Recent progress in realistic simulations of solar convection have
enabled us to evaluate the robustness of solar interior structures
and dynamics obtained by methods of local helioseismology. We
present results of testing the time-distance method using realistic
simulations. By computing acoustic wave propagation time and distance
relations for different depths of the simulated data, we confirm that
acoustic waves propagate into the interior and then turn back to the
photosphere. For the surface gravity waves (f-mode), we calculate
perturbations of their travel times, caused by localized downdrafts,
and demonstrate that the spatial pattern of these perturbations
(representing so-called sensitivity kernels) is similar to the
patterns obtained from the real Sun, displaying characteristic
hyperbolic structures. We then test the time-distance measurements
and inversions by calculating acoustic travel times from a sequence
of vertical velocities at the photosphere of the simulated data, and
inferring a mean 3D flow fields by performing inversion based on the
ray approximation. The inverted horizontal flow fields agree very well
with the simulated data in subsurface areas up to 3 Mm deep, but differ
in deeper areas. These initial tests provide important validation of
time-distance helioseismology measurements of supergranular-scale
convection, illustrate limitations of this technique, and provide
guidance for future improvements.
Title: Numerical Simulations of Solar Acoustic Waves in the Whole Sun
Authors: Hartlep, Thomas; Mansour, N. N.; Kosovichev, A. G.
Bibcode: 2007AAS...210.2204H
Altcode: 2007BAAS...39..125H
Helioseismology provides important tools for studying solar internal
structure and dynamics, and for space weather forecast. For testing
and correct interpretation of inversion results, as well as advancing
our general understanding of wave propagation in stars, numerical
simulations of wave propagation promise to be immensly valuable. We
present a new numerical method and code for studying the 3D propagation
of acoustic and MHD waves in the Sun as a full sphere. The waves are
excited randomly, and are propagated through a statitionary solar model
which includes localized structures such as models of sunspots. Here, we show results from simulations for structures with localized
variations of the sound speed compared with the averaged, quiet Sun. For
instance, we show how the oscillation power varies from the quiet Sun
model, and compare with observations. The simulation results also
provide artificial data for testing time-distance helioseismology
inferences including far-side imaging and probing the tachocline
structure. This research was supported by an appointment to the
NASA Postdoctoral Program at Ames Research Center, administered by
Oak Ridge Associated Universities through a contract with NASA.
Title: Study Of Excitation Mechanism Of Solar Oscillation Modes
Using Realistic Numerical Simulations
Authors: Jacoutot, Laetitia; Kosovichev, A. G.; Mansour, N. N.;
Wray, A.
Bibcode: 2007AAS...210.4603J
Altcode: 2007BAAS...39..161J
Investigation of the excitation mechanisms of solar oscillations is
important for understanding the interaction between turbulence and
waves in the Sun and for developing helioseismology diagnostics. The objective of this research is to study the sources which drive the
waves in the Sun by means of realistic numerical simulations. We used
a 3D, compressible, non-linear radiative-hydrodynamics code developed
by Wray et al. (2005) for simulating the upper solar photosphere
and lower atmosphere. This code takes into account several physical
phenomena: compressible fluid flow in a highly stratified medium,
radiative energy transfer between the fluid elements, and a real-gas
equation of state. Strong fluctuations in the outer convective layers
of the sun generate dominant acoustic sources in this region. Therefore
we simulate the upper layers of the convection zone using 64^3 grid
cells. The region extends 6x6 Mm horizontally and from 4.5 Mm below
the visible surface to 1 Mm above the surface. We calculate the
oscillation power spectra of radial and non-radial modes, extract
the eigenfunctions, and calculate the work integral to estimate the
depth of the oscillation sources; we then compare with the previous
studies of Stein and Nordlund (2001) and Georgobiani et al. (2006)
and with the observed properties of solar oscillations.
Title: Modeling of solar wave damping
Authors: Parchevsky, Konstantin; Kosovichev, A. G.
Bibcode: 2007AAS...210.4604P
Altcode: 2007BAAS...39..161P
The damping mechanism of solar modes below the acoustic cut-off
frequency is not yet completely understood. Both the scattering on
turbulence in subsurface layers and partial escaping waves into the
chromosphere may play significant role. Using 3D numerical simulations
of wave propagation in the solar atmosphere with the realistic top
boundary condition we studied both mechanisms. The wave sources were
modeled by stochastic randomly distributed perturbations of vertical
component of force. In both cases we were able to reproduce the observed
characteristics of the acoustic spectrum (line-widths and relative
amplitude distribution). We masked the sources in the central circle
to simulate the absence of sources in sunspots, and we found that the
oscillation amplitude in regions of suppressed excitation only weakly
depends on the wave damping mechanism in the upper convection zone
and chromosphere.
Title: Investigation of Magnetic Properties of Emerging Active Regions
Authors: Kosovichev, Alexander G.; Stenflo, J. O.
Bibcode: 2007AAS...210.9214K
Altcode: 2007BAAS...39..210K
Magnetic flux emerging from the Sun's interior carries information
about the physical processes of magnetic field generation and transport
in the convection zone. The current paradigm is that solar magnetic
fields are generated in the tachocline region and emerge in the form
of toroidal flux tubes forming bipolar active regions, and that because
of the Coriolis force the flux tubes emerge with a particular tilt with
respect to the equator, observationally known as Joy's law. In order to
test this and other properties of emerging magnetic flux we have carried
out a systematic study of all, more than 500, emerging active regions
observed by SOHO/MDI during 1996-2006, using 96-min cadence full-disk
MDI magnetograms. The results reveal new interesting properties of
emerging magnetic flux. In particular, they indicate that the
initial tilt may be quite different from Joy's law, but that after
emergence the polarities quickly rearrange themselves to better conform
to this law, something that presents a challenge to current theoretical
models. We investigate statistics of the emerging flux properties,
their changes during the solar cycle and discuss implications for
dynamo theories and models of magnetic flux emergence and formation
of active regions.
Title: Helioseismic inferences on subsurface solar convection
Authors: Kosovichev, Alexander G.
Bibcode: 2007IAUS..239..113K
Altcode:
No abstract at ADS
Title: Imaging Far-Side Solar Active Regions by Use of Time-Distance
Helioseismology and Its Validation
Authors: Zhao, Junwei; Hartlep, T.; Kosovichev, A. G.; Mansour, N. N.
Bibcode: 2007AAS...210.4606Z
Altcode: 2007BAAS...39..161Z
Helioseismic holography has succeeded in imaging the solar far-side
active regions, providing a useful tool for space weather forecast by
monitoring large active regions in the back of the Sun. In order to
check the robustness of the holography results, as well as to provide
another far-side imaging technique, we developed the code to image
the far-side of the Sun by use of time-distance helioseismology. In
addition to the four-skip acoustic signals that were used by holography
analysis, five-skip acoustic signals were also taken into computation
in this new time-distance technique. The combination of both far-side
images, computed from four-skip and five-skip signals, greatly enhances
the picture of active regions, and reduces spurious signals. To
validate our technique of far-side imaging, we performed medium-l
numerical simulation of the Sun as a whole sphere, and tested the
time-distance far-side imaging technique on this simulated data set. It
was found that this technique could successfully identify the location
and size of the far-side active regions which typically have a faster
interior sound-speed.
Title: The Cause Of Sunquakes: Electrons Or Protons?
Authors: Kosovichev, Alexander G.
Bibcode: 2007AAS...210.9326K
Altcode: 2007BAAS...39..215K
Sunquakes, the helioseismic response to solar flares, are caused
by strong localized hydrodynamic impacts in the photosphere, which
are observed directly in SOHO/MDI Dopplergrams during the impulsive
phase. These impacts correlate very well both temporary and spatially
with hard X-ray sources, and in some cases are close to gamma-ray
sources imaged by RHESSI. A common paradigm is the sunquake events
are caused by accelerated protons because protons carry more momentum
and penetrate much deeper into the solar atmosphere than electrons
which loose most of their energy in the upper chromosphere. I present
the results of analysis of the hydrodynamic response for the
sunquake events when both hard X-rays and gamma-rays were observed
by RHESSI. In one event, X5.6 flare of July 23, 2002, the hard X-ray
and gamma-ray sources were significantly separated from each other,
approximately by 20" (with 5-sigma confidence). Analysis of MDI
Dopplergrams reveals hydrodynamic and seismic responses associated the
hard X-ray source and shows no significant signal in the gamma-source
area. These observations effectively rule out high-energy protons as
a source of sunquakes. Furthermore, detailed analysis of the dynamics
of sunquake sources reveals their close association with expanding
flare ribbons, and thus with the magnetic reconnection process. The
fast motion of these sources results in strong anisotropy of the
seismic waves, clearly observed in the MDI data. The general picture
that comes from the analysis of MDI and RHESSI is consistent with
the previously developed hydrodynamic thick-target model, in which
electrons heat the upper chromosphere to high temperatures generating
a high-pressure region, expansion of which causes a high compression
shock, which reaches the photosphere and excites the seismic waves. I
discuss how the observations and modeling of sunquakes and their sources
help to understand processes of the energy release and transport in
solar flares.
Title: Modeling Of Magnetic Effects In Local Helioseismology
Authors: Parchevsky, Konstantin; Kosovichev, A. G.
Bibcode: 2007AAS...210.2209P
Altcode: 2007BAAS...39..125P
Investigation of magnetic field effects in wave propagation on the
Sun is very important for developing robust local helioseismology
techniques and diagnostics of subsurface magnetic fields. We have
developed a complete 3D MHD numerical model to investigate how magnetic
fields affect wave properties and helioseismology measurements in
realistic solar conditions. We investigate the variations of wave
amplitudes, wavelengths, phase shifts and travel times in various
models of magnetic field structures in the solar convection zone and
atmosphere. A particular attention is paid to the so-called "inclined
field effect", which is of significant interest for time-distance and
holography techniques. The simulation results are compared with the
observational data.
Title: Analytical Models for Cross-Correlation Signal in Time-Distance
Helioseismology
Authors: Nigam, R.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 2007ApJ...659.1736N
Altcode: 2007astro.ph..2499N
In time-distance helioseismology, the time signals (Doppler shifts) at
two points on the solar surface separated by a fixed angular distance
are cross-correlated, and this leads to a wave packet signal. Accurately
measuring the travel times of these wave packets is crucial for
inferring the subsurface properties in the Sun. The observed signal
is quite noisy, and to improve the signal-to-noise ratio and make
the cross-correlation more robust, the temporal oscillation signal is
phase-speed filtered at the two points in order to select waves that
travel a fixed horizontal distance. Hence a new formula to estimate the
travel times is derived in the presence of a phase-speed filter, and it
includes both the radial and horizontal component of the oscillation
displacement signal. It generalizes the previously used Gabor wavelet
that was derived without a phase-speed filter and included only the
radial component of the displacement. This is important since it will be
consistent with the observed cross-correlation that is computed using
a phase-speed filter, and it also accounts for both the components of
the displacement. The new formula depends on the location of the two
points on the solar surface that are being cross-correlated and accounts
for the travel time shifts at different locations on the solar surface.
Title: Validation of Time-Distance Helioseismology by Use of Realistic
Simulations of Solar Convection
Authors: Zhao, Junwei; Georgobiani, Dali; Kosovichev, Alexander G.;
Benson, David; Stein, Robert F.; Nordlund, Åke
Bibcode: 2007ApJ...659..848Z
Altcode: 2006astro.ph.12551Z
Recent progress in realistic simulations of solar convection have
given us an unprecedented opportunity to evaluate the robustness of
solar interior structures and dynamics obtained by methods of local
helioseismology. We present results of testing the time-distance
method using realistic simulations. By computing acoustic wave
propagation time and distance relations for different depths of the
simulated data, we confirm that acoustic waves propagate into the
interior and then turn back to the photosphere. This demonstrates
that in numerical simulations properties of acoustic waves (p-modes)
are similar to the solar conditions, and that these properties can be
analyzed by the time-distance technique. For surface gravity waves
(f-modes), we calculate perturbations of their travel times caused
by localized downdrafts and demonstrate that the spatial pattern of
these perturbations (representing so-called sensitivity kernels)
is similar to the patterns obtained from the real Sun, displaying
characteristic hyperbolic structures. We then test time-distance
measurements and inversions by calculating acoustic travel times from
a sequence of vertical velocities at the photosphere of the simulated
data and inferring mean three-dimensional flow fields by performing
inversion based on the ray approximation. The inverted horizontal
flow fields agree very well with the simulated data in subsurface
areas up to 3 Mm deep, but differ in deeper areas. Due to the cross
talk effects between the horizontal divergence and downward flows,
the inverted vertical velocities are significantly different from the
mean convection velocities of the simulation data set. These initial
tests provide important validation of time-distance helioseismology
measurements of supergranular-scale convection, illustrate limitations
of this technique, and provide guidance for future improvements.
Title: Helioseismic Test of Nonhomologous Solar Radius Changes with
the 11 Year Activity Cycle
Authors: Lefebvre, S.; Kosovichev, A. G.; Rozelot, J. P.
Bibcode: 2007ApJ...658L.135L
Altcode: 2007astro.ph..2497L
Recent models of variations of the Sun's structure with the 11 year
activity cycle by Sofia et al. predict strong nonhomologous changes
of the radius of subsurface layers, due to subsurface magnetic fields
and field-modulated turbulence. According to their best model, the
changes of the surface radius may be 1000 times larger than those at
the depth of 5 Mm. We use f-mode oscillation frequency data from the
SOHO MDI and measurements of the solar surface radius variations from
SOHO and ground-based observatories during solar cycle 23 (1996-2005)
to put constraints on the radius changes. The results show that the
above model overestimates the change of the radius at the surface
relative to the change at 5 Mm.
Title: Comparison of Large-Scale Flows on the Sun Measured by
Time-Distance Helioseismology and Local Correlation Tracking
Authors: Švanda, Michal; Zhao, Junwei; Kosovichev, Alexander G.
Bibcode: 2007SoPh..241...27S
Altcode: 2007astro.ph..1717S
We present a direct comparison between two different techniques:
time-distance helioseismology and a local correlation tracking
method for measuring mass flows in the solar photosphere and in
a near-surface layer. We applied both methods to the same dataset
(MDI high-cadence Dopplergrams covering almost the entire Carrington
rotation 1974) and compared the results. We found that, after necessary
corrections, the vector flow fields obtained by these techniques are
very similar. The median difference between directions of corresponding
vectors is 24°, and the correlation coefficients of the results for
mean zonal and meridional flows are 0.98 and 0.88, respectively. The
largest discrepancies are found in areas of small velocities where the
inaccuracies of the computed vectors play a significant role. The good
agreement of these two methods increases confidence in the reliability
of large-scale synoptic maps obtained by them.
Title: Commission 12: Solar Radiation & Structure
Authors: Bogdan, Thomas. J.; Martínez Pillet, Valentin; Asplund,
M.; Christensen-Dalsgaard, J.; Cauzzi, G.; Cram, L. E.; Dravins, D.;
Gan, W.; Henzl, P.; Kosovichev, A.; Mariska, J. T.; Rovira, M. G.;
Venkatakrishnan, P.
Bibcode: 2007IAUTA..26...89B
Altcode:
Commission 12 covers research on the internal structure and dynamics
of the Sun, the "quiet" solar atmosphere, solar radiation and its
variability, and the nature of relatively stable magnetic structures
like sunspots, faculae and the magnetic network. There is considerable
productive overlap with the other Commissions of Division II as
investigations move progressively toward the fertile intellectual
boundaries between traditional research disciplines. In large part,
the solar magnetic field provides the linkage that connects these
diverse themes. The same magnetic field that produces the more subtle
variations of solar structure and radiative output over the 11 yr
activity cycle is also implicated in rapid and often violent phenomena
such as flares, coronal mass ejections, prominence eruptions, and
episodes of sporadic magnetic reconnection.The last three years have
again brought significant progress in nearly all the research endeavors
touched upon by the interests of Commission 12. The underlying causes
for this success remain the same: sustained advances in computing
capabilities coupled with diverse observations with increasing levels
of spatial, temporal and spectral resolution. It is all but impossible
to deal with these many advances here in anything except a cursory and
selective fashion. Thankfully, the Living Reviews in Solar Physics; has
published several extensive reviews over the last two years that deal
explicitly with issues relevant to the purview of Commission 12. The
reader who is eager for a deeper and more complete understanding of
some of these advances is directed to http://www.livingreviews.org
for access to these articles.
Title: Helioseismology program for Solar Dynamics Observatory
Authors: Kosovichev, A. G.; HMI Science Team
Bibcode: 2007AN....328..339K
Altcode:
An overview of the science investigation program for the Helioseismic
and Magnetic Imager (HMI) of the Solar Dynamics Observatory (SDO)
space mission scheduled for launch in 2008 is presented. The HMI
investigation encompasses three primary objectives of the Living With a
Star Program: first, to determine how and why the Sun varies; second,
to improve our understanding of how the Sun drives global change and
space weather; and third, to determine to what extent predictions of
space weather and global change can be made and to prototype predictive
techniques. Helioseismology provides unique tools to study the basic
mechanisms of the Sun's magnetic activity and variability. It plays a
crucial role in all HMI investigations, which include convection-zone
dynamics and the solar dynamo; origin and evolution of sunspots,
active regions and complexes of activity; sources and drivers
of solar activity and disturbances; links between the internal
processes and dynamics of the corona and heliosphere; and precursors
of solar disturbances for space-weather forecasts. We describe new
unique opportunities for helioseismology studies with HMI data, in
combination with data from the other SDO instruments, Atmospheric
Imaging Assembly (AIA) and Extreme-ultraviolet Variability Experiment
(EVE), and also from various space and ground-based observatories. The
complete HMI science investigation and data analysis plan is available
at http://hmi.stanford.edu.
Title: Local Helioseismology and Correlation Tracking Analysis of
Surface Structures in Realistic Simulations of Solar Convection
Authors: Georgobiani, Dali; Zhao, Junwei; Kosovichev, Alexander G.;
Benson, David; Stein, Robert F.; Nordlund, Åke
Bibcode: 2007ApJ...657.1157G
Altcode: 2006astro.ph..8204G
We apply time-distance helioseismology, local correlation tracking, and
Fourier spatial-temporal filtering methods to realistic supergranule
scale simulations of solar convection and compare the results with
high-resolution observations from the Solar and Heliospheric Observatory
(SOHO) Michelson Doppler Imager (MDI). Our objective is to investigate
the surface and subsurface convective structures and test helioseismic
measurements. The size and grid of the computational domain are
sufficient to resolve various convective scales from granulation to
supergranulation. The spatial velocity spectrum is approximately a
power law for scales larger than granules, with a continuous decrease
in velocity amplitude with increasing size. Aside from granulation
no special scales exist, although a small enhancement in power at
supergranulation scales can be seen. We calculate the time-distance
diagram for f- and p-modes and show that it is consistent with the SOHO
MDI observations. From the simulation data we calculate travel-time
maps for surface gravity waves (f-mode). We also apply correlation
tracking to the simulated vertical velocity in the photosphere to
calculate the corresponding horizontal flows. We compare both of these
to the actual large-scale (filtered) simulation velocities. All three
methods reveal similar large-scale convective patterns and provide an
initial test of time-distance methods.
Title: Active Region Dynamics
Authors: Kosovichev, A. G.; Duvall, T. L., Jr.
Bibcode: 2007sdeh.book....1K
Altcode:
No abstract at ADS
Title: The change of solar shape in time and depth. Some consequences
for space climate
Authors: Lefebvre, S.; Rozelot, J. P.; Kosovichev, A. G.
Bibcode: 2007AdSpR..40.1000L
Altcode:
During the last five years, studies of the Sun and Sun-Earth
relationships have dramatically changed our view on solar terrestrial
physics. We will here focus on new views on the solar interior. The
internal non-homogeneous mass distribution and non-uniform angular
velocity (function of the radial distance to the center and of the
latitude) yield a complex outer shape. Beyond a "spherical" Sun is a
new approach of solar-physics taking into account the gravitational
energy which triggers the various layers. Such energy has been
skipped in many ways up to now in theoretical models describing
the solar output variability. In spite of many works on solar
variations, there is not yet a consensus on the global experimental
phenomenology. For instance, it is not yet known if this gravitational
energy may explain faint observed irradiance variations, and the way
the asphericity-luminosity parameter W acts on our stratosphere. Such
issues must be solved to understand how the solar output variability
may influence the Earth's environment (helioclimatology). We will
emphasize the key role of the subsurface layers (the leptocline,
recently put in evidence by helioseismology) for a better prediction
of the solar cycles. Regarding the solar core dynamics, the subject
is of high priority for new investigations. We will conclude by giving
some imprints on space-dedicated missions: GOLF-NG/DynaMICS in a joint
effort with SDO (Solar Dynamics Observatory).
Title: Direct Observations of Acoustic Waves Excited by Solar Flares
and their Propagation in Sunspot Regions
Authors: Kosovichev, A. G.
Bibcode: 2006ASPC..354..154K
Altcode:
I present new results from SOHO-MDI of direct observations and analysis
of ``sunquakes'', seismic acoustic waves excited by solar flares and
propagating below the surface through surrounding sunspot regions. The
main results are the following: the sources of the seismic waves
are associated with hydrodynamic response of the solar atmosphere to
high-energy electrons (`thick-target' model); the seismic sources are
identified in MDI Dopplergrams as compact regions of strong Doppler
shift during the flare impulsive phase; the seismic sources are close
to the sources of hard X-ray emission and located in flare ribbons;
the expanding waves are often anisotropic and non-circular reflecting
the properties of the hydrodynamic impact controlled by magnetic
field topology in the energy-release regions and subphotospheric
condition; the waves have periods of three -- five minutes and are
best observed about 20 minutes after the impulsive phase; the fronts
of the sunquake waves propagating through surrounding sunspots do not
show significant distortion and decay, thus giving no evidence for
mode conversion in magnetic fields; time-distance diagrams of the
seismic waves show only small deviations caused by sunspots in the
travel times from the quiet Sun data, which are consistent with the
standard travel time measurements from cross-covariance functions. The
direct observations of acoustic waves excited by flares open new
perspectives for high-resolution helioseismic diagnostics of sunspots
and understanding the hydrodynamic processes in solar flares.
Title: Anisotropy of Helioseismic Waves Excited by Solar Flares
Authors: Kosovichev, A.
Bibcode: 2006AGUFMSH23A0335K
Altcode:
Recent observations of helioseismic response to solar flares
("sunquakes") revealed strong anisotropy in the seismic wave
amplitude. It appears that the acoustic waves excited by solar flares
and propagating through the solar interior have the strongest amplitude
in the direction of the expansion of the flare ribbons. Our analysis
of the flare data from SOHO/MDI and RHESSI, and modeling of sunquakes
show that this anisotropy is likely to be caused by moving seismic
sources. These sources correspond to the places where high-energy
particles penetrate into the lower atmosphere of the Sun, and their
motion is related to the dynamics of the magnetic reconnection process
in the corona.
Title: Suppression of amplitude of 5-min oscillations in sunspots
due to the lack of acoustic sources
Authors: Parchevsky, K.; Kosovichev, A.
Bibcode: 2006ESASP.624E..39P
Altcode: 2006soho...18E..39P
No abstract at ADS
Title: The DynaMICS perspective
Authors: Turck-Chièze, S.; Schmutz, W.; Thuillier, G.; Jefferies,
S.; Pallé; Dewitt, S.; Ballot, J.; Berthomieu, G.; Bonanno, A.;
Brun, A. S.; Christensen-Dalsgaard, J.; Corbard, T.; Couvidat, S.;
Darwich, A. M.; Dintrans, B.; Domingo, V.; Finsterle, W.; Fossat,
E.; Garcia, R. A.; Gelly, B.; Gough, D.; Guzik, J.; Jiménez, A. J.;
Jiménez-Reyes, S.; Kosovichev, A.; Lambert, P.; Lefebvre, S.; Lopes,
I.; Martic, M.; Mathis, S.; Mathur, S.; Nghiem, P. A. P.; Piau, L.;
Provost, J.; Rieutord, M.; Robillot, J. M.; Rogers, T.; Roudier, T.;
Roxburgh, I.; Rozelot, J. P.; Straka, C.; Talon, S.; Théado, S.;
Thompson, M.; Vauclair, S.; Zahn, J. P.
Bibcode: 2006ESASP.624E..24T
Altcode: 2006soho...18E..24T
No abstract at ADS
Title: Sunquake sources and wave propagation
Authors: Kosovichev, A. G.
Bibcode: 2006ESASP.624E.134K
Altcode: 2006soho...18E.134K
No abstract at ADS
Title: Effect of the subsurface shear layer on solar supergranulation
Authors: Green, C. A.; Kosovichev, A. G.; Miesch, M. S.
Bibcode: 2006ESASP.624E..13G
Altcode: 2006soho...18E..13G
No abstract at ADS
Title: Properties of Flares-Generated Seismic Waves on the Sun
Authors: Kosovichev, A. G.
Bibcode: 2006SoPh..238....1K
Altcode: 2006astro.ph..1006K; 2006SoPh..tmp...33K
The helioseismic waves excited by solar flares ("sunquakes") are
observed as circular, expanding waves on the Sun's surface. The first
sunquake was observed for a flare on July 9, 1996, by the Solar and
Heliospheric Observatory (SOHO) space mission. This paper presents
results of new observations and a detailed qualitative analysis of
the basic properties of the helioseismic waves generated by four solar
flares in 2003 - 2005. For two of these flares, the X17 flare of October
28, 2003, and the X1.2 flare of January 15, 2005, the helioseismology
observations are compared with simultaneous observations of flare
X-ray fluxes measured from the RHESSI satellite. These observations
show a close association between the flare seismic waves and the
hard X-ray source, indicating that high-energy electrons accelerated
during the flare impulsive phase produced strong compression waves in
the photosphere, causing the sunquake. The results also reveal new
physical properties such as strong anisotropy of the seismic waves,
the amplitude of which varies significantly with the direction of
propagation. The waves travel through surrounding sunspot regions to
large distances, up to 120 Mm, without significant distortion. These
observations open new perspectives for helioseismic diagnostics of
flaring active regions on the Sun and for understanding the mechanisms
of the energy release and transport in solar flares.
Title: Cyclic variability of the seismic solar radius from SOHO/MDI
and related physics
Authors: Lefebvre, S.; Kosovichev, A. G.; Nghiem, P.; Turck-Chièze,
S.; Rozelot, J. P.
Bibcode: 2006ESASP.624E...9L
Altcode: 2006soho...18E...9L
No abstract at ADS
Title: The internal structure of the Sun inferred from g modes and
low-frequency p modes
Authors: Elsworth, Y. P.; Baudin, F.; Chaplin, W; Andersen, B;
Appourchaux, T.; Boumier, P.; Broomhall, A. -M.; Corbard, T.;
Finsterle, W.; Fröhlich, C.; Gabriel, A.; García, R. A.; Gough,
D. O.; Grec, G.; Jiménez, A.; Kosovichev, A.; Provost, J.; Sekii,
T.; Toutain, T.; Turck-Chièze, S.
Bibcode: 2006ESASP.624E..22E
Altcode: 2006soho...18E..22E
The Phoebus group is an international collaboration of
helioseismologists, its aim being to detect low-frequency solar g
modes. Here, we report on recent work, including the development and
application of new techniques based on the detection of coincidences
in contemporaneous datasets and the asymptotic properties of the g-mode
frequencies. The length of the time series available to the community is
now more than ten years, and this has reduced significantly the upper
detection limits on the g-mode amplitudes. Furthermore, low-degree p
modes can now be detected clearly at frequencies below 1000 μHz.
Title: Solar dynamics, asphericities and gravitational moments:
present state of the art
Authors: Rozelot, J. -P.; Lefebvre, S.; Kosovichev, A.; Pireaux, S.
Bibcode: 2006IAUJD..17E..10R
Altcode:
Solar gravitational moments J[n] reflect the internal non-homogeneous
mass distribution and non-uniform angular velocity (function
of the radial distance to the center and of the latitude). The
result is the complex outer shape of the Sun, described by shape
coefficients c[n] , also referred to as asphericities. The study of
solar gravitational moments is not only crucial for solar physics, but
also for astrometry (when computing light deflection in the vicinity
of the Sun), celestial mechanics: relativistic precession of planets,
planetary orbit inclination and spin-orbit couplings) and for future
tests of alternative theories of gravitation (correlation of J[2]
with Post-Newtonian parameters). A variability of c[n] and J[n]
might be due to the temporal variation of the internal structure
and the angular velocity which is known at the surface down to the
tachocline. Applying helioseismic inversions just below the surface,
new results have been obtained showing temporal variations with solar
activity inside the surface layers. Regarding the solar core dynamics,
the subject is of high priority for new investigations. Space-dedicated
missions, such as Golf-NG/Dynamics in a joint effort with SDO (Solar
Dynamics Observatory), should provide a new insight on the question.
Title: Local helioseismology techniques and results
Authors: Kosovichev, A. G.
Bibcode: 2006IAUJD..17E..12K
Altcode:
Local helioseismology techniques (time-distance helioseismology,
acoustic imaging and holography, and ring-diagram analysis) use
measurements of variations of acoustic travel times, phase shifts
and local oscillation frequencies to infer the internal properties
of the Sun. These techniques provide 3D maps of subsurface wave speed
variations and flow velocities. The anisotropy of wave propagation in
magnetic field is used to estimate properties of subsurface magnetic
fields. The techniques are verified by using realistic numerical
simulations of solar dynamics and wave propagation. The results reveal
complicated dynamical structure of the solar interior. They provide
new insight into large-scale convection and global circulation of the
Sun, emergence and evolution of active regions, structure and dynamics
sunspots and twisting and sheering flows in flaring regions.
Title: Scientific Objectives of the Novel Formation Flying Mission
Aspiics
Authors: Turck-Chièze, S.; Schmutz, W.; Thuillier, G.; Jefferies,
S.; Pallé; Dewitt, S.; Ballot, J.; Berthomieu, G.; Bonanno, A.;
Brun, A. S.; Christensen-Dalsgaard, J.; Corbard, T.; Couvidat, S.;
Darwich, A. M.; Dintrans, B.; Domingo, V.; Finsterle, W.; Fossat,
E.; Garcia, R. A.; Gelly, B.; Gough, D.; Guzik, J.; Jiménez, A. J.;
Jiménez-Reyes, S.; Kosovichev, A.; Lambert, P.; Lefebvre, S.; Lopes,
I.; Martic, M.; Mathis, S.; Mathur, S.; Nghiem, P. A. P.; Piau, L.;
Provost, J.; Rieutord, M.; Robillot, J. M.; Rogers, T.; Roudier, T.;
Roxburgh, I.; Rozelot, J. P.; Straka, C.; Talon, S.; Théado, S.;
Thompson, M.; Vauclair, S.; Zahn, J. P.
Bibcode: 2006ESASP.617E.164L
Altcode: 2006soho...17E.164L
No abstract at ADS
Title: Helioseismic Measurements of Solar Radius Changes from SOHO/MDI
Authors: Lefebvre, S.; Kosovichev, A. G.; Rozelot, J. P.
Bibcode: 2006ESASP.617E..43L
Altcode: 2006soho...17E..43L
No abstract at ADS
Title: The EUV Variability Experiment (EVE) on the Solar Dynamics
Observatory (SDO): Science Plan and Instrument Overview
Authors: Turck-Chièze, S.; Schmutz, W.; Thuillier, G.; Jefferies,
S.; Pallé; Dewitt, S.; Ballot, J.; Berthomieu, G.; Bonanno, A.;
Brun, A. S.; Christensen-Dalsgaard, J.; Corbard, T.; Couvidat, S.;
Darwich, A. M.; Dintrans, B.; Domingo, V.; Finsterle, W.; Fossat,
E.; Garcia, R. A.; Gelly, B.; Gough, D.; Guzik, J.; Jiménez, A. J.;
Jiménez-Reyes, S.; Kosovichev, A.; Lambert, P.; Lefebvre, S.; Lopes,
I.; Martic, M.; Mathis, S.; Mathur, S.; Nghiem, P. A. P.; Piau, L.;
Provost, J.; Rieutord, M.; Robillot, J. M.; Rogers, T.; Roudier, T.;
Roxburgh, I.; Rozelot, J. P.; Straka, C.; Talon, S.; Théado, S.;
Thompson, M.; Vauclair, S.; Zahn, J. P.
Bibcode: 2006ESASP.617E.165W
Altcode: 2006soho...17E.165W
No abstract at ADS
Title: Analysis of a Relation between Subphotospheric Plasma Flows
and Photospheric Current Kernels
Authors: Kulinová, A.; Dzifčáková, E.; Kosovichev, A. G.; Duvall,
T. L.
Bibcode: 2006ESASP.617E..69K
Altcode: 2006soho...17E..69K
No abstract at ADS
Title: Recent Progresses on g-Mode Search
Authors: Appourchaux, T.; Andersen, B.; Baudin, F.; Boumier, P.;
Broomhall, A. -M.; Chaplin, W.; Corbard, T.; Elsworth, Y.; Finsterle,
W.; Fröhlich, C.; Gabriel, A.; Garcia, R.; Gough, D. O.; Grec, G.;
Jiménez, A.; Kosovichev, A.; Provost, J.; Sekii, T.; Toutain, T.;
Turck-Chièze, S.
Bibcode: 2006ESASP.617E...2A
Altcode: 2006soho...17E...2A
No abstract at ADS
Title: The Dynamics Project
Authors: Turck-Chièze, S.; Schmutz, W.; Thuillier, G.; Jefferies,
S.; Pallé; Dewitt, S.; Ballot, J.; Berthomieu, G.; Bonanno, A.;
Brun, A. S.; Christensen-Dalsgaard, J.; Corbard, T.; Couvidat, S.;
Darwich, A. M.; Dintrans, B.; Domingo, V.; Finsterle, W.; Fossat,
E.; Garcia, R. A.; Gelly, B.; Gough, D.; Guzik, J.; Jiménez, A. J.;
Jiménez-Reyes, S.; Kosovichev, A.; Lambert, P.; Lefebvre, S.; Lopes,
I.; Martic, M.; Mathis, S.; Mathur, S.; Nghiem, P. A. P.; Piau, L.;
Provost, J.; Rieutord, M.; Robillot, J. M.; Rogers, T.; Roudier, T.;
Roxburgh, I.; Rozelot, J. P.; Straka, C.; Talon, S.; Théado, S.;
Thompson, M.; Vauclair, S.; Zahn, J. P.
Bibcode: 2006ESASP.617E.162T
Altcode: 2006soho...17E.162T
No abstract at ADS
Title: Helioseismic measurements of solar radius changes from SOHO/MDI
Authors: Lefebvre, S.; Kosovichev, A. G.; Rozelot, J. P.
Bibcode: 2006sf2a.conf..551L
Altcode:
The sub-surface of the Sun is much more complex than it does
appear up to now. Analysing the SOHO/MDI (SOlar and Heliospheric
Observatory/Michelson Doppler Imager) f-mode frequencies and their
temporal variation for the last 9 years, we computed the variation of
the radius in the subsurface layers of the Sun by applying helioseismic
inversions. We have found a variability of the ``helioseismic''
radius in antiphase with the solar activity, with the strongest
variations of the stratification being just below the surface around
0.995R⊙. In addition, the radius of the deeper layers
of the Sun, between 0.975R⊙ and 0.99R⊙
changes in phase with the 11-year cycle. These results imply a
non-homogeneous variation of the radius with depth and time and may
explain discrepancies in ground-based observations.
Title: Active Region Dynamics
Authors: Kosovichev, A. G.; Duvall, T. L.
Bibcode: 2006SSRv..124....1K
Altcode: 2007SSRv..tmp...56K
New methods of local helioseismology and uninterrupted time series
of solar oscillation data from the Solar and Heliospheric Observatory
(SOHO) have led to a major advance in our understanding of the structure
and dynamics of active regions in the subsurface layers. The initial
results show that large active regions are formed by repeated magnetic
flux emergence from the deep interior, and that their roots are at least
50 Mm deep. The active regions change the temperature structure and
flow dynamics of the upper convection zone, forming large circulation
cells of converging flows. The helioseismic observations also indicate
that the processes of magnetic energy release, flares and coronal mass
ejections, might be associated with strong (1 2 km/s) shearing flows,
4 6 Mm below the surface.
Title: Modeling of Suppression of Acoustic Oscillation Power in
Sunspots
Authors: Parchevsky, Konstantin V.; Kosovichev, A. G.
Bibcode: 2006SPD....37.3204P
Altcode: 2006BAAS...38..258P
In this paper we make an attempt to explain the suppression of
amplitude of 5-min. acoustic oscillations inside sunspot. A program for
numerical simulation of propagation of acoustic waves in 3D has been
developed. Realistic top boundary condition is a characteristic feature
of this program. Nonreflecting boundary condition based on PML layer is
established above the temperature minimum, and absorbs all waves not
reflected by photosphere. Acoustic sources with random amplitudes and
phases were distributed below the photosphere. Sources were masked in
a central circle, simulating the absence of acoustic sources inside
sunspots. Large scale simulations in rectangular box of size 120Mm
x 120Mm x 50Mm showed that acoustic amplitude is approximately 4
times lower in the region without sources in comparison with the
quiet Sun. This is in a good agreement with observations. Hence, the
observed suppression of oscillations in sunspots can be explained by
the lack of acoustic sources in sunspot areas where strong magnetic
field inhibits convection.
Title: Investigating Mechanisms of Wave-Like Behaviour of
Supergranulation
Authors: Green, Cristina A.; Kosovichev, A. G.; Miesch, M. S.
Bibcode: 2006SPD....37.0503G
Altcode: 2006BAAS...38R.223G
Recent time-distance helioseismology results suggest that
supergranulation has properties of travelling waves. The supergranular
pattern rotates faster than the surface plasma or any other features
on the Sun. We investigate possible mechanisms of the wave-like
behaviour of supergranulation. In particular, we suggest that this may
be caused by the steep gradient of the subsurface shear layer. Using
a linear model, we calculated the phase speed of unstable modes of
convection. The speeds were found to depend on the stratification
of the convective layer. The phase speeds of the linear convective
modes were greater than the surface plasma speed, though lower than
observations. An alternate suggested explanation is a non-linear
effect of the Coriolis force. We analyzed the results of non-linear
3D simulations of the convection zone including solar rotation, and
found no evidence of wave-like behaviour. These results suggest that
the subsurface shear layer is a plausible explanation for the wave-like
behaviour, but the non-linear effects need to be studied.
Title: Time-Distance and Correlation Tracking Analysesof Convective
Structures using Realistic Large-ScaleSimulations of Solar Convection
Authors: Georgobiani, Dali; Zhao, J.; Kosovichev, A. G.; Benson, D.;
Stein, R. F.; Nordlund, A.
Bibcode: 2006SPD....37.0509G
Altcode: 2006BAAS...38..224G
Recent large-scale simulations of solar turbulentconvection and
oscillations produce a wealth of realisticdata and provide a great
opportunity to study solaroscillations and test various techniques,
such aslocal helioseismology or local correlation trackingmethods,
widely used for the analysis of the realobserved solar data.The
application of the time-distance analysis to theartificial data produced
with a realistic 3D radiativehydrodynamic code successfully reproduces
thetime-distance diagram and travel time maps. Resultingtravel times are
similar to the travel times obtainedfrom the SOHO/MDI observations. To
further validatethe model, the inversion will be performed in
orderto infer the interior velocities at various depthsand compare
them with the simulated data.f-mode time-distanceanalysis as well as
local correlation tracking can be usedto study the morphology of the
simulated convection. Bothmethods reveal the large-scale convective
structures, whichare also directly visible in the time-averaged
simulatedflow fields.
Title: Surface Magnetism Effects in Time-Distance Helioseismology
Authors: Zhao, Junwei; Kosovichev, Alexander G.
Bibcode: 2006ApJ...643.1317Z
Altcode: 2006astro.ph..2260Z
Recent observations of helioseismic holography revealed that magnetic
fields that are inclined relative to the line-of-sight direction
could cause systematic variations in measured acoustic phase shifts
(hereafter, inclined magnetic field effect) and that the presence of
surface magnetic field may shift the phases and impair the coherence of
acoustic waves (known as the showerglass effect), thus complicating
the interpretation of acoustic wave propagation time through the
solar interior. In this paper we examine how these two observational
effects affect time-distance helioseismology measurements in magnetic
regions. It is confirmed that the inclined magnetic field could
cause variations in time-distance measured acoustic travel times
inside sunspot penumbra as well; however, inversions of the measured
times for the wave propagation show that this effect only slightly
shifts the location of negative sound-speed variations near the solar
surface but basically does not change the inverted deeper interior
structures. Further measurements using continuum intensitygrams and
line-depth data from the MDI on board SOHO illustrate that the inclined
magnetic field does not cause any obvious systematic travel-time
variations in these observations. Regarding to the showerglass effect,
we find that outgoing and ingoing travel-time perturbations through
sunspots from our typical time-distance measurements are significantly
smaller than those reported from helioseismic holography and also
strongly depend on the propagation depth indicating deep changes. In
addition, our second-skip cross-correlation experiments demonstrate
that inside sunspots, the half of the double-skip travel times are
very similar to the mean single-skip travel times, indicating that
acoustic signals observed inside sunspots do not introduce detectable
phase shifts after applying proper phase-speed filtering.
Title: Measuring Surface Magnetism Effects in Time-Distance
Helioseismology
Authors: Zhao, Junwei; Kosovichev, A. G.
Bibcode: 2006SPD....37.0507Z
Altcode: 2006BAAS...38..224Z
It was found recently by helioseismic holography analysis that
magneticfields that are inclined relative to the line-of-sight
direction couldcause systematic variations in measured acoustic
phase shifts, andthe presence of surface magnetic field may shift
the phases and impairthe coherence of acoustic waves, known as
"showerglass effect". Weexamine how these two observational effects
affect time-distancemeasurements in magnetic regions. It is confirmed
that the inclinedmagnetic field could cause variations in measured
travel times insidesunspot penumbra, however, inversions of the
measured times showthat this effect only slightly shift the location of
negativesound-speed variations near the solar surface, but basically
does notchange the inverted deeper interior structures. Measurements
by useof continuum intensitygrams and line-depth data illustrate
that theinclined magnetic field does not cause any obvious systematic
traveltime variations. For the showerglass effect, we find that the
outgoingand ingoing travel time perturbations through sunspots from our
typicaltime-distance measurements are significantly smaller than those
reportedfrom holography, and also strongly depend on the propagation
depths.Additionally, our second-skip travel time measurements
demonstrate thatthe signals inside sunspots do not introduce detectable
phase shiftsafter applying phase-speed filtering. We finally conclude
that thesesurface magnetism effects do not cause considerable systematic
errorsin time-distance helioseismology of active regions.
Title: Helioseismic and Photospheric Effects of Solar Flares
Authors: Kosovichev, Alexander G.
Bibcode: 2006SPD....37.0825K
Altcode: 2006BAAS...38..235K
I present a detailed analysis of new observations ofhelioseismic waves
('sunquakes') generated by solarflares, using SOHO/MDI and RHESSI
data. Theseobservations show a close association between theflare
seismic waves and the hard X-ray source,indicating that high-energy
electrons acceleratedduring the flare impulsive phase produced
strongcompression waves in the photosphere, causing thesunquake. The
results also reveal new physicalproperties such as strong anisotropy
of the seismicwaves, the amplitude of which varies significantlywith
the direction of propagation. The waves travelthrough surrounding
sunspot regions to largedistances, up to 120 Mm, without significant
decay.They provide new diagnostics of sunspot regions. Inaddition,
I discuss whether high-energy electrons orprotons are the prime cause
of these effects.
Title: Traveling Convective Modes in the Sun's Subsurface Shear Layer
Authors: Green, C. A.; Kosovichev, A. G.
Bibcode: 2006ApJ...641L..77G
Altcode:
Observations reveal that supergranular convective cells on the Sun
rotate faster than the plasma on the solar surface. Recent time-distance
helioseismology results from the Solar and Heliospheric Observatory
Michelson Doppler Imager suggest that supergranulation has properties
of traveling waves. We consider the idea that these properties are
related to the steep increase of the angular rotation rate with
depth, obtained by helioseismic inversions. The subsurface shear
layer causes the convective modes to travel faster than the surface
plasma. We calculate the phase speed of unstable modes of convection
in the linear approximation for a standard solar model as a function
of the velocity gradient in the shear layer. We find that for the
helioseismically determined gradient, the calculated phase speed is
significantly smaller than the observed speed of the supergranular
pattern relative to the surface plasma. This suggests that, while
the subsurface shear layer provides a plausible explanation for the
wavelike behavior, it is necessary to study nonlinear effects in the
dynamics of supergranulation.
Title: Three-dimensional Inversion of Sound Speed below a Sunspot
in the Born Approximation
Authors: Couvidat, S.; Birch, A. C.; Kosovichev, A. G.
Bibcode: 2006ApJ...640..516C
Altcode:
We revise the inversion of acoustic travel times for the
three-dimensional sound-speed structure below the solar NOAA
Active Region 8243 of 1998 June. We benefit from recent progress in
time-distance helioseismology that provides us with more reliable
tools to infer subsurface solar properties. Among the improvements
we implement here are the use of Born approximation-based travel-time
sensitivity kernels that take into account finite-wavelength effects
and thus are more accurate than the previously employed ray-path
kernels, the inclusion of solar noise statistical properties in the
inversion procedure through the noise covariance matrix, and the use
of the actual variance of the noise in the temporal cross-covariances
in the travel-time fitting procedure. Of these three improvements,
the most significant is the application of the Born approximation to
time-distance helioseismology. This puts the results of this discipline
at the same level of confidence as those of global helioseismology based
on inversion of normal-mode frequencies. Also, we compare inversion
results based on ray-path and Born approximation kernels. We show
that both approximations return a similar two-region structure for
sunspots. However, the depth of inverted structures may be offset by 1
or 2 Mm, and the spatial resolution of the results is more accurately
estimated with the more realistic Born sensitivity kernels. Finally,
using artificial realizations of Doppler velocities of the quiet Sun,
we are now able to estimate the statistical uncertainties of these
inversion results.
Title: Comparison of subsurface sound-speed structures of three
active regions
Authors: Couvidat, S.; Birch, A. C.; Rajaguru, S. P.; Kosovichev, A. G.
Bibcode: 2006IAUS..233...75C
Altcode:
We analyze three solar active regions observed with the MDI instrument
onboard SoHO (Scherrer et al. 1995). We apply the time-distance
helioseismology formalism to derive the travel times of acoustic
waves propagating through these active regions. The inversion of
these acoustic travel times gives us access to the 3D sound-speed
structure below the sunspots. We compare the main characteristics of
these inversion results as a function of the active region size and
magnetic field strength.
Title: Subsurface characteristics of sunspots
Authors: Kosovichev, Alexander G.
Bibcode: 2006AdSpR..38..876K
Altcode:
Time distance helioseismology provides 3-D subphotospheric maps of
sunspots, emerging magnetic structures and associated convective
flows. This gives us new insights into the basic physical processes
inside the Sun, formation of magnetic structures in the solar plasma and
mechanisms of solar activity. Recent results from the MDI instrument
on SOHO reveal complicated stratification and flow patterns of solar
plasma beneath sunspots, with a characteristic temperature increase
in deeper layers and multiple vortex systems, providing evidence for
a cluster model of sunspots. The helioseismic observations provide
important clues for understanding the mechanisms of the formation
of sunspots and active regions, and the subphotospheric dynamics of
convective flows shearing and twisting magnetic fields in the corona.
Title: Subsurface flows of solar active regions
Authors: Kosovichev, A. G.
Bibcode: 2006cosp...36.1489K
Altcode: 2006cosp.meet.1489K
Mass flows below the solar surface are likely to play significant
role in evolution of solar active regions their magnetic topology and
dynamics Recently new methods of local helioseismology have provided
three-dimensional maps of subsurface flows in active regions These maps
have revealed a great variety of complicated flow patterns of various
scales In particular local helioseismology discovered around active
regions large-scale circulation flows converging in the upper convective
layer and diverging at greater depths These persistent flows affect the
global meridional circulation on the Sun and magnetic flux transport
during the solar cycle These flows may also influence the convective
energy transport and large-scale zonal flows - torsional oscillations
Another class of subsurface flows associated with horizontal vortices
is probably important for twisting magnetic field of active regions
and generating magnetic helicity In addition local helioseismology has
found initial evidence for strong shearing flows below flaring active
regions which may be significant for initiation of solar flares and CMEs
Title: Probing solar and stellar interior dynamics and dynamo
Authors: Kosovichev, A. G.
Bibcode: 2006cosp...36.3458K
Altcode: 2006cosp.meet.3458K
Solar and stellar activity is a result of complex interaction between
magnetic field turbulent convection and differential rotation in
a star s interior Magnetic field is believed to be generated by
a dynamo process in the convection zone It emerges on the surface
forming sunspots and starspots Localization of the magnetic spots and
their evolution with the activity cycle is determined by large-scale
interior flows Thus the internal dynamics of the Sun and other stars
hold the key to understanding the dynamo mechanism and activity
cycles Recently significant progress has been made for modeling
magnetohydrodynamics of the stellar interiors and probing the internal
rotation and large-scale dynamics of the Sun by helioseismology Also
asteroseismology is beginning to probe interiors of distant stars I
review key achievements and challenges in our quest to understand the
basic mechanisms of solar and stellar activity
Title: Probing plasma properties and dynamics inside the Sun
Authors: Kosovichev, Alexander G.
Bibcode: 2006AdSpR..37.1455K
Altcode:
Helioseismology experiment MDI on board the SOHO spacecraft has provided
tremendous amount of new information about the thermodynamic properties
and dynamics of plasma in the Sun’s interior, uncovering remarkable
new perspectives for studying complex interactions between turbulent
convection, rotation and magnetism below the solar surface, and their
relationship to solar irradiance variations and coronal activity. In
particular, the new results have revealed the deep structure of
sunspots and associated complicated patterns of plasma flows, the
dynamics of the emerging magnetic flux and formation of active regions,
the supergranular structure and dynamics of the upper convection zone,
as well as the global structures and circulation patterns in the deep
interior, evolving with the activity cycle. In addition, first attempts
are made to find the links between the internal dynamics and processes
of magnetic energy release in the solar corona. Understanding these
results, often puzzling and counter-intuitive, is a major challenge
for MHD theories of astrophysical plasma.
Title: Helioseismic observations of magnetic flux emergence and
flare effects
Authors: Kosovichev, A. G.; Duvall, T. L.
Bibcode: 2006IAUS..233..365K
Altcode:
Time-distance helioseismology and data from SOHO/MDI are used for
obtaining 3D images of subsurface sound-speed perturbations and maps
of plasma flows, associated with emerging magnetic flux and flaring
activity of large active regions in October 2003. The results reveal
extremely complicated dynamical processes in the upper convection
zone and indicate that subsurface shear flows may play an important
role in magnetic energy release in solar flares. Strong X-class flares
generated impulsive seismic waves (“sunquakes”), traveling through
surrounding sunspots, thus providing new insight into the interaction
of seismic waves with magnetic fields.
Title: New Observations of Flare-Generated Sunquakes
Authors: Kosovichev, A.
Bibcode: 2005AGUFMSH11A0244K
Altcode:
Solar flares may produce "sunquakes", strong localized compressions
in the photosphere, which generate acoustic waves propagating through
the Sun's interior. These seismic waves are observed on the solar
surface as expanding wave rings. Several strong sunquake events were
observed by the Solar and Heliospheric Observatory (SOHO) in 2003-2005,
long after the first sunquake detected in 1996. Detailed analysis of
these events SOHO and RHESSI space missions reveals new interesting
properties of the seismic response to solar flares, such as close
association between the hard X-ray flare emission and the hydrodynamic
impacts on the solar surface, strong anizotropy of the seismic waves,
and their interaction with sunspots. These results are of significant
interest for understanding the physics of energy release and transport
in solar flares, and for helioseismic diagnostics of flaring active
regions. An intriguing question is why the sunquakes have been observed
only during the declining phases of the solar activity cycle.
Title: Variability of the solar interior and irradiance
Authors: Kosovichev, A.
Bibcode: 2005AGUFMSH33C..03K
Altcode:
Solar variability is controlled by dynamo processes in the Sun's
interior. The interior properties are measured by helioseismology. These
measurements are based on observations of solar oscillations and waves
which are excited by turbulent convection near the surface and propagate
through the interior. Oscillation frequencies and travel times provide
information about variations of the structure and large-scale dynamics
inside the Sun. Recently developed methods of acoustic tomography
provide 3D images of the solar interior. Variations of the global
internal properties of the Sun have been measured for two solar cycles,
and for the last 10 years the space mission SOHO and ground-based
networks (GONG, BiSON, TON) provided continuous monitoring of the
global properties including total irradiance. I review recent results
of the helioseismic measurements of the structure and dynamics of the
Sun, and discuss their relation for understanding mechanisms of solar
variability and activity.
Title: Numerical Simulations of Solar Acoustic Field
Authors: Parchevsky, K. V.; Kosovichev, A. G.
Bibcode: 2005AGUFMSH41A1118P
Altcode:
We present numerical simulations of propagation of acoustic waves in
the upper convection zone using a standard solar model and realistic
equation of state (OPAL model). The main goals are to study properties
of solar waves for various excitation sources and interaction of these
waves with spatial inhomogeneities, and also to generate artificial wave
fields for testing local helioseismic diagnostics of the solar interior,
currently used for SOHO/MDI and GONG data. In our numerical model,
non-reflecting boundary conditions based on absorbing 3D perfectly
matched layer (PML) are imposed at all boundaries of the computational
domain in Cartesian geometry. This prevents spurious reflection of
acoustic waves from boundaries back to the computational domain. The
top non-reflecting boundary is set in the solar atmosphere above the
temperature minimum. This allowed us to realistically model the wave
reflection from the solar atmosphere. We have developed a special
PLM model, numerically stable in the case of a stratified medium
with gravity, and investigated and tested various numerical schemes
(including high-order dispersion-relation-preserving scheme). Numerical
simulations have been carried out on parallel computers for different
kinds of acoustic sources(force and energy sources). Single point
sources are used to calculate realistic Green functions required for
holographic seismic imaging. Simulated acoustic field from multiple
sources randomly distributed below the photosphere is used as artificial
data for testing helioseismic inversions, accuracy of Born and ray
approximations.
Title: Time-distance analysis of realistic simulations of solar
convection
Authors: Georgobiani, D.; Zhao, J.; Benson, D.; Stein, R. F.;
Kosovichev, A. G.; Nordlund, A.
Bibcode: 2005AGUFMSH41A1117G
Altcode:
The results of the new realistic large-scale simulations of solar
turbulent convection provide an unprecedented opportunity to study
solar oscillations and perform similar local helioseismology techniques
as for the real solar data. The results offer an unique opportunity
to compare the simulated flow fields with the flows and sounds speed
variations inferred from the time-distance analysis. Applying some
of the existing local helioseismology methods to the simulated solar
convection and comparing to the observed results, one can validate
the accuracy of these methods. We apply the time-distance analysis
to the simulated data and successfully obtain the time-distance
curve and travel time maps. Our travel times are consistent with the
SOHO/MDI observations. The next step is to perform inversion to infer
the interior flow fields at various depths and compare them with the
simulated data in order to validate the model. This work is currently
in progress.
Title: Changes in the Subsurface Stratification of the Sun with the
11-Year Activity Cycle
Authors: Lefebvre, S.; Kosovichev, A. G.
Bibcode: 2005ApJ...633L.149L
Altcode: 2005astro.ph.10111L
We report on changes in the Sun's subsurface stratification inferred
from helioseismology data. Using data from the Solar and Heliospheric
Observatory (SOHO) Michelson Doppler Imager (MDI) for the last 9 years
and, more precisely, the temporal variation of f-mode frequencies, we
have computed the variation in the radius of subsurface layers of the
Sun by applying helioseismic inversions. We have found a variability
of the ``helioseismic'' radius in antiphase with the solar activity,
with the strongest variations of the stratification being just below
the surface, around 0.995 Rsolar. In addition, the radius of
the deeper layers of the Sun, between 0.975 and 0.99 Rsolar,
changes in phase with the 11-year cycle.
Title: Solar Polar Imager: Observing Solar Activity from a New
Perspective
Authors: Alexander, D.; Sandman, A.; Liewer, P.; Ayon, J.; Goldstein,
B.; Murphy, N.; Velli, M.; Floyd, L.; Moses, D.; Socker, D.; Vourlidas,
A.; Garbe, G.; Suess, S.; Hassler, D.; Kosovichev, A.; Mewaldt, R.;
Neugebauer, M.; Ulrich, R.; Zurbuchen, T.
Bibcode: 2005ESASP.592..663A
Altcode: 2005soho...16E.131A; 2005ESASP.592E.131A
No abstract at ADS
Title: Properties of the Solar Acoustic Source Inferred from
Nonadiabatic Oscillation Spectra
Authors: Wachter, R.; Kosovichev, A. G.
Bibcode: 2005ApJ...627..550W
Altcode:
Severino et al. suggested in 2001 that observed power and cross spectra
of medium-degree p-modes in velocity and intensity can be described
by splitting the solar background noise into correlated, coherent,
and uncoherent components. We account for the nonadiabatic nature of
solar oscillations by including the perturbations of the radiative
energy flux in our model for the oscillations. Our calculations show
the potential to explain the observations without the ad hoc phase
differences between velocity and intensity oscillations introduced in
the model of Severino et al. The phases and amplitudes of the correlated
noise components are obtained by fitting our nonadiabatic model to the
SOHO MDI power and cross spectra. These parameters provide information
about the p-mode excitation process. We show that the type and location
of the source can not be uniquely determined by the properties of
the resonant p-modes in power and cross spectra of velocity and
intensity oscillations. However, we obtain estimates for the phases
and amplitudes of the correlated noise, which we interpret in terms of
isolated rapid downdrafts in intergranular lanes. This idea is supported
by three-dimensional simulations of the upper solar convection zone.
Title: Time-Distance Helioseismology: Inversion of Noisy Correlated
Data
Authors: Couvidat, S.; Gizon, L.; Birch, A. C.; Larsen, R. M.;
Kosovichev, A. G.
Bibcode: 2005ApJS..158..217C
Altcode:
In time-distance helioseismology most inversion procedures ignore the
correlations in the data errors. Here we simulate the travel-time
perturbations of wavepackets that result from known distributions
of sound speed inhomogeneities. The forward and inverse problems are
carried out using recently developed Born approximation sensitivity
kernels. A realistic solar noise component, with the correct
statistics, is added to the data. We then apply a three-dimensional
inversion procedure based on an improved multichannel deconvolution
algorithm that includes the full covariance matrix of the simulated
data and constrains the solution both in the vertical and horizontal
directions. The validation of the inversion is achieved through
comparison of the inferred sound speed distributions with the exact
solutions. We show that including the covariance matrix matters for
sound speed inhomogeneities varying on a length scale smaller than
the correlation length. We also find that the inversion procedure is
improved by adding horizontal regularization.
Title: Multi-Wavelength Investigations of Solar Activity (IAU S223)
Authors: Stepanov, Alexander V.; Benevolenskaya, Elena E.; Kosovichev,
Alexander G.
Bibcode: 2005misa.book.....S
Altcode:
Section I: Solar Cycle in the Interior, Atmosphere and Heliosphere;
Section II-III: Structure and Evolution of Active Regions from the
Sub-Photospheric Layers to the Corona, Filaments and Prominences;
Section IV: Multi-Scale Coronal Structures and Links to Photospheric
Magnetic Field; Section V: Energy Transport, Storage and Release in
the Solar Atmosphere and Corona; Section VI: Heliospheric Effects and
Space Weather Research; Section VII: Multi-Wavelength Observations of
the Sun from Ground and Space.
Title: Large-Scale Solar Photosphere Simulations
Authors: Wray, A.; Mansour, N. N.; Kosovichev, A.
Bibcode: 2005AGUSMSP11C..01W
Altcode:
We have developed a 3D, compressible radiative-hydrodynamics code
for simulating the upper solar photosphere and lower atmosphere,
from depths of a few 10's of megameters below to an altitude of a few
hundred kilometers above the visible surface. Real gas opacities and
equation of state are used, and the radiation is captured through full
3D solution of the radiative transfer equation in opacity bands. The
code is fully parallelized using the Message-Passing Interface (MPI)
standard, allowing execution on both distributed and shared-memory
architectures. We have benchmarked the code on the NASA Columbia
system at Ames (an SGI Altix computer), obtaining near-ideal, linear
scaling for a 500× 500× 500 node spatial mesh using from 1 to
500 processors. This scaling behavior gives us confidence that the
code can be used to run large meshes effectively on large numbers of
processors. We will show preliminary results from a simulation of a
box 20 Mm × 20 Mm × 18 Mm at a resolution (Δ x) of approximately
40km, with a 5123 grid. Granular and intergranular lane structure and
dynamics, turbulent statistics, thermal, and radiative properties will
be described and compared to observations. Testing of subgrid-scale
turbulence models will be discussed.
Title: Effect of phase speed filters on time-distance correlations
of acoustic waves on the Sun.
Authors: Nigam, R.; Rajaguru, P.; Kosovichev, A. G.
Bibcode: 2005AGUSMSP11B..02N
Altcode:
Use of phase-speed filters in time-distance helioseismic measurements
is crucial to obtain spatially resolved information about localised
sub-surface structures. These filters have to be chosen such that
the travel times of the waves that are filtered in are themselves
not affected by the filtering process. Here we derive analytically the
cross-correlation signal that results from phase-speed filtered signals,
assuming plane wave conditions. The resulting wavelet explicitly depends
on the parameters of the filters, such as the phase-speed and its
dispersion, in contrast to the currently used Gabor wavelet, and hence
accounts for any filter induced changes in travel times. Alternatively,
this new wavelet allows the determination of optimum parameters for
the filters.
Title: The Effects of Inclined Magnetic Field over Time-Distance
Measurements
Authors: Zhao, J.; Kosovichev, A. G.
Bibcode: 2005AGUSMSP13A..02Z
Altcode:
Recent observation by phase-sensitive acoustic holography has shown
that the inclined magnetic field has significant effects over the
local helioseismology measurements, and it was thus suggested that the
acoustic wave phase shift is substantially caused by the photospheric
magnetic field. We make the similar measurements by use of time-distance
helioseismology technique over a sunspot when it rotated from the
solar east limb to the west. We find that the wave travel time is often
longer in the part of sunspot penumbra that is closer to the solar disk
center, similar to what was found by acoustic holography analysis. We
investigate the other possible reasons that may cause this effect,
e.g., projection effect, Doppler velocity masking effect, and find
these effects do not account for all the travel time variations. We
perform time-distance inversions to investigate how these measurements
affect interior structures inferred from inversions.
Title: Systematic errors in Dopplergrams of active regions
Authors: Wachter, R.; Schou, J.; Kosovichev, A. G.; Rajaguru, P.
Bibcode: 2005AGUSMSP13A..07W
Altcode:
Dopplergrams of magnetic regions are used in time-distance
helioseismology and for the investigation of oscillations in
sunspots. MDI Dopplergrams are produced onboard SOHO by measuring four
filtergrams which are sensitive to intensity fluctuations in the wings
of the Ni i absorption line at 6768 Å. The line of sight velocity
is inferred from a lookup table which is based on the shape of the
line in the quiet sun. In magnetic regions, however, the line shape
changes drastically and molecular absorption lines appear close to the
line's wavelength. This problem is equally relevant for the upcoming
Helioseismic and Magnetic Imager (HMI) which measures velocities based
on the same principles as MDI, using however a different absorption
line (Fe i at 6173 Å). Based on high spectral resolution images of
magnetic regions obtained by the ASP (Advanced Stokes Polarimeter)
instrument we show that the MDI Doppler velocities are systematically
underestimated in magnetic regions. We discuss possibilities to correct
the velocity measurements in magnetic regions using intensity and
magnetic field data.
Title: A Search for the Relationship Between Subphotospheric Dynamics
of Active Regions and Flaring Activity
Authors: Kosovichev, A. G.; Duvall, T. L.
Bibcode: 2005AGUSMSP51C..04K
Altcode:
MHD models of solar flares and CMEs suggest the magnetic energy for
these events can be accumulated and released in magnetic structures
sheared and twisted by plasma motions. We use time-distance
helioseismology for investigating subphotospheric structures and
dynamics of active regions that might be related to their flaring
activity. In particular, we present a detailed study of active region
NOAA 10486, which produced a series of X-class flares, during its
passage on the solar disk for 8 days, Oct.25-Nov.1, 2003. The maps
of subsurface flows and sound-speed perturbations are obtained from
the SOHO/MDI data every 2-hours during this period, with 8-hour
resolution and for the depth range of 0-12 Mm, and compared with the
MDI magnetograms and X-ray data from RHESSI and GOES. The results
reveal interesting dynamics at the depth of 4-6 Mm, which is compared
with the restructuring, emergence and cancellation the magnetic field
in this region.
Title: Solar Supergranulation as Propagating Waves
Authors: Green, C.; Kosovichev, A. G.
Bibcode: 2005AGUSMSP11C..03G
Altcode:
It has been observed that the supergranulation pattern on the surface
of the Sun appears to rotate faster than the photospheric plasma
and magnetic features. It is postulated that this could be due to
instabilities in the subsurface convective shear layer. This behaviour
is modelled, starting with a linearized system of differential equations
describing a convectively unstable region containing a horizontal
shear flow. The system is solved numerically, using parameters drawn
from the solar model and helioseismic inversions and assuming linear
and non-linear shear flow profiles, for a range of wavenumbers. The
phase speeds of the resulting wave solutions are found to be greater
than the surface speed, possibly explaining the observed behaviour.
Title: Study of the Solar Cycle Dependence of Low-Degree p-Modes
with Michelson Doppler Imager and VIRGO
Authors: Toutain, T.; Kosovichev, A. G.
Bibcode: 2005ApJ...622.1314T
Altcode:
Studying variations of observational properties of low-degree solar
acoustic p-modes (radial, dipole, and quadrupole) with the activity
cycle is important for understanding potential effects of the cycle
in the deep interior, obtaining more accurate inversions for the sound
speed and rotation, and also for estimating prospective investigation
of stellar activity by asteroseismology. We have analyzed changes
of low-degree p-mode parameters during the current solar cycle
(number 23) by using velocity and intensity oscillation data from two
instruments on the Solar and Heliospheric Observatory (SOHO): the
Michelson Doppler Imager (MDI) and Variability of Solar Irradiance
and Gravity Oscillations (VIRGO). We show a clear dependence of
mode frequency, line width, frequency splitting, and line asymmetry
on the solar cycle. Moreover, we demonstrate that these properties
change differently for modes of different frequency, angular degree,
and order. The greatest changes are observed for sectorial modes of
frequencies higher than 3 mHz, corresponding to the fact that the most
significant variations of the solar structure occur in low-latitude
regions close to the solar surface.
Title: Sun's global property measurements: helioseismic probing of
solar variability
Authors: Kosovichev, A. G.
Bibcode: 2005MmSAI..76..743K
Altcode:
Solar variability is controlled by dynamo processes in the Sun's
interior. Global and local interior properties are measured by
helioseismology. These measurements are based on observations of solar
oscillations and waves which are excited by turbulent convection near
the surface and propagate through the interior. Oscillation frequencies
and travel times provide information about variations of the structure
and large-scale dynamics inside the Sun. Recently developed methods of
acoustic tomography provide 3D images of the solar interior. Variations
of the global internal properties of the Sun have been measured for
two solar cycles, and for the last 10 years the space mission SOHO
and ground-based networks (GONG, BiSON, TON) provided continuous
monitoring of the global properties. Recent helioseismic measurements
of the structure and dynamics of the Sun provide new information about
variations of the solar radius, asphericity, internal structure of
sunspots and active regions, important for understanding mechanisms
of the solar irradiance variations. The results provide support for a
picture of a smaller and cooler, on average, Sun during the activity
maxima, the higher irradiance of which is explained by a corrugated
surface due to magnetic fields.
Title: The Solar Energetic Particle Event of 16 August 2001: ~
400 MeV Protons Following an Eruption at ~ W180
Authors: Cliver, E. W.; Thompson, B. J.; Lawrence, G. R.; Zhukov,
A. N.; Tylka, A. J.; Dietrich, W. F.; Reames, D. V.; Reiner, M. J.;
MacDowall, R . J.; Kosovichev, A. G.; Ling, A. G.
Bibcode: 2005ICRC....1..121C
Altcode: 2005ICRC...29a.121C
No abstract at ADS
Title: Helioseismology for SolarB and Joint Investigations with
SDO/HMI Project
Authors: Kosovichev, A.
Bibcode: 2004ASPC..325...75K
Altcode:
New methods of local helioseismology provide information about
subsurface convective and shear flows, thermal and magnetic
structures, which is critical for accomplishing the SolarB scientific
objectives. I review the current status of acoustic tomography,
observational requirements and limitations, and discuss observations
of sub-photospheric vortex and shear motions, subsurface transport and
flux emergence, and links between subphotospheric dynamics, magnetic
topology, and coronal activity.
Title: Large-Scale Flow Fields Beneath Major Solar Active Regions
Authors: Zhao, J.; Kosovichev, A. G.
Bibcode: 2004AGUFMSH13A1143Z
Altcode:
By use of MDI dynamic campaign data and by employing time-distance
helioseismology, we have continued our efforts to map synoptic flow
charts in the solar interior to deeper depths, especially under some
major active regions, such as AR9393 and AR10486. Two dimensional
horizontal flow fields, as well as vorticity and kinetic helicity
distributions, are presented for a few Carrington rotations. Flow
fields, vorticity and kinetic helicity distributions are studied in
more details under major solar active regions, in particular, before
and after major solar flares. We try to identify connections between
solar flares with these subsurface dynamical properties.
Title: Diagnostics of Subphotospheric Sources of Solar Variability
Authors: Kosovichev, A. G.; Duvall, T. L.; Zhao, J.
Bibcode: 2004AGUFMSH13A1144K
Altcode:
Local helioseismology provides new tools for studying subphotospheric
processes that are related to solar variability of various spatial
and temporal scales. Large-scale flow patterns beneath active
regions and sunspots affect the solar energy transport in the upper
convection zone. Smaller-scale shearing and twisting flows may trigger
instabilities of magnetic configurations that lead to flares and
CMEs. We discuss the recent progress in the local helioseismology
diagnostic tools and in our understanding of the subphotospheric
dynamics and sources of variability.
Title: High Degree Solar Oscillations in 3d Numerical Simulations
Authors: Georgobiani, D.; Stein, R. F.; Nordlund, Å.; Kosovichev,
A. G.; Mansour, N. N.
Bibcode: 2004ESASP.559..267G
Altcode: 2004soho...14..267G
No abstract at ADS
Title: The Current Status of Analyzing High-Degree Modes
Authors: Reiter, J.; Rhodes, E. J., Jr.; Kosovichev, A. G.; Schou, J.
Bibcode: 2004ESASP.559...61R
Altcode: 2004soho...14...61R
No abstract at ADS
Title: Oscillation Spectra of Line Depth, Intensity and Velocity
from Radiative Transfer Calculations
Authors: Wachter, R.; Haberreiter, M.; Kosovichev, A. G.
Bibcode: 2004ESASP.559..668W
Altcode: 2004soho...14..668W
No abstract at ADS
Title: Turbulence Convection and Oscillations in the Sun
Authors: Mansour, N. N.; Kosovichev, A. G.; Georgobiani, D.; Wray,
A.; Miesch, M.
Bibcode: 2004ESASP.559..164M
Altcode: 2004soho...14..164M
No abstract at ADS
Title: Properties of the Acoustic Source Inferred from Nonadiabatic
Oscillation Spectra
Authors: Wachter, R.; Kosovichev, A. G.
Bibcode: 2004ESASP.559...55W
Altcode: 2004soho...14...55W
No abstract at ADS
Title: Probing Solar Dynamics in the Upper Convection Zone by
Time-Distance Helioseismology
Authors: Zhao, J.; Kosovichev, A. G.
Bibcode: 2004ESASP.559..672Z
Altcode: 2004soho...14..672Z
No abstract at ADS
Title: Erratum: ``Looking for Gravity-Mode Multiplets with the GOLF
Experiment aboard SOHO'' (ApJ,
604, 455 [2004])
Authors: Turck-Chièze, S.; García, R. A.; Couvidat, S.; Ulrich,
R. K.; Bertello, L.; Varadi, F.; Kosovichev, A. G.; Gabriel, A. H.;
Berthomieu, G.; Brun, A. S.; Lopes, I.; Pallé, P.; Provost, J.;
Robillot, J. M.; Roca Cortés, T.
Bibcode: 2004ApJ...608..610T
Altcode:
As a result of an error at the Press, the second panel of Figure 9
was repeated twice in the top row of the printed, black-and-white
version of this figure, and the first panel was omitted. This error
appears in the print edition and the PDF and postscript (PS) versions
available with the electronic edition of the journal, although the
panels of the color figure displayed in the electronic article itself
are correct. Please see below for the corrected print version of Figure
9. The Press sincerely regrets the error.
Title: Sensitivity of Acoustic Wave Travel Times to Sound-Speed
Perturbations in the Solar Interior
Authors: Birch, A. C.; Kosovichev, A. G.; Duvall, T. L., Jr.
Bibcode: 2004ApJ...608..580B
Altcode:
For time-distance helioseismology, it is important to establish the
relationship between the travel times of acoustic waves propagating
between different points on the solar surface through the solar interior
and local perturbations to the sound speed in the propagation region. We
use the Born approximation to derive a general expression for the
linear sensitivity of travel times to local sound-speed perturbations
in plane-parallel solar models with stochastic wave sources. The results
show that the sensitivity of time-distance measurements to perturbations
in sound speed depends on the details of the measurement procedure,
such as the phase-speed filter used in typical time-distance data
analysis. As a result, the details of the measurement procedure should
be taken into account in the inversion of time-distance data. Otherwise,
the inferred depths of perturbations may be incorrect.
Title: On the Relationship between the Rotational Velocity and the
Field Strength of Solar Magnetic Elements
Authors: Zhao, Junwei; Kosovichev, Alexander G.; Duvall, Thomas L., Jr.
Bibcode: 2004ApJ...607L.135Z
Altcode:
By tracking various solar surface tracers, previous studies have found
that magnetic structures, such as plages and sunspots, rotate faster
than the quiet solar regions. We investigate how the rotational speed of
these magnetic features is related to their magnetic field strength. By
use of near-surface horizontal velocities inferred from time-distance
helioseismology, we have studied a few Carrington rotations covering
the years 1997-2002, from near the solar minimum to the maximum. It
is found that the residual rotational velocity of magnetic elements
nearly linearly depends on their magnetic field strength: the stronger
the magnetic field strength, the faster the magnetic elements rotate
relative to the quiet solar regions. It is also found that the
magnetic elements rotate faster in the solar maximum years than the
elements of the same magnetic strength but in years with moderate solar
activity. For all Carrington rotations studied, magnetic elements of the
following polarity rotate faster than the leading polarity elements of
the same magnetic strength. Possible interpretations of the observed
relations are discussed. Prograde supergranular advection may cause
the faster rotation of the following polarity elements.
Title: Three-dimensional Inversion of Time-Distance Helioseismology
Data: Ray-Path and Fresnel-Zone Approximations
Authors: Couvidat, S.; Birch, A. C.; Kosovichev, A. G.; Zhao, J.
Bibcode: 2004ApJ...607..554C
Altcode:
Time-distance helioseismology has provided important new insight into
the subphotospheric structure and dynamics of sunspots, active regions,
supergranular cells, and large-scale flows. These inferences have been
made by using either the ray-path or Fresnel-zone approximations. We
present inversion results of travel-time perturbations of wavepackets
propagating inside the Sun, using both ray-path and Fresnel-zone
kernels for real and artificial data. The ray approximation was
the first approximation used in time-distance helioseismology for
deriving the sensitivity of travel times to perturbations in the solar
interior. However, new types of sensitivity kernels are being developed
to take into account finite-wavelength effects (such as kernels based
on a Fresnel-zone approximation) and thus improve the resolution and
accuracy of the inversions. Since many results have been obtained with
the ray-path approximation, it is important to compare them with the
new Fresnel-zone inversions to quantify their accuracy. We have applied
the two approximations to artificial and real data and concluded that
both approximations provide similar results for structures lying within
the scope of the kernels. Nonetheless, the vertical structure can be
inferred at greater depth with the Fresnel-zone kernels than with the
ray-path ones, using the same travel-time data. Applying Fresnel-zone
inversion to the MDI time-distance data of 1998 June 20 we confirm
the two-part structure of the sunspots previously derived with the
ray approximation.
Title: Subphotospheric Dynamics During the Period of Massive Solar
Flares
Authors: Kosovichev, A. G.; Duvall, T. L., Jr.
Bibcode: 2004AAS...204.4703K
Altcode: 2004BAAS...36Q.737K
Unstable coronal structures that lead to impulsive energy release in
solar flares are created by magnetic flux emergence and by photospheric
and subphotospheric motions shearing and twisting magnetic field
lines. We present results of investigation of subsurface plasma flows
and emerging structures associated with AR 10484, 10486, and 10488,
which produced the majority of massive flares of October-November
2003. The results representing 3-D maps of sound-speed perturbations
and flow velocities are obtained from SOHO/MDI full-disk Doppler data
by time-distance helioseismology with the horizontal resolution of 3
Mm, vertical resolution of 0.7-4 Mm (in the depth range of 0-40 Mm),
and temporal resolution of 8 hours. These maps are compared with the
corresponding MDI magnetograms, and also with TRACE and RHESSI images
to determine the role of subphotospheric dynamics in the development
of the active regions and their flaring activity.
Title: Structure Properties of Supergranulation and Granulation
Authors: Berrilli, F.; Del Moro, D.; Consolini, G.; Pietropaolo, E.;
Duvall, T. L., Jr.; Kosovichev, A. G.
Bibcode: 2004SoPh..221...33B
Altcode:
We investigate spatial dislocation ordering of the solar structures
associated with supergranulation and granulation scales. The
supergranular and granular structures are automatically segmented
from time-distance divergence maps and from broad-band images,
respectively. The spatial dislocation ordering analysis is accomplished
by applying the statistical method of Pair Correlation Function,
g2(r), to segmented features in the solar fields. We
compare the computed g2(r) functions obtained from both
single and persistent, i.e., time-averaged, fields associated with
supergranulation and granulation. We conclude that supergranulation
and granulation patterns present a different topological order both
in single and persistent fields. The analysis carried out on single
fields suggests that the granulation behaves as an essentially random
distribution of soft plasma features with a very broad distribution
in size, while supergranulation behaves as a random distribution of
close packed, coherent stiff features with a rather defined mean size.
Title: Dynamics and Structure of Supergranulation
Authors: Del Moro, D.; Berrilli, F.; Duvall, T. L., Jr.; Kosovichev,
A. G.
Bibcode: 2004SoPh..221...23D
Altcode:
In this paper we investigate the temporal evolution and geometric
properties of solar supergranular features. For this purpose we apply
an automatic feature-tracking algorithm to a 6-day time series of 18
near-surface flowmaps containing 548 target objects. Lifetimes are
calculated by measuring the time elapsing between the birth and death
of each target. Using an exponential fit on the lifetime distribution
of single supergranules we derived a mean lifetime of 22 hours. Based
on the application of segmentation numerical procedures, we estimated
characteristic geometric parameters such as area distributions of
supergranular cells. We also derive the relationship between measured
lifetime and the area of the supergranules.
Title: Probing Plasma Structures and Dynamics Inside the Sun
Authors: Kosovichev, A. G.
Bibcode: 2004AIPC..703..209K
Altcode:
Helioseismology has provided tremendous amount of new information
about the thermodynamic properties and dynamics of plasma in the Sun's
interior. This gives us new insights into the basic physical processes
inside the Sun, formation of magnetic structures in the solar plasma
and mechanisms of solar and stellar activity. In particular, the new
results have revealed the deep structure of sunspots and associated
complicated patterns of plasma flows, the dynamics of the emerging
magnetic flux and formation of active regions, the supergranular
structure and dynamics of the upper convection zone, as well as
the global structures and circulation patterns in the deep interior,
evolving with the activity cycle. In addition, first attempts are made
to find links between the internal dynamics and processes of magnetic
energy release in the solar corona. Understanding of these results,
often puzzling and counter-intuitive, represents a major challenge
for MHD theories of astrophysical plasma.
Title: Looking for Gravity-Mode Multiplets with the GOLF Experiment
aboard SOHO
Authors: Turck-Chièze, S.; García, R. A.; Couvidat, S.; Ulrich,
R. K.; Bertello, L.; Varadi, F.; Kosovichev, A. G.; Gabriel, A. H.;
Berthomieu, G.; Brun, A. S.; Lopes, I.; Pallé, P.; Provost, J.;
Robillot, J. M.; Roca Cortés, T.
Bibcode: 2004ApJ...604..455T
Altcode:
This paper is focused on the search for low-amplitude solar gravity
modes between 150 and 400 μHz, corresponding to low-degree, low-order
modes. It presents results based on an original strategy that looks
for multiplets instead of single peaks, taking into consideration
our knowledge of the solar interior from acoustic modes. Five years
of quasi-continuous measurements collected with the helioseismic GOLF
experiment aboard the SOHO spacecraft are analyzed. We use different
power spectrum estimators and calculate confidence levels for the
most significant peaks. This approach allows us to look for signals
with velocities down to 2 mm s-1, not far from the limit
of existing instruments aboard SOHO, amplitudes that have never been
investigated up to now. We apply the method to series of 1290 days,
beginning in 1996 April, near the solar cycle minimum. An automatic
detection algorithm lists those peaks and multiplets that have a
probability of more than 90% of not being pure noise. The detected
patterns are then followed in time, considering also series of 1768 and
2034 days, partly covering the solar cycle maximum. In the analyzed
frequency range, the probability of detection of the multiplets
does not increase with time as for very long lifetime modes. This is
partly due to the observational conditions after 1998 October and the
degradation of these observational conditions near the solar maximum,
since these modes have a ``mixed'' character and probably behave as
acoustic modes. Several structures retain our attention because of
the presence of persistent peaks along the whole time span. These
features may support the idea of an increase of the rotation in the
inner core. There are good arguments for thinking that complementary
observations up to the solar activity minimum in 2007 will be decisive
for drawing conclusions on the presence or absence of gravity modes
detected aboard the SOHO satellite.
Title: Torsional Oscillation, Meridional Flows, and Vorticity
Inferred in the Upper Convection Zone of the Sun by Time-Distance
Helioseismology
Authors: Zhao, Junwei; Kosovichev, Alexander G.
Bibcode: 2004ApJ...603..776Z
Altcode:
By applying time-distance helioseismology measurements and inversions
to Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager
(MDI) dynamics campaign data, we obtain synoptic maps of subsurface
plasma-flow fields at a depth of 0-12 Mm for seven solar Carrington
rotations, covering the years 1996-2002, from solar-activity minimum to
maximum. Vorticity distribution and both zonal and meridional flows are
derived from such synoptic flow maps, which contain an enormous amount
of information about solar dynamics. The results for the zonal flows
agree well with previous results. The meridional flows of an order of 20
m s-1 are found to remain poleward during the whole period of
observations. In addition to the poleward meridional flows observed at
the solar minimum, extra meridional circulation cells of flow converging
toward the activity belts are found in both hemispheres, which may imply
plasma downdrafts in the activity belts. These converging flow cells
migrate toward the solar equator together with the activity belts as
the solar cycle evolves. The vorticity distributions are largely linear
with latitude, and the deviations from the vorticity caused by the mean
differential rotation are presented. Patterns of large-scale flows are
investigated for a large active region at different depths. Converging
flows toward the center of the active region are found near the solar
surface, and divergent flows in this large active region are found
to be rooted much deeper than similar flows observed in individual
sunspots. We conclude that the extremely rich and complicated dynamics
of the upper convection zone reveal remarkable organization on the
large scale, which can be correlated with the magnetic activity zones.
Title: Subphotospheric Structure of Sunspots and Active Regions
Authors: Kosovichev, Alexander G.
Bibcode: 2004IAUS..223..171K
Altcode: 2005IAUS..223..171K
New methods of local area helioseismology provide three-dimensional maps
of sound-speed variations and mass flows in the upper convection zone,
giving important insight into the internal structure and dynamics of
sunspots and active regions. Most of these results are obtained from
SOHO/MDI data using the method of time-distance helioseismology (or
acoustic tomography). Robustness of this method has been significantly
improved by incorporating most important wave propagation effects
and stochastic properties of solar oscillations. Time-distance
helioseismology reveals that developed sunspots have a two layer
structure: a relatively thin sub-photospheric layer of lower sound
speed, and a deeper layer of higher sound speed. The mass flows in the
upper layer are typically converging and directed downward, while in the
deeper interior the flows are mostly diverging. These results support
the cluster model of sunspots suggested by Parker. New observations
also provide interesting information about emerging magnetic flux,
formation and evolution of active regions and complexes of activity,
and allow us to investigate effects of sub-photospheric dynamics of
active regions on the global circulation of the Sun and also small-scale
rapid shear flows associated with flares and CME.
Title: Multi-wavelength investigations of solar activity : proceedings
of the 223th symposium of the International Astronomical Union held
in Saint Petersburg, Russia, June 14-19, 2004
Authors: Stepanov, Alexander V.; Benevolenskaya, Elena E.; Kosovichev,
Alexander G.
Bibcode: 2004IAUS..223.....S
Altcode: 2005IAUS..223.....S
No abstract at ADS
Title: Probing Plasma Properties and Dynamics Inside the Sun
Authors: Kosovichev, A. G.
Bibcode: 2004cosp...35.1908K
Altcode: 2004cosp.meet.1908K
Helioseismology experiment MDI on board the SOHO spacecraft has provided
tremendous amount of new information about the thermodynamic properties
and dynamics of plasma in the Sun's interior, uncovering remarkable new
perspectives for studying the complex interactions between turbulent
convection, rotation and magnetism below the solar surface, and their
relationship to solar irradiance variations and coronal activity. In
particular, the new results have revealed the deep structure of
sunspots and associated complicated patterns of plasma flows, the
dynamics of the emerging magnetic flux and formation of active regions,
the supergranular structure and dynamics of the upper convection zone,
as well as the global structures and circulation patterns in the deep
interior, evolving with the activity cycle. In addition, first attempts
are made to find the links between the internal dynamics and processes
of magnetic energy release in the solar corona. Understanding these
results, often puzzling and counter-intuitive, is a major challenge
for MHD theories of astrophysical plasma.
Title: The Sub-surface Characteristics of Sunspots
Authors: Kosovichev, A. G.
Bibcode: 2004cosp...35.1907K
Altcode: 2004cosp.meet.1907K
New methods of helioseismology provide 3-D subphotospheric maps of
sunspots, emerging magnetic structures and associated convective
flows. This gives us new insights into the basic physical processes
inside the Sun, formation of magnetic structures in the solar plasma
and mechanisms of solar and stellar activity. The current results
from the MDI instrument on SOHO reveal complicated stratification and
flow patterns of solar plasma beneath sunspots, with a characteristic
temperature increase in deeper layers and multiple vortex systems,
providing evidence for a cluster model of sunspots. The helioseismic
observations provide importants clues for understanding the mechanisms
of formation of sunspots and active regions, and the subphotospheric
dynamics of convective flows shearing and twisting magnetic fields in
the corona.
Title: The Sun's Surface and Subsurface. Investigating Shape and
Irradiance
Authors: Kosovichev, Alexander
Bibcode: 2004EOSTr..85...32K
Altcode:
Since the Sun's radiative output establishes the Earth's thermal
environment, knowing the source and nature of its variability
is essential for understanding and predicting the interactions
in the Earth-Sun system, among which are climate changes and the
energy balance, photochemistry, and dynamics of the middle and upper
atmosphere. The Sun's Surface and Subsurface brilliantly demonstrates
how precise measurements of the Sun's properties, such as the solar
diameter, oblateness, irradiance, and oscillation frequencies of
normal modes provide insight into the structure and dynamics of the
deep interior and the physical mechanisms of solar variability. This
relatively compact book provides a comprehensive review of the
basic principles, methodology, and tools for studying the solar
variability. The main focus is on measurements and interpretation
of the solar properties rather than on theory. It provides both an
excellent introduction to the field and an exciting review of the
recent advances in solar observations from the ground and space.
Title: Sensitivity of Time-Distance Measurements to Local Changes
in Sound Speed and Source Properties
Authors: Birch, A.; Kosovichev, A.
Bibcode: 2003AGUFMSH21B0163B
Altcode:
In order to interpret time-distance measurements of travel times it
is important to understand the sensitivity of these measurements to
different types of perturbations to a solar model. We will show example
sensitivity functions for local perturbations to the sound-speed,
source strength, and source correlation time. These examples show
that the effect of a sound-speed perturbation on the time-distance
travel-times depends on the details of the filtering. In particular,
for narrow phase-speed filters, the sensitivity functions do not
resemble the now traditional banana-doughnut kernels.
Title: Solar Subsurface Synoptic Flow Maps by Time-Distance
Helioseismology
Authors: Zhao, J.; Kosovichev, A. G.
Bibcode: 2003AGUFMSH42B0537Z
Altcode:
The synoptic flow maps have been made up to 12 Mm beneath the solar
surface by use of time-distance helioseismology measurements and
inversions. The study selects one Carrington rotation each year from
1996 to 2002, covering from solar minimum to maximum. The synoptic flow
map has a high spatial resolution of 0.24 heliographic degree per pixel,
and supergranular flows are able to be shown. Zonal and meridional
flows, as well as the vorticity distribution are derived from such
maps. It is found that after the subtraction of the solar minimum data,
the residual meridional flows converge toward the activity belts, and
migrate toward the equator together with the sunspot zones. Results
are to be compared with those obtained from previous studies such as
frequency splitting and ring-diagram analysis.
Title: Evolution of Active Regions in the Solar Interior
Authors: Kosovichev, A. G.; Duvall, T. L.
Bibcode: 2003AGUFMSH22A0186K
Altcode:
Using data from SOHO and TRACE, we investigate the emergence,
evolution and dissipation of magnetic active regions in the
solar interior and atmosphere by comparing 3D maps of subsurface
structures and plasma flows, obtained by acoustic tomography, with the
corresponding photospheric magnetograms and coronal EUV images. We
find that the growth of active regions is characterized by multiple
emergence of magnetic flux structures propagating very rapidly in the
upper convection zone and by the formation of large-scale converging
flows. During the decay, we have observed mostly diverging flows, and
have attempted to detect submergence of magnetic flux. We look at some
details of the dynamics of active regions, and discuss initial results
of a search for the relationship between subphotospheric shearing flows,
and changes in magnetic topology and flaring activity in the corona.
Title: Time-Distance Helioseismology: How The Inversion Results
Depend On The Approximation Used
Authors: Couvidat, S.; Birch, A. C.; Kosovichev, A. G.; Zhao, J.
Bibcode: 2003AGUFMSH42B0534C
Altcode:
During the last decade, time-distance helioseismology has provided
important new insight into the solar sub-photospheric structure
and dynamics of sunspots, active regions, supergranular cells, and
large-scale flows. These results were based either on the ray-path or
on the Fresnel-zone approximations. We present inversion results of
travel-time perturbations of wavepackets propagating inside the Sun,
using both ray-path and Fresnel-zone kernels for real and artificial
data. The ray approximation was the first approximation in time-distance
helioseismology for deriving the travel-times. However new types of
kernels are being developed to take into account the finite-wavelength
effects of the wavepackets (such as Fresnel-zone kernels), and thus
improve the resolution and accuracy of the inversions. Since many
results have been obtained with the ray-path approximation, it is
important to compare them with the new Fresnel-zone inversions to
quantify their accuracy.
Title: Solar Seismic Models and the Neutrino Predictions
Authors: Couvidat, S.; Turck-Chièze, S.; Kosovichev, A. G.
Bibcode: 2003ApJ...599.1434C
Altcode: 2002astro.ph..3107C
This paper focuses on the solar neutrino fluxes, the g-mode predictions,
and the possible impact of the magnetic fields on the neutrino emission
and transport. The Solar and Heliospheric Observatory (SOHO) spacecraft
has allowed astrophysicists to achieve a major breakthrough in the
knowledge of the solar core. Both GOLF and MDI instruments on SOHO
have significantly improved the accuracy of the sound speed profile,
mainly by the detection of low-degree low-order p-modes. Our study
(Turck-Chièze and coworkers) has lead to precise neutrino predictions
through constructing a seismic solar model that is in good agreement
with the sound speed profile inferred by helioseismology in the
radiative interior of the Sun. In this paper we present the details
of this study and investigate new solar models validated by the
acoustic modes. These new models are primarily used to derive the
emitted neutrino fluxes. We show that these fluxes do not depend
strongly on the modified physics as far as the model is consistent
with the helioseismic observations in the core. We also show that an
internal large-scale magnetic field cannot exceed a maximum strength
of ~=3×107 G in the radiative zone and may increase the
emitted 8B neutrino flux only by ~=2%. All the neutrino
predictions here are compatible with the Sudbury Neutrino Observatory
results, assuming three neutrino flavors. We deduce the electron and
neutron radial densities that are needed to calculate the neutrino
oscillation properties. Finally, we discuss how the magnetic fields
may influence the neutrino transport through the RSFP process, for
different values of Δm2.
Title: Fast photospheric flows and magnetic fields in a flaring
active region
Authors: Meunier, N.; Kosovichev, A.
Bibcode: 2003A&A...412..541M
Altcode:
We present new results from the coordinated observations between
the THEMIS telescope (in the multi-line spectropolarimetric mode)
and Michelson Doppler Imager (MDI) on SOHO obtained in November 2000
for active region NOAA 9236 which was the source of several X-class
flares. The goal of these observations was twofold: to verify MDI
measurements of the line-of-sight components of flow velocity and
magnetic field, and to obtain more information about the photospheric
flows and magnetic fields in flaring regions. Using the simultaneous
observational data in several lines we have analyzed the structure and
dynamics of this active region at the photospheric level before and
after a X4.0 flare of November 26, the last major flare produced by
this very active region. Vector magnetic field maps are computed from
the THEMIS data by full inversion of the Stokes line profiles. In
the Doppler velocity maps from THEMIS and MDI, we observe fast
photospheric flows which appear to be supersonic in two regions
located close to the region where the flare occurred. These flows
seem to be long-lived (several hours at least). In one position,
we observe a supersonic downflow strongly inclined with respect to
the vertical (by 51deg), while in another position, a flow
suggesting a strong shear with a supersonic component as well, although
almost horizontal upflows and downflows cannot be ruled out in that
case. These flows seem to be present at least 8 hours before the flare,
and the amplitude in the second case appeared to be modified during
the flare, especially, during the first minutes. In the MDI data, we
observed strong permanent changes of the longitudinal magnetic flux,
associated with the flare. The role of the strong flows and their
interaction with the magnetic field in the development of the active
region and the flare is not understood yet.
Title: Analysis of relationship between flaring activity and
subphotospheric flows in NOAA 9393
Authors: Kulinová, A.; Dzifcáková, E.; Duvall, T. L., Jr.;
Kosovichev, A. G.
Bibcode: 2003ESASP.535..125K
Altcode: 2003iscs.symp..125K
The relationship between the subphotospheric flows and flaring activity
is not well understood. It is believed that subphotospheric shearing
flows play important role in creating unstable magnetic topology
that leads to initiation of flares and CMEs. In this paper, we study
subphotospheric flows and their relationship with two flares observed
in active region NOAA 9393. One of the flares is connected with halo
CME. SOHO/MDI and helioseismology data are used for determining the
changes in morphology and are compared with changes of the topology
as observed by TRACE. We find evidence of some connections between
subphotospheric flows within 12 Mm below the photosphere and changes
of photospheric magnetic fields and also the flaring activity.
Title: Characterization of supergranular features via topological
measures
Authors: Berrilli, F.; del Moro, D.; Giordano, S.; Consolini, G.;
Kosovichev, A.
Bibcode: 2003ESASP.535...47B
Altcode: 2003iscs.symp...47B
The spatial configuration of enhanced magnetic field (active regions)
in the outer layers of the Sun derives from the interaction between
convective flows and solar magnetic field. Temporal evolution of
active regions is considered the main responsible of radiative
output variations. Particularly, solar irradiance variations are
explained in terms of temporal and spatial evolution of solar
surface magnetic fields. A key role in this evolution is played by
supergranular convective flow that, advecting magnetic flux tubes,
is also responsible of the creation of the magnetic network. This
latter results located on the boundaries of supergranular cells. In
order to measure both the spatial correlation length in supergranular
structures and the degree of near neighbor order in such structures, we
employ two topological analysis methods, the Pair Correlation Function
g2(r) and the Information Entropy H'(l). More in detail,
we apply these statistical methods to segmented images of divergence
maps derived from the application of the time-distance technique to
MDI/SOHO data. We assume that segmented images are representative of
the mass flows associated to convective supergranular motions.
Title: What helioseismology teaches us about the Sun
Authors: Kosovichev, A. G.
Bibcode: 2003ESASP.535..795K
Altcode: 2003iscs.symp..795K
Helioseismology uses observations of oscillations of the solar surface
to determine the internal structure and dynamics of the Sun, providing
critical knowledge about the mechanisms of solar variability and
activity cycles. The recent advances based on observations from SOHO
spacecraft and GONG network have allowed us to study both long-term
changes of the global structure and circulation and short-term
variations associated with developing active regions, sunspots
and coronal activity. In particular, the global helioseismology
results have revealed 1.3-year variations of the rotation rate in the
tachocline, but found no indication of 11-year variations. Studies
of the meridional circulation have shown formation of additional
meridional cells of flows converging toward the activity belts, thus,
questioning the flux-transport theories of the solar cycle. It is
found that sunspots as cool objects appear to be only 4-5 Mm deep,
but accumulate significant heat in the deeper layers, and also form
converging downflows. Large active regions are formed as a result of
multiple flux emergence, and no evidence of large-scale emerging Ω-loop
has been found. This paper presents a brief review of these and some
other results of helioseismology, analysis techniques, and perspectives.
Title: Helioseismic Observation of the Structure and Dynamics of a
Rotating Sunspot Beneath the Solar Surface
Authors: Zhao, Junwei; Kosovichev, Alexander G.
Bibcode: 2003ApJ...591..446Z
Altcode:
Time-distance helioseismology is applied to study the subphotospheric
structures and dynamics of an unusually fast-rotating sunspot observed
by the Michelson Doppler Imager on board SOHO in 2000 August. The
subsurface sound speed structures and velocity fields are obtained for
the sunspot region at different depths from 0 to 12 Mm. By comparing
the subsurface sound speed variations with the surface magnetic field,
we find evidence for structural twists beneath the visible surface of
this active region, which may indicate that magnetic twists often seen
at the photosphere also exist beneath the photosphere. We also report
on the observation of subsurface horizontal vortical flows that extend
to a depth of 5 Mm around this rotating sunspot and present evidence
that opposite vortical flows may exist below 9 Mm. It is suggested that
the vortical flows around this active region may build up a significant
amount of magnetic helicity and energy to power solar eruptions. Monte
Carlo simulation has been performed to estimate the error propagation,
and in addition the sunspot umbra is masked to test the reliability of
our inversion results. On the basis of the three-dimensional velocity
fields obtained from the time-distance helioseismology inversions, we
estimate the subsurface kinetic helicity at different depths for the
first time and conclude that it is comparable to the current helicity
estimated from vector magnetograms.
Title: Optimal Masks for Solar g-Mode Detection
Authors: Wachter, R.; Schou, J.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 2003ApJ...588.1199W
Altcode:
The detection of gravity (g) modes of solar oscillations is important
for probing the physical conditions in the Sun's energy-generating
core. We have developed a new method of spatial masks optimized to
reveal solar g-modes of angular degree l=1-3 and applied it to Michelson
Doppler Imager data in the frequency range of 50-500 μHz. These
masks take into account the horizontal component of g-mode velocity
eigenfunctions and the variations in the level of noise across the
solar disk and adjust for the time-dependent mode projection properties
caused by the inclination of the Sun's axis of rotation. They allow us
to optimize the signal-to-noise ratio in the oscillation power spectra
for potential g-modes of various angular order and degree. The peaks
in the resulting spectra are analyzed in terms of their instrumental
origin, long-term stability, and correspondence to the theoretically
predicted g-mode spectrum. As a consequence of failing to detect
any g-mode candidates, new upper limits for the surface amplitude of
g-modes are obtained. The lowest upper limits in the range of 5-6 mm
s-1 are found for sectorial g-modes (l=m). These limits
are an order of magnitude higher than the theoretical prediction of
Kumar et al. in 1996.
Title: Searching for Deep Changes of the Solar Structure
Authors: Kosovichev, A. G.
Bibcode: 2003SPD....34.2611K
Altcode: 2003BAAS...35..856K
A relation between normal-mode and multiple-reflection time-distance
data is derived by applying the principle of stationary phase to
solar acoustic oscillations. This relation links the global and local
helioseismology data, and provides estimates of acoustic travel
times in terms of mode frequencies. It is employed for searching
solar-cycle variations in the tachocline, by using GONG and MDI data of
1996-2002. The time-distance signature of magnetic field at the base
of the convection zone recently reported by Chou and Serebryanskiy
(2002) and the current precision of the deep changes are discussed.
Title: Travel-Time Sensitivities for Time-Distance Helioseismology
Authors: Birch, A. C.; Kosovichev, A. G.
Bibcode: 2003SPD....34.0807B
Altcode: 2003BAAS...35Q.823B
Time-distance helioseismology is an important tool for studying the
interior of the Sun in three dimensions. The primary data products in
time-distance helioseismology are the times for wave packets (p mode
or f mode) to travel between separate points on the solar surface. In
order to invert travel times to obtain subsurface structure and flows
it is necessary to understand the forward problem: the dependence
of travel-times on small perturbations to conditions in the solar
interior. We will present calculations, in the Born approximation,
of the dependence of travel times on subsurface flows, perturbations
to the sound speed, and spatial variations in source strength. We show
that travel-time sensitivities can depend strongly on the details of
the filtering that is performed during the data analysis.
Title: Comparison of Solar Subsurface Weather Obtained with
Time-Distance Tomography and Ring Analysis
Authors: Hindman, B. W.; Zhao, J.; Haber, D. A.; Kosovichev, A. G.;
Toomre, J.
Bibcode: 2003SPD....34.0806H
Altcode: 2003BAAS...35R.822H
The near-surface shear layer exhibits a rich medley of flows that
vary in size from granular and supergranular flows to flows of global
scale. The largest of these flows have been dubbed Solar Surface Weather
(SSW), and have been detected with both time-distance tomography and
ring analysis. We present comparisons of synoptic maps of SSW flows
obtained with both techniques from SOI-MDI Dynamics Program data. Both
techniques provide measurements of the flows as a function of depth
through inversion. The time-distance method utilizes only p-mode
oscillations, while the ring analysis uses f modes as well. We find that
the flows obtained with the two helioseismic techniques are remarkably
similar, with common inflow and outflow sites as well as agreement in
the general flow direction. At a depth of roughly 1.5 Mm the Spearman
rank correlation coefficient between maps is on the order of 0.80. As
the depth increases the correlation become weaker. The reduction in the
correlation coefficient with depth is due to the increasing difference
between the vertical resolution kernel of the two seismic techniques.
Title: Synoptic Maps of Mass Flows Beneath the Solar Surface by
Time-Distance Helioseismology
Authors: Zhao, J.; Kosovichev, A. G.
Bibcode: 2003SPD....34.2609Z
Altcode: 2003BAAS...35..855Z
We have derived the subsurface horizontal flow velocity fields
for several solar Carrington rotations from 1996 to 2001, covering
the period from the activity minimum to maximum of solar cycle 23,
by analyzing SOHO/MDI Dynamics data. The synoptic flow maps have
been made from the near-surface down to 12 megameters below the
photosphere. The torsional oscillation and meridional flows, as well
as the flow vorticity have been calculated from the flow maps as a
function of latitude. It is found that after the subtraction of the
solar minimum data, the residual meridional flows converge toward
the activity belts, and migrate toward the equator together with
the sunspot zones. In addition, the corresponding residual vorticity
peaks in the activity belts, having the opposite signs in Northern
and Southern hemispheres. Comparing the synoptic flow maps with the
synoptic magnetic field maps, it is found that the solar plasma rotates
faster where the photospheric magnetic field is stronger. An approximate
linear relation between the magnetic strength and the rotational speed
has been obtained from these data.
Title: Inversions of artificial time-distance data using mainly Born
approximation kernels and the MCD algorithm
Authors: Couvidat, S.; Birch, A. C.; Zhao, J.; Kosovichev, A. G.
Bibcode: 2003SPD....34.0809C
Altcode: 2003BAAS...35..823C
Local helioseismology, more specifically time-distance analysis,
is a recent development in solar physics that gives us invaluable
insight into the upper layers of the Sun. In this poster we show
the results of a hare-and-hounds exercise concerning the inversion
of time-distance data for perturbations to the sound-speed. We base
our analysis on the Born approximation, which is expected to be more
accurate than the usual ray-path approximation. We produce artificial
time-distance data by solving the forward problem for travel times
in the Born approximation. To invert these data, we apply the MCD
(Multi-Channel Deconvolution) with Born approximation kernels, and
the MCD and LSQR algorithms with ray theory kernels. We will present
a detailed comparison between the different inversion results.
Title: Autocorrelation analysis of MDI high-frequency data
Authors: Sekii, T.; Shibahashi, H.; Kosovichev, A. G.
Bibcode: 2003ESASP.517..385S
Altcode: 2003soho...12..385S
We have analyzed MDI data using time-distance autocorrelation function,
in a high-frequency range above the acoustic cut-off frequency of the
solar atmosphere. The MDI velocity, intensity and line-depth signals
were looked at. The wave reflection rate at the photosphere has been
found to be around 10 per cent for l = 125 and ν = 6.75mHz. The result
is compared with a previous measurement.
Title: Imaging of the solar interior: possibilities and limitations
Authors: Kosovichev, Alexander G.; Duvall, Thomas L., Jr.
Bibcode: 2003SPIE.4853..327K
Altcode:
Helioseismic tomography is a promising new method for probing
3-D structures and flows beneath the solar surface. It is based on
observation of solar acoustic waves, and provides great possibilities
for studying the birth of active regions in the Sun's interior and
for understanding the relation between the internal dynamics of
active regions and chromospheric and coronal activity. We discuss
observational requirements, challenges and limitations of this technique
for investigating physical processes in the solar interior on their
intrinsic spatial and temporal scales.
Title: On the inference of supergranular flows by time-distance
helioseismology
Authors: Zhao, Junwei; Kosovichev, Alexander G.
Bibcode: 2003ESASP.517..417Z
Altcode: 2003soho...12..417Z
We have attempted to derive the internal flow fields of supergranules
by inverting time-distance helioseismology measurements based on the
ray theoretical approximation. Due to the "cross-talk" effect between
the contributions of the vertical flows and horizontal divergent flows
and also due to the measurement errors propagation in the inversion,
we can derive reliably only the horizontal velocity distribution in
the supergranules, but the vertical velocity which is much smaller
than the horizontal velocity is rather uncertain. The preliminary
results from our inversion show that the divergent flows extend to
a few megameters below solar surface, and also present evidence of
converging flows at the depth of ~10Mm. A simple estimation gives
the average depth of supergranules is approximately 15 Mm. Another
inversion technique Multi-Channel Deconvolution is also developed. The
subsurface flow fields of a sunspot derived from this technique are
compared with the results from algorithm LSQR, and a correlation of 95%
and above is found.
Title: A search for the relationship between flaring activity and
subphotospheric flows
Authors: Dzifcáková, E.; Kulinová, A.; Kosovichev, A. G.
Bibcode: 2003ESASP.517..263D
Altcode: 2003soho...12..263D
It is believed that subphotospheric shearing flows play important
role in creating unstable magnetic topology that leads to initiation
of flares and CME. However, the relationship between the flows and
flaring activity is not well understood. Using the flow maps obtained
by local helioseismology and magnetic and coronal data we attempt to
search for this relationship. In particular, we study the evolution of
active region NOAA 9393 in the context of changes in magnetic topology
and flaring activity. SOHO/MDI and helioseismology data are used for
determining the changes in morphology and are compared with changes
of the topology as observed by TRACE.
Title: Accurate measurements of SOI/MDI high-degree frequencies and
frequency splittings
Authors: Reiter, J.; Kosovichev, A. G.; Rhodes, E. J., Jr.; Schou, J.
Bibcode: 2003ESASP.517..369R
Altcode: 2003soho...12..369R
We present accurate measurements of high-degree p-mode frequencies
and frequency splittings obtained from the Full-Disk Program of
the Michelson Doppler Imager (MDI) experiment onboard the Solar
and Heliospheric Observatory (SOHO). The frequencies and frequency
splittings are computed from unaveraged zonal, tesseral, and sectoral
power spectra using a new fitting method of Reiter et al. (2002)
based upon a maximum-likelihood fitting approach. In this method,
both the spectral power distribution and contributions of the
various observational and instrumental effects to the spatial leakage
matrices are modelled accurately. We demonstrate that one of the most
long-standing problems in high-degree helioseismology, viz. the jumps
in the frequency splitting coefficients, can be solved by taking into
account the distortion of the leakage matrix by the solar differential
rotation. The results of inversion of the initial frequency set
determined using this new method in the range of angular degree l =
45-300 show a substantially better resolution of the subsurface layers
compared to the previous studies with l below 220.
Title: Granule and Supergranule properties derived from solar
timeseries
Authors: Del Moro, D.; Berrilli, F.; Bonet, J. A.; Consolini, G.;
Kosovichev, A.; Pietropaolo, E.
Bibcode: 2003MmSAI..74..584D
Altcode:
In this paper we mainly aim at the understanding of temporal evolution
and spatial characterization of solar granular and supergranular
features. For this purpose we apply an automatic feature-tracking
algorithm to three different solar granulation timeseries and to a
supergranular timeseries of near-surface divergence fields. The single
lifetimes are calculated measuring the time elapsing between the birth
and death of each target. In addition, we investigate spatial order of
surface flows studying the g2(r) function of time-averaged
supergranular fields.
Title: Helioseismic tomography
Authors: Kosovichev, A. G.
Bibcode: 2003dysu.book...78K
Altcode:
Helioseismic tomography extends the capabilities of helioseismology
by providing three-dimensional images of sound-speed variations and
mass flows associated with sunspots, active regions, emerging magnetic
flux, convective cells and other solar phenomena. The initial results
reveal the structure of supergranulation and meridional flows beneath
the solar surface as well as large-scale mass motions around sunspots
and active regions, provide a clue for the mechanism of sunspots,
and even show the presence of active regions on the far side of the Sun.
Title: Telechronohelioseismology
Authors: Kosovichev, Alexander
Bibcode: 2003safd.book..279K
Altcode:
Telechronohelioseismology (or time-distance helioseismology) is a new
diagnostic tool for three-dimensional structures and flows in the solar
interior. Along with the other methods of local-area helioseismology,
the ring diagram analysis, acoustic holography and acoustic imaging,
it provides unique data for understanding turbulent dynamics of
magnetized solar plasma. The technique is based on measurements
of travel time delays or wave-form perturbations of wave packets
extracted from the stochastic field of solar oscillations. It is
complementary to the standard normal mode approach which is limited to
diagnostics of two-dimensional axisymmetrical structures and flows. I
discuss theoretical and observational principles of the new method,
and present some current results on large-scale flows around active
regions, the internal structure of sunspots and the dynamics of emerging
magnetic flux.
Title: Is Ni I 676.8 nm line affected by electron beams in flaring
atmospheres?
Authors: Zharkova, V. V.; Kosovichev, A. G.
Bibcode: 2002ESASP.506.1031Z
Altcode: 2002ESPM...10.1031Z; 2002svco.conf.1031Z
Non-LTE simulations for a model Ni I atom in the ambient plasma with
solar element abundances are carried out in order to investigate the
effects of electron beams on the resulting Ni I 676.8 nm emission during
solar flares. The Ni I 676.8 nm line profiles, source functions and
departure coefficients were calculated for a period of 10 s after an
injection of beams with a variety of their initial parameters. The Ni I
676.8 nm line was found to decrease its depth in the core by about 30%
in response to the increase of hydrodynamic heating of the atmosphere
during the electron beam precipitation. At the maximum electron flux
(6 s in our models), the line profile changes to emissive and stays
as such for several seconds returning to the normal absorption profile
after the beam is switched off (10 s). Therefore, the line measurements
within a timescale of 1 min or longer are more likely not to be strongly
affected by the line inversion while more precise (within seconds)
temporal measurements on Ni 676.8 nm line profiles of the impulsive
phase of flares have to be carefully investigated.
Title: Evolution of Large-scale Coronal Structure with the Solar
Cycle from EUV Data
Authors: Benevolenskaya, E. E.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 2002ASPC..277..419B
Altcode: 2002sccx.conf..419B
No abstract at ADS
Title: Solar cycle in the photosphere and corona
Authors: Benevolenskaya, E. E.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 2002ESASP.506..831B
Altcode: 2002svco.conf..831B; 2002ESPM...10..831B
EIT/SOHO data in four EUV lines and MDI/SOHO (1996-2002), and soft X-ray
YOHKOH data (1991-2001) are analyzed in the form of coronal synoptic
maps for the investigation of solar cycle variations of the corona
and magnetic field. The evolution of coronal structures is closely
related to sunspot activity, photospheric magnetic field, and topology
of the large-scale magnetic field. The coronal structures visible in
extreme ultraviolet and soft X-rays as extended bright loops, reflect
the non-axisymmetrical magnetic structure of the Sun, changing with the
solar cycle. The long-living coronal structures are related to complexes
of solar activity and display the quasiperiodic behavior (impulses of
coronal activity) with periods of 1.0-1.5 year in the axisymmetrical
distribution of EUV and X-ray fluxes during the current cycle.
Title: Acoustic Tomography of the Sun's Interior with SDO:
Possibilities and Limitations
Authors: Kosovichev, A. G.; Duvall, T. L.; Birch, A. C.; Gizon, L.;
Zhao, J.; Sekii, T.; Shibahashi, H.
Bibcode: 2002AGUFMSH21C..06K
Altcode:
Helioseismic and Magnetic Imager on board SDO will significantly expand
the possibilities for imaging plasma flows and structures in the solar
interior. It will provide for the first time high-resolution data for
continuous monitoring of emerging flux and developing active regions in
the upper convection zone. It will also allow us to look for localized
structures and flows in the tachocline region and deeper interior,
and also investigate the near-polar regions. The expected results may
give important clues of how the solar dynamo works and active regions
develop. HMI will observe the entire spectrum of the solar acoustic
and surface gravity waves, and provide the most comprehensive data
for global and local helioseismology. The high-frequency part of the
oscillation spectrum will be used for studying seismic properties of
the solar atmosphere in the quiet Sun and active regions. The method
of acoustic tomography or time-distance helioseismology is one of the
primary tools of the HMI investigation. It is based on measurements and
inversions of travel-time delays of solar waves, caused by plasma flows
and variations of temperature and magnetic field. The data analysis
requires deep understanding of the physics of wave propagation in the
Sun and substantial computer resources. One of the important goals
is to provide the flow and sound-speed maps of the upper convection
zone in near-real time for space weather applications. We present the
current status of the field, and discuss plans and challenges for the
HMI data analyses and interpretation.
Title: Inferring Subphotospheric Supergranular Flows by Time-Distance
Helioseismology
Authors: Zhao, J.; Kosovichev, A. G.
Bibcode: 2002AGUFMSH52A0460Z
Altcode:
Time-distance helioseismology has provided us a tool to investigate
the interior structures and flow fields of the Sun. Combining
the measurements from surface- and deep-focusing schemes, we have
attempted to derive the flow maps of supergranules by using the
LSQR algorithm. The divergent horizontal flow fields are obtained
near the surface, and there is evidence of converging flows below 12
megameters or so. The main difficulty is in inferring the vertical
component of the flow field because of strong cross-talk between the
horizontal divergence and the vertical velocity in the travel-time
data. A Multi-Channel Deconvolution technique was also employed to
derive the velocity fields, and the results agree well with those from
LSQR inversion. We discuss the systematic and random errors of the
measurements, and implications of the initial results for understanding
the supergranular convection.
Title: Helioseismic observations of subphotospheric dynamics of
sunspots and developing active regions
Authors: Kosovichev, A. G.; Duvall, T. L., Jr.; Zhao, Junwei
Bibcode: 2002ESASP.505...79K
Altcode: 2002IAUCo.188...79K; 2002solm.conf...79K
New methods of time-distance helioseismology provide us unique
information about the structure and dynamics of sunspots and active
regions in the upper convection zone. We present three-dimensional maps
of the sound-speed perturbations and flow velocities obtained from the
SOHO/MDI data for sunspots, emerging flux events and evolving active
regions. The results reveal complex dynamics of magnetic structures
below the solar surface, and shed light on the mechanisms of sunspots
and active regions, and magnetic field dynamics. One interesting case
that includes a fast spinning sunspot accompanied with subphotospheric
vortex motions and twisting coronal loops represents an intriguing
example of magnetic coupling between the subphotospheric processes and
the atmospheric activity. The evolution of a large active region, NOAA
9393, has been studied for almost 3 solar rotations in March-April 2001,
including the periods of emergence, maximum activity and decay. It is
concluded that this active region was formed by fragmented magnetic
flux tubes emerging during an extended period of time rather than by
a single large Ω-loop broken into smaller flux tubes near the surface.
Title: Fast photospheric flows in a flaring active region
Authors: Meunier, N.; Kosovichev, A.
Bibcode: 2002ESASP.505..505M
Altcode: 2002IAUCo.188..505M; 2002solm.conf..505M
We present new results from coordinated observations with THEMIS
(multi-line spectropolarimetric mode) and MDI/SOHO obtained in Nov
2000. We observed active region AR 9236 using several photospheric
lines before and after the last main flare produced by this very
active region. Vector magnetic fields are computed from THEMIS data
and a full inversion of the interesting profiles is performed. We
observe fast flows which are probably supersonic in two regions located
where this flare occurs. These flows seem to be long-lived structures
(several hours). One of them is related to a strong shear with flows
almost horizontal while the other is a downflow strongly inclined
with respect to the vertical. They are present 9 hours before the
flare, however their amplitude appears to be modified by the flare
(especially during the first minutes).
Title: Subsurface structure of sunspots
Authors: Kosovichev, A. G.
Bibcode: 2002AN....323..186K
Altcode:
The subsurface structure of sunspots is probed by measuring travel-time
delays of acoustic waves propagating beneath the spots, and using
a helioseismic inversion method to infer the internal sound-speed
perturbations and mass flows. The initial results obtained from the
MDI instrument on SOHO reveal under sunspots zones of the relatively
low sound speed, extended to a depth of approximately 4 Mm. These zones
are associated with cool areas of sunspots. In the deeper interior of
sunspots the sound speed is higher than in the surrounding plasma. The
regions of the higher sound speed are at least 60 Mm deep. These
observations also show complicated flow patterns that include
converging vortex flows in the upper 4 Mm deep layer. These flows play
important role for maintaining the structure of sunspots. Developing
active regions are associated with complicated evolving sound-speed
perturbations beneath the surface, which are probably caused by
multiple flux tubes emerging from the deep interior. A study of a
rapidly rotating sunspot revealed strong shear flows beneath the spot.
Title: Global low frequency acoustic modes after half a solar cycle
aboard SOHO: an improved view of the nuclear core
Authors: Turck-Chièze, S.; Garcí, R. A.; Couvidat, S.; Kosovichev,
A. G.; Bertello, L.; Corbad, T.; Berthomieu, G.; Provost, J.;
Eff-Darwich, A.
Bibcode: 2002ESASP.508..593T
Altcode: 2002soho...11..593T
Solar global oscillations have now been measured for more than 20
years. The study of these modes has contributed to improve, along
time, the description of the solar core. We have now a proper access
to this part of the Sun, with ground networks observing for more than
10 years and the three instruments aboard SOHO in a quasi continuous
mode for now half a cycle. In this talk, we show the advantages of
the global acoustic modes measured at low frequency. They are due to
their longer lifetime and the reduced influence of the turbulent and
variable surface effects. As a consequence, we have converged last
year, after 30 years of unsuccess, to a boron-8 emitted neutrino flux
in perfect agreement with the better understood detection of these
neutrinos on earth. The splitting at low frequency is also now properly
determined but the extracted rotation information is still limited in
the core. It contains nevertheless the first dynamical vision of this
part of the radiative zone. We will focus on it up to the end of the
SOHO mission, together with the gravity mode region and the possible
internal signature of the magnetic field. Some limits are given on
these observables. Further improvements of their detectability are
under study and will be mentioned.
Title: Observations of solar-cycle effects by local- area
helioseismology
Authors: Kosovichev, A. G.
Bibcode: 2002ESASP.508..131K
Altcode: 2002soho...11..131K
The local-area helioseismology is a set of new developing tools for
3D acoustic imaging of the Sun's interior structure and dynamics. It
considerably extends our capability for observing and understanding
solar-cycle effects and dynamo processes of various scales inside
the Sun. It also helps to interpret the solar-cycle variations
that are observed in oscillation frequencies and other properties
of global modes. Three basic tools: the ring-diagram analysis, the
time-distance helioseismology and the acoustic holography, provide
complementary diagnostics of the internal processes associated with
solar activity. The ring-diagram analysis based on measurements of the
local frequency-wavenumber dispersion relation for high-degree sound
waves provides synoptic maps of large-scale subsurface flows. The
time-distance helioseismology is capable of providing the synoptic
maps for the interior sound-speed distribution in addition to the
flow maps. This technique has also revealed intriguing images of
developing active regions and sunspots and associated flow patterns
in the upper convection zone. The method of acoustic holography
has proved to be very efficient for mapping active regions on the
far side of the Sun. Most results of the local-area helioseismology
obtained so far deal with the solar-cycle effects in the upper half
of the convection zone. However, there has been an attempt to obtain
longitudinal synoptic maps of sound-speed variations in the tachocline
by the time-distance technique. The recent major achievements of
the local-area helioseismology also include studies of the meridional
circulation and its variation with the solar cycle and an investigation
of the relation between the torsional oscillations and large-scale flows
around active regions. These results are obtained using Doppler-shift
data from the MDI instrument on SOHO (Scherrer et al. 1995).
Title: A new method for measuring frequencies and splittings of
high-degree modes
Authors: Reiter, Johann; Rhodes, E. J., Jr.; Kosovichev, A. G.; Schou,
J.; Scherrer, P. H.
Bibcode: 2002ESASP.508...91R
Altcode: 2002soho...11...91R
A novel peak-bagging method is presented that operates by fitting a
theoretical profile (symmetric or asymmetric) to the separate peaks of
each multiplet within each unaveraged power spectrum. This new approach
allows a separate frequency, width, and amplitude to be obtained for
each m value at each value of l, n. Hence, the frequency splittings
due to solar rotation for each multiplet can be measured directly. We
present some of our initial results obtained with this new method
in the range 45 <= l <= 300, ν <= 7 mHz when applied to
data from the Michelson Doppler Imager (MDI) onboard the Solar and
Heliospheric Observatory (SOHO). Also discussed are some instrumental
and methodological problems of high-degree mode measurements.
Title: The solar cycle dependence of low-degree p-mode parameters
Authors: Toutain, Thierry; Kosovichev, A. G.
Bibcode: 2002ESASP.508..103T
Altcode: 2002soho...11..103T
Thanks to the extension of the SOHO mission it is possible to analyze
with a good accuracy the dependence of the low-degree p-mode parameters
on the solar cycle. We have constructed from the LOI-proxy data
(MDI) two time series of 784 days each, one taken at low activity
and the other one at high activity. We have focused on analyzing the
solar-cycle dependence of the linewidth, the splitting, the asymmetry
and the frequency shift of the low-degree p modes. It turns out that
some of these parameters exhibit a significant dependence on solar
cycle moreover there is also a dependence on m the azimuthal order.
Title: Effect of line asymmetry on determination of high-degree
mode frequencies
Authors: Reiter, Johann; Rhodes, E. J., Jr.; Kosovichev, A. G.; Schou,
J.; Scherrer, P. H.
Bibcode: 2002ESASP.508...87R
Altcode: 2002soho...11...87R
Accurate measurements of frequencies of high-degree p-modes are
important for diagnostics of the structure and dynamics of the
upper convective boundary layer, and understanding the nature of
the solar-cycle variations detected in low- and medium-degree mode
frequencies. Neglecting line asymmetry in the peak-bagging approach
may lead to systematic errors in the determination of the mode
characteristics and, hence, may affect the results of inversions. Here
we demonstrate how the p-mode frequencies are systematically changed in
the range of l <= 1000, ν <= 7mHz when line asymmetry is taken
into account in the fitting of the spectral power peaks. The results
reported are based upon spectra that were created from observations
obtained from the MDI Full-Disk Program during the 1996 SOHO/MDI
Dynamics Run.
Title: Large-Scale Solar Coronal Structures in Soft X-Rays and Their
Relationship to the Magnetic Flux
Authors: Benevolenskaya, E. E.; Kosovichev, A. G.; Lemen, J. R.;
Scherrer, P. H.; Slater, G. L.
Bibcode: 2002ApJ...571L.181B
Altcode:
We have investigated the relationship between magnetic activity
and coronal structures using soft X-ray data from the Yohkoh soft
X-ray telescope and magnetic field data from the Kitt Peak Solar
Observatory for the period of 1991-2001 and EUV data from the Solar
and Heliospheric Observatory EUV Imaging Telescope for 1996-2001. The
data are reduced to Carrington synoptic maps, which reveal two types of
migrating structures of coronal activity at low and high latitudes in
the time-latitudinal distribution. The low-latitude coronal structures,
migrating equatorward, correspond to photospheric sunspot activity,
and the high-latitude structures migrating toward the poles reflect
polar activity of the Sun. We present the following new results:1. The
migrating high-latitude coronal magnetic structures are revealed in the
soft X-ray data as complete bright giant loops connecting the magnetic
field of the following part of active regions with the polar field. They
appear during the rising phase and maximum of the solar cycle and show
quasi-periodic impulsive variations with a 1-1.5 yr period.2. The soft
X-ray intensity of these loops has a strong power-law correlation with
the photospheric magnetic flux. The power-law index, which on average
is close to 2, shows variations with the solar cycle: it is higher
for the period of the declining phase and minimum of solar activity
than for the rising phase and maximum.
Title: Large-scale coronal structures in EUV and soft X-rays in
solar cycle 23
Authors: Benevolenskaya, E. E.; Kosovichev, A. G.; Scherrer, P. H.;
Lemen, J. R.; Slater, G. L.
Bibcode: 2002ESASP.508..367B
Altcode: 2002soho...11..367B
We have analyzed the EIT/SOHO data in four EUV lines (171 Å, 195 Å,
284 Å and 304 Å) and soft X-ray YOHKOH data in two filters (AlMg and
Al) in the form of coronal synoptic maps for the period 1996 - 2001
yrs. Two types of the bright structures have been detected in EUV in
the axisymmetrically averaged synoptic maps. The structures of the first
type migrate equatorward as the solar cycle progresses. They are related
to complexes of sunspot activity and display the "butterfly"-type
distribution. The structures of the second type migrate polarward
and are associated with footpoints of giant coronal loops, which
connect the polar regions and the following parts of the active
complexes. These structures of coronal activity are also pronounced in
the soft X-ray maps. However, the whole structure of the giant polar
loops is visible in X-rays, and reveals connections to the low-latitude
coronal structures. The relationship between the soft X-rays emission
and the photospheric magnetic flux obtained from SOHO/MDI and Kitt
Peak Solar Observatory has been investigated. It has been found that
the relationship depends on the phase of the solar cycle. We discuss
the role of the magnetic flux in the formation and evolution of the
stable coronal structures during the rising phase of cycle 23.
Title: Fast magnetic field variations associated with solar flares
Authors: Zharkova, V. V.; Kosovichev, A. G.
Bibcode: 2002ESASP.508..159Z
Altcode: 2002soho...11..159Z
The SOHO/MDI observations of fast magnetic field variations associated
with flares reveal a close temporal correlation of the irreversible
magnetic field changes occurring in the bipolar flare region with
the temporal behaviour of hard and soft X-ray emission. These are
expected to be accountable for a primary magnetic energy release in
the flare while the released energy is about 1% of the estimated
energy being stored in the magnetic rope structure reconstructed
for this active region (Yan et al., 2001). There are also reversible
changes of magnetic field, called earlier as "magnetic transients" or
wave-like magnetic field variations, occurring in some unipolar flare
regions. The "transient" magnetic field changes are accompanied by
the Moreton waves in the chromosphere and EIT waves in the corona. In
some flares the magnetic field variations measured with MDI look like
waves following the Moreton wave's fronts. These changes are likely
to reflect the mechanisms of energy transport into deeper atmospheric
layers by charged particles (electrons and/or protons).
Title: Optimal masks for g-mode detection in MIDI velocity data
Authors: Wachter, R.; Schou, J.; Kosovichev, A.; Scherrer, P. H.
Bibcode: 2002ESASP.508..115W
Altcode: 2002soho...11..115W
We are applying spatial masks to MDI velocity data that are optimized
for revealing g-modes in the frequency range 50 through 500 μHz. These
masks take into account the horizontal component of g-mode velocity
eigenfunctions as well as the time dependent mode projection properties
due to the changing solar B angle, and the varying noise level across
the solar disk. The solar noise, which is likely to be caused by
supergranulation in this frequency range is assumed to be uniformly
distributed over the solar surface, consisting of a dominant horizontal
component and a small radial component. The resulting time series are
examined for possible g-mode candidates and new upper limits for the
surface amplitude of g-modes are obtained.
Title: Optimal masks for g-mode detection in MDI velocity data
Authors: Wachter, R.; Schou, J.; Kosovichev, A.; Scherrer, P. H.;
Phoebus Team
Bibcode: 2002AAS...200.0412W
Altcode: 2002BAAS...34..645W
We are applying spatial masks for MDI velocity data that are optimized
for revealing g-modes in the frequency range 50 through 500 μ
Hz. These masks take into account the horizontal component of g-mode
velocity eigenfunctions as well as the time dependent mode projection
properties due to the changing solar B angle, and the varying noise
level across the solar disk. The solar noise, which is likely to be
caused by supergranulation in this frequency range is assumed to be
uniformly distributed over the solar surface, consisting of a dominant
horizontal component and a smaller radial component. The optimal masks
are applied to the image and the resulting time series are examined
for possible g-mode candidates. Because no mode peak has been detected,
firm upper limits for the surface visibility of individual low degree
modes can be given.
Title: Helioseismic observations of developing active regions in
the solar convection zone
Authors: Kosovichev, A.; Duvall, T., Jr.
Bibcode: 2002AAS...200.8903K
Altcode: 2002BAAS...34Q.791K
Time-distance helioseismology provides unique opportunities for studying
the formation and evolution of regions of solar activity. The key
questions are: How deep in the convection zone are the active regions
formed? How fast do they erupt? What is the role of convective mass
flows in the formation and evolution of active regions? Is there
retraction of magnetic flux during the decay of active regions? Why
do new active regions tend to appear in places where previous active
regions existed? We discuss the current techniques for probing the
subphotospheric structure and dynamics of active regions, uncertainties
and limitations of these studies, and present results of observation of
three developing active regions using Michelson Doppler Imager on SOHO.
Title: Advances in Time-Distance Helioseismology
Authors: Duvall, T. L., Jr.; Beck, J. G.; Gizon, L.; Kosovichev, A. G.
Bibcode: 2002AAS...200.7902D
Altcode: 2002BAAS...34..780D
Time-distance helioseismology is a way to measure travel times between
surface locations for waves traversing the solar interior. Coupling
the travel time measurements with an extensive modeling effort has
proven to be a powerful tool for measuring flows and other wave speed
inhomogeneities in the solar interior. Problems receiving current
attention include studying the time variation of the meridional
circulation and torsional oscillation and active region emergence and
evolution. Current results on these topics will be presented.
Title: Study of a Rotating Sunspot and Statistics of Kinetic Helicity
Near Sunspots' Surface By Local Helioseismology
Authors: Zhao, J.; Kosovichev, A.
Bibcode: 2002AAS...200.0413Z
Altcode: 2002BAAS...34..645Z
Time-distance helioseismology has provided a unique tool for studying
interior structures of the Sun. The structure of sound speed variations
and flow fields beneath the sunspot surface have been obtained by
the use of inversion techniques in some previous studies. By applying
time-distance measurements and an inversion technique to active region
NOAA 9114 observed by SOHO/MDI, which showed unusual fast rotation
around its center during its passage on the solar disk, we obtained
the sound-speed structure and plasma flow fields up to 10 megameters
below the photosphere. The subsurface sound-speed structure revealed
apparent structural twists relative to the surface magnetic structure,
which may suggest that the magnetic twists have existed below the
visible surface. For the flow fields, strong vortical flows can be
seen near the surface and opposite vortex was found about 10 megameters
below the surface, which may provide an explanation of magnetic twists
in sunspots. Same technique was applied to 86 active regions observed
by SOHO/MDI in order to study the hemispherical preference of kinetic
helicities of active regions. It was found that about 55% of active
regions in northern hemisphere showed positive kinetic helicity,
while 59% of southern hemisphere active regions showed negative
kinetic helicity. This provides observational evidences to figure
out where or how the observed surface magnetic helicity hemispherical
preference forms.
Title: A new insight into the energy release and transport in
solar flares
Authors: Zharkova, V. V.; Kosovichev, A. G.
Bibcode: 2002ESASP.477...35Z
Altcode: 2002scsw.conf...35Z
The SOHO/MDI observations of fast magnetic field variations associated
with flares reveal a close temporal correlation of the irreversible
magnetic field changes occurring in the bipolar flare region with
the temporal behaviour of hard and soft X-ray emission. These are
expected to be accounted for a primary magnetic energy release during
the flare event. There are also reversible changes of magnetic field,
called earlier as "magnetic transients", occurring in some unipolar
flare regions. These changes are likely to reflect the mechanisms of
energy transport into deeper atmospheric layers by charged particles
(electrons and/or protons). These "transient" magnetic field changes
are also accompanied by Moreton waves in the chromosphere and EIT waves
in the corona. The attempt to interpret these events using our previous
kinetic models for beam electrons revealed that the energy momentum of
electron beam delivered to the photospheric levels is marginally close
to the limit derived from the recent solar quake observations. This
encourages to consider a neutralised beam as the most possible source
of both magnetic and helioseismic waves.
Title: Coronal Patterns of Activity from Yohkoh and SOHO/EIT Data
Authors: Benevolenskaya, E. E.; Kosovichev, A. G.; Scherrer, P. H.;
Lemen, J. R.; Slater, G. L.
Bibcode: 2002mwoc.conf..329B
Altcode:
We have studied the evolution of large-scale coronal structures using
soft X-ray data from YOHKOH and EUV data from SOHO/EIT during the
rising phase of the current solar cycle 23, and compared with the
evolution of the photospheric magnetic field. During this period the
distribution of the coronal structures generally reflects the evolution
of the magnetic fields. However, the data from EIT and YOHKOH reveal
large-scale magnetic connections in the corona which probably play
significant role in the solar cycle. In particular, we have found that
coronal structures such as high-latitude giant loops may be important
for the topological evolution of magnetic structures during the solar
cycle and for polar magnetic field reversals. We discuss possible
mechanisms of the polar magnetic field reversals and their relations
to the observed coronal structures.
Title: Solar Coronal Activity and Evolution of the Magnetic Field
Authors: Benevolenskaya, E. E.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 2002stma.conf...27B
Altcode:
No abstract at ADS
Title: Active longitudes and coronal structures during the rising
phase of the solar cycle
Authors: Benevolenskaya, E. E.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 2002AdSpR..29..389B
Altcode:
The longitudinal structure of the solar corona has been investigated
during the transition period between solar cycles 22 and 23 and at the
beginning of the current cycle 23 using the SOHO/EIT data obtained
in 171 Å, 195 Å, 284 Å and 304 Å EUV lines. The EIT images
were transformed into synoptic maps for each of the spectral lines,
and for the 195Å/171Å line ratio, which is an index of the coronal
temperature. The synoptic maps reveal stable longitudinal structures in
the coronal intensities and temperature, that are related to large-scale
magnetic field structures. We discuss the relation between the coronal
and photospheric magnetic structures obtained from the SOHO and Kitt
Peak Solar Observatory data, and compare the rotation rates of these
structures with the rotation profile of the solar interior in order
to determine the possible origin of the coronal structures.
Title: Local-area helioseismology as a diagnostic tool for solar
variability
Authors: Kosovichev, A. G.; Duvall, T. L.; Birch, A. C.; Gizon, L.;
Scherrer, P. H.; Zhao, Junwei
Bibcode: 2002AdSpR..29.1899K
Altcode:
Dynamical and thermal variations of the internal structure of the Sun
can affect the energy flow and result in variations in irradiance
at the surface. Studying variations in the interior is crucial for
understanding the mechanisms of the irradiance variations. "Global"
helioseismology based on analysis of normal mode frequencies, has helped
to reveal radial and latitudinal variations of the solar structure
and dynamics associated with the solar cycle in the deep interior. A
new technique, - "local-area" helioseismology or heliotomography,
offers additional potentially important diagnostics by providing
three-dimensional maps of the sound speed and flows in the upper
convection zone. These diagnostics are based on inversion of travel
times of acoustic waves which propagate between different points on the
solar surface through the interior. The most significant variations
in the thermodynamic structure found by this method are associated
with sunspots and complexes of solar activity. The inversion results
provide evidence for areas of higher sound speed beneath sunspot regions
located at depths of 4-20 Mm, which may be due to accumulated heat or
magnetic field concentrations. However, the physics of these structures
is not yet understood. Heliotomography also provides information about
large-scale stable longitudinal structures in the solar interior,
which can be used in irradiance models. This new diagnostic tool for
solar variability is currently under development. It will require both
a substantial theoretical and modeling effort and high-resolution
data to develop new capabilities for understanding mechanisms of
solar variability.
Title: Noise reduction in helioseismic power spectra\ by
non-orthogonal wavelets
Authors: Solanki, S. K.; Régulo, C.; Fligge, M.; Kosovichev, A. G.
Bibcode: 2001A&A...379.1039S
Altcode:
We present a method to reduce noise in helioseismic power spectra
using a non-orthogonal wavelet transform based on quadratic spline
functions. The quality of our method is tested by applying it to
artificially generated time-series approximating solar p-modes. The
mode frequencies and line widths obtained from least-squares fits to
the smoothed spectra are compared with the corresponding parameters
deduced from maximum likelihood fits to the original spectra. The
results from both approaches are very similar and suggest that there
is no major bias in either of these rather independent approaches. As a
practical example we denoise parts of the power spectrum obtained from
the two first years of operation of the GOLF instrument onboard SOHO.
Title: The Largest Active Region of the Solar Cycle
Authors: Kosovichev, A. G.; Bush, R. I.; Duvall, T. L.; Scherrer, P. H.
Bibcode: 2001AGUFMSH11C0730K
Altcode:
The largest and most active sunspot region of the current solar
cycle (known as AR 9393) was observed by the MDI instrument on SOHO
continuously during three solar rotations in March-May 2001. On April
2 this active region produced the largest solar flare of the last 25
years. By using time-distance helioseismology we have investigated
the development of the active region in the solar interior during that
period starting from the processes of emergence. We present tomographic
images of the sound-speed structures associated with this active region
up to 100 Mm below the solar surface, and discuss their relation to
the evolution of the surface magnetic field.
Title: Accuracy of Born and Ray Approximations in Time-Distance
Helioseismology
Authors: Price, G. H.; Birch, A. C.; Kosovichev, A. G.; Schlottmann,
R. B.
Bibcode: 2001AGUFMSH11B0709P
Altcode:
Time-distance helioseismology attempts to infer localized departures
from a nominal state of the solar interior from comparison of
observed travel times of acoustic wave packets to those anticipated
from the model. Until recently, such time-distance measurements
have generally been modeled in the ray approximation, which neglects
finite-wavelength effects. Concern that these effects can be important
has prompted interest in the Born approximation, which is sensitive
to them. In order to elucidate the ranges of validity of the ray and
Born approximations and the nature of their limitations, we compare
travel-time pertrubations calculated using these approximations to
exact travel times for a simple problem, namely the propagation of
adiabatic acoustic waves through the center of a spherically symmetric
sound-speed perturbation to an otherwise uniform medium. We show that
the Born and first-order ray approximations converge to the same result
as the spatial scale of the medium perturbation becomes large compared
to the first Fresnel zone, with a fractional error in the travel-time
perturbation equal to the fractional strength of the perturbation,
while a full ray calculation converges to the exact solution in this
limit. For a uniform perturbation having a size the order of the
first Fresnel zone, interference between direct and diffracted wave
produces travel-time fluctuations that are entirely absent in the ray
approximation; these fluctuations are only qualitatively replicated
by the Born approximation for moderately weak (e.g., 5%) perturbation
strengths. Such fluctuations are, however, largely suppressed for
a smoothly-varying perturbation expected to be more representative
of solar structures. The so-called banana-doughnut (here, ventilated
cigar) form of the Born sensitivity kernels, i.e., a greatly reduced
sensitivity of the travel-time perturbation to small-scale medium
perturbations that fall near the unperturbed ray path that is absent
in the ray approximation, is also shown to be consistent with the
exact results.
Title: Time-distance Helioseismology Study Over a Rotating Sunspot
Authors: Zhao, J.; Kosovichev, A. G.; Duvall, T. L.
Bibcode: 2001AGUFMSH11B0708Z
Altcode:
Time-distance helioseismology has provided a unique tool in studying
interior structures of the Sun. The structure of sound speed variations
and flow fields beneath the sunspot surface have been obtained by
use of inversion technique in some previous studies. In this study
we have applied the time-distance measurements from SOHO/MDI and the
inversion technique to investigate a sunspot which showed unusually fast
rotation around its center for a couple of days from Aug 7 to Aug 8,
2000. The sound speed structure which is related to the magnetic field
structures beneath the surface and associated temperature variations was
obtained. The results revealed some twists in the sound-speed internal
structure of the spot relative to the surface magnetic structure. This
kind of subsurface twist was not seen 2 days after the start of
rotation. This is consistent with the surface observation showing
a reduction of transverse magnetic field twists after the surface
rotation stopped. It could be explained as the magnetic field lines
were twisted beneath the surface and the untwisting of field lines
caused the surface rotation. Flow fields beneath the sunspot surface
were also obtained. A strong vortex was found near the surface and
a few megameters below the surface. Whether the subsurface vortical
flows caused the magnetic field twists or the untwisting of field
lines caused the subsurface vortical flows will be discussed.
Title: Solar Coronal Structures in Extreme Ultraviolet and Soft
X-rays and Their Relation to Magnetic Flux
Authors: Benevolenskaya, E. E.; Kosovichev, A. G.; Lemen, J. R.;
Scherrer, P. H.; Slater, G. L.
Bibcode: 2001AGUFMSH11C0720B
Altcode:
The large-scale coronal structures are ultimately related to internal
magnetic fields and thus provide important information about the solar
dynamo. We have investigated the relationship between magnetic activity
and coronal structures using EUV data from SOHO/EIT and X-ray data
from Yohkoh/SXT, and magnetic field data from Kitt Peak and Wilcox
Solar Observatories for the period 1996-2000 years. We discuss the
non-uniform distribution of coronal heating and its connection with
long-lived complexes of solar activity during the current cycle. EUV
images reveal two sets of migrating structures of coronal activity
in the time-latitudinal distribution of the EUV intensity in 171A,
195A, 284A and 304A EIT wavelength channels. The low-latitude coronal
structures, migrating equatorward, correspond to photospheric sunspot
activity, and the high-latitude structures migrating towards the poles
reflect polar activity of the sun. The polar branches are cooler then
the equatorial branches. This is reflected in the time-latitudinal
distribution of the soft X-rays in two filters (Al and AlMg). We
discuss the physical properties and nature of these structures of
coronal activity and their role in the solar cycle.
Title: Probing Deep Structure of the Sun by Time-Distance
Helioseismology
Authors: Birch, A. C.; Duvall, T. L.; Kosovichev, A. G.
Bibcode: 2001AGUFMSH11B0710B
Altcode:
Time-distance helioseismology is a method for inferring sound-speed
perturbations and flow velocities by measuring the travel times for
acoustic wave packets as they move between points on the solar surface
through the solar interior. It has been successfully applied to infer
structures and flows in the upper convection zone. However, probing
the deep convection zone and, in particular, the tachocline region at
the bottom of the convection zone where the solar dynamo is believed
to be operating is quite challenging. Using the solar oscillation
data from SOHO/MDI we have attempted to detect deep structures in
a low-latitude band of the convection zone. For inversion of the
travel-time measurements we used the theoretical sensitivity, in the
first Born approximation, of travel times to sound speed inhomogeneities
in the solar convection zone. We have obtained synoptic sound-speed maps
for two solar rotations in 2000. The results show resolved structures
in the lower convection zone. We compare the sound-speed maps with
surface magnetic field synoptic maps and discuss possible relations
between the deep structures and the surface field.
Title: Observed and Predicted Ratios of the Horizontal and Vertical
Components of the Solar p-Mode Velocity Eigenfunctions
Authors: Rhodes, Edward J., Jr.; Reiter, Johann; Schou, Jesper;
Kosovichev, Alexander G.; Scherrer, Philip H.
Bibcode: 2001ApJ...561.1127R
Altcode:
We present evidence that the observed ratios of the horizontal
and vertical components of the solar intermediate-degree p-mode
velocity eigenfunctions closely match theoretical predictions of
these ratios. This evidence comes from estimates of the observed
eigenfunction component ratios that were obtained from the fitting
of the p-mode oscillation peaks in low- and intermediate-degree
(l<=200) m-averaged power spectra computed from two different
60.75 day time series of Global Oscillation Network Group (GONG)
project Dopplergrams obtained in late 1996 and early 1998. These fits
were carried out using a peak-fitting method in which we fitted each
observed p-mode multiplet with a model profile that included both the
target mode and its six nearest spatial sidelobes and which incorporated
the effects of the incomplete observational time series through the
convolution of the fitted profiles with the temporal window functions,
which were computed using the two actual GONG observing histories. The
fitted profile also included the effects of the spatial leakage of
the modes of differing degrees into the target spectrum through the
use of different sets of m-averaged spatial leakage matrices. In
order to study the sensitivity of the estimated component ratios to
the details of the computation of the m-averaged power spectra and of
the image-masking schemes employed by the GONG project, we generated
a total of 22 different sets of modal fits. We found that the best
agreement between the predicted and inferred ratios came from the use
of unweighted averaged power spectra that were computed using so-called
n-averaged frequency-splitting coefficients, which had been computed
by cross-correlating the 2l+1 zonal, tesseral, and sectoral power
spectra at each l over a wide range of frequencies. This comparison
yielded a total of 1906 pairs of predicted ct,theory and
fitted ct,fit eigenfunction component ratios. A linear
regression analysis of these pairs of ratios resulted in the following
regression equation: ct,fit=(0.0088+/-0.0013)+(0.9940+/-
0.0044)ct,theory. The resulting correlation coefficient
was 0.9817. This agreement between the predicted and inferred ratios
suggests that the predicted ratios should be used in the fitting of
high-degree power spectra where the ratios cannot be inferred because
of the blending together of individual modal peaks into broad ridges
of power.
Title: The Accuracy of the Born and Ray Approximations in
Time-Distance Helioseismology
Authors: Birch, A. C.; Kosovichev, A. G.; Price, G. H.; Schlottmann,
R. B.
Bibcode: 2001ApJ...561L.229B
Altcode:
Time-distance helioseismology measures the time for acoustic
wave packets to travel, through the solar interior, from one
location on the solar surface to another. Interpretation of travel
times requires an understanding of their dependence on subsurface
inhomogeneities. Traditionally, time-distance measurements have been
modeled in the ray approximation. Recent efforts have focused on the
Born approximation, which includes finite-wavelength effects. In order
to understand the limitations and ranges of validity of the ray and Born
approximations, we apply them to a simple problem-adiabatic acoustic
waves in a uniform medium with a spherical inclusion-for which a
numerical solution to the wave equation is computationally feasible. We
show that, for perturbations with length scales large compared to
the size of the first Fresnel zone, both the Born and first-order ray
approximations yield the same result and that the fractional error in
the travel time shift, computed by either approximation, is proportional
to the fractional strength of the sound speed perturbation. Furthermore,
we demonstrate that for perturbations with length scales smaller
than the first Fresnel zone the ray approximation can substantially
overestimate travel time perturbations while the Born approximation
gives the correct order of magnitude. The main cause of the inaccuracy
of the Born approximation travel times is the appearance of a diffracted
wave. This wave, however, has not yet been observed in the solar data.
Title: Time-distance helioseismology and the Solar Orbiter mission
Authors: Gizon, L.; Birch, A. C.; Bush, R. I.; Duvall, T. L., Jr.;
Kosovichev, A. G.; Scherrer, P. H.; Zhao, Junwei
Bibcode: 2001ESASP.493..227G
Altcode: 2001sefs.work..227G
No abstract at ADS
Title: Investigation of Mass Flows beneath a Sunspot by Time-Distance
Helioseismology
Authors: Zhao, Junwei; Kosovichev, Alexander G.; Duvall, Thomas L., Jr.
Bibcode: 2001ApJ...557..384Z
Altcode:
A time-distance helioseismic technique is employed to analyze a set of
high-resolution Dopplergram observations of a large sunspot by SOHO/MDI
on 1998 June 18. A regularized, damped least-squares inversion is
applied to the measurements of travel times to infer mass flows around
the sunspot below the solar surface. Powerful converging and downward
directed flows are detected at depths of 1.5-5 Mm, which may provide
observational evidence for the downdrafts and vortex flows that were
suggested by Parker for a cluster model of sunspots. Strong outflows
extending more than 30 Mm are found below the downward and converging
flows. It is suggested that the sunspot might be a relatively shallow
phenomenon, with a depth of 5-6 Mm, as defined by its thermal and
hydrodynamic properties. A strong mass flow across the sunspot is found
at depths of 9-12 Mm, which may provide more evidence in support of the
cluster model, as opposed to the monolithic sunspot model. We suggest
that a new magnetic emergence that was found 5 hr after our analysis
period is related to this mass flow.
Title: Solar Neutrino Emission Deduced from a Seismic Model
Authors: Turck-Chièze, S.; Couvidat, S.; Kosovichev, A. G.; Gabriel,
A. H.; Berthomieu, G.; Brun, A. S.; Christensen-Dalsgaard, J.; García,
R. A.; Gough, D. O.; Provost, J.; Roca-Cortes, T.; Roxburgh, I. W.;
Ulrich, R. K.
Bibcode: 2001ApJ...555L..69T
Altcode:
Three helioseismic instruments on the Solar and Heliospheric Observatory
have observed the Sun almost continuously since early 1996. This
has led to detailed study of the biases induced by the instruments
that measure intensity or Doppler velocity variation. Photospheric
turbulence hardly influences the tiny signature of conditions in the
energy-generating core in the low-order modes, which are therefore very
informative. We use sound-speed and density profiles inferred from GOLF
and MDI data including these modes, together with recent improvements
to stellar model computations, to build a spherically symmetric
seismically adjusted model in agreement with the observations. The
model is in hydrostatic and thermal balance and produces the present
observed luminosity. In constructing the model, we adopt the best
physics available, although we adjust some fundamental ingredients,
well within the commonly estimated errors, such as the p-p reaction
rate (+1%) and the heavy-element abundance (+3.5%); we also examine the
sensitivity of the density profile to the nuclear reaction rates. Then,
we deduce the corresponding emitted neutrino fluxes and consequently
demonstrate that it is unlikely that the deficit of the neutrino fluxes
measured on Earth can be explained by a spherically symmetric classical
model without neutrino flavor transitions. Finally, we discuss the
limitations of our results and future developments.
Title: Detection of High-Latitude Waves of Solar Coronal Activity in
Extreme-Ultraviolet Data from the Solar and Heliospheric Observatory
EUV Imaging Telescope
Authors: Benevolenskaya, E. E.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 2001ApJ...554L.107B
Altcode:
We present the results of an investigation of EUV coronal structures
in 1996-2000 using the Solar and Heliospheric Observatory EIT data
in 171, 195, 284, and 304 Å lines. During this period, poleward-
and equatorward-migrating waves of solar activity have been found
in axisymmetrical distributions of EUV intensity in all four
lines. In the axisymmetrical distribution of the ratio of 195 Å
to 171 Å intensities, which is a proxy of coronal temperature from
1×106 to 2×106 K, the polar branches are less
prominent. The high-latitude activity waves are caused by giant coronal
magnetic loops connecting the polar magnetic field (formed during the
preceding solar cycle) with the magnetic field of the ``following''
parts of active regions that emerged during the rising phase of the
current cycle. We suggest that these coronal loops play an important
role in the topological evolution of the magnetic structure of the
Sun during the solar cycle.
Title: Low-Degree Low-Order Solar p Modes As Seen By GOLF On
board SOHO
Authors: García, R. A.; Régulo, C.; Turck-Chièze, S.; Bertello,
L.; Kosovichev, A. G.; Brun, A. S.; Couvidat, S.; Henney, C. J.;
Lazrek, M.; Ulrich, R. K.; Varadi, F.
Bibcode: 2001SoPh..200..361G
Altcode:
Data recovered from the GOLF experiment on board the ESA/NASA SOHO
spacecraft have been used to analyze the low-order low-degree
solar velocity acoustic-mode spectrum below ν=1.5 mHz (i.e.,
1≤n≤9,l≤2). Various techniques (periodogram, RLAvCS,
homomorphic-deconvolution and RLSCSA) have been used and compared to
avoid possible biases due to a given analysis method. In this work,
the acoustic resonance modes sensitive to the solar central region
are studied. Comparing results from the different analysis techniques,
10 modes below 1.5 mHz have been identified.
Title: Studying Solar Variability by Local-Area Helioseismology
Authors: Kosovichev, A. G.
Bibcode: 2001AGUSM..SP22A01K
Altcode:
Local-area helioseismology provides diagnostics of 3D structures and
flows in the solar interior. It substantially extends diagnostic
capabilities of the `global' helioseismology which is based on
normal mode frequencies and can recover only 2D azimuthally averaged
structures and flows. Generally, very long time series of observations
of solar oscillations are required to probe the deep interior. Recently
developed approaches to the local-area helioseismology (time-distance
helioseismology, acoustic imaging, holography, ring-diagram analysis)
study evolution of particular regions on the Sun by tracking them with
solar rotation. These methods use relatively short time series but so
far have been limited to inferences in shallow subsurface layers of
the Sun. Studying the deeper layers requires substantially longer time
series, and represents a significant challenge for these techniques. I
will discuss the current status and potentials of 3D diagnostics
of internal processes inside the Sun, and their significance for
understanding solar variability.
Title: Polar Activity Wave in the Solar Corona
Authors: Benevolenskaya, E. E.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 2001AGUSM..SP61A05B
Altcode:
We present the results of investigation of EUV coronal structures in
1996-2000 using SOHO/EIT data in 171A, 195A, 284A and 304A. During
this period polarward and equatorward migrating waves of activity
have been found in axysymmetrical distributions of EUV intensity in
all four lines. In the ratio of 195A and 171A intensities, which is
a proxy of coronal temperature from 1 MK to 2 MK, the polar branch
is not present. We discuss the physical nature of the polar wave
of activity and conclude that it is related to dense plasma loops
which are cooler then the loops related to the equatorward migrating
wave. The latter reflects coronal structures connected with active
regions and complexies of solar activity. The polar activity wave
is caused by reconnection between the polar magnetic field and the
magnetic field of `following' parts of active regions, and, probably,
plays an important role in the solar cycle.
Title: Deep Focusing in Time-Distance Helioseismology
Authors: Duvall, T. L.; Jensen, J. M.; Kosovichev, A. G.; Birch, A. C.
Bibcode: 2001AGUSM..SP22A03D
Altcode:
Much progress has been made by measuring the travel times of solar
acoustic waves from a central surface location to points at equal arc
distance away. Depth information is obtained from the range of arc
distances examined, with the larger distances revealing the deeper
layers. This method we will call surface-focusing, as the common
point, or focus, is at the surface. To obtain a clearer picture of the
subsurface region, it would, no doubt, be better to focus on points
below the surface. Our first attempt to do this used the ray theory to
pick surface location pairs that would focus on a particular subsurface
point. This is not the ideal procedure, as Born approximation kernels
suggest that this focus should have zero sensitivity to sound speed
inhomogeneities. However, the sensitivity is concentrated below the
surface in a much better way than the old surface-focusing method,
and so we expect the deep-focusing method to be more sensitive. A
large sunspot group was studied by both methods. Inversions based on
both methods will be compared.
Title: Mass Flows Beneath the Sunspot from Inversion of Time-distance
Helioseismology
Authors: Zhao, J.; Kosovichev, A. G.; Duvall, T. L.
Bibcode: 2001AGUSM..SP22A04Z
Altcode:
Time-distance helioseismic technique has provided a useful tool to
study the interior structure of the Sun. The inversion of time-distance
measurements can help us reveal mass flows and sound speed perturbation
beneath the solar surface. We have applied time-distance measurements
to a set of high resolution Dopplergram observations of a sunspot by
SOHO/MDI, and a regularized damped least-squares inversion was used
to infer the mass flows beneath this sunspot. Powerful converging
and downward flows are detected at a depth of 1.5 to 5 Mm, which may
provide observational evidence for the cluster sunspot model. Strong
outflows which extend more than 30Mm outside the center of the sunspot
are found below 5Mm. A full disk observation of an interesting event in
August 2000, an apparent spin of a sunspot, was also analyzed by the
same approach but with lower resolution. For this event a vortex flow
has been detected in subsurface layers. This may provide an insight
into the study of helicity below the photosphere of the Sun.
Title: Using a Wave-Theory Approach to Time-Distance Helioseismology
Authors: Birch, A. C.; Duvall, T. L.; Kosovichev, A. G.
Bibcode: 2001AGUSM..SP31A21B
Altcode:
Time-distance helioseismology is a method for measuring the travel
times for acoustic wave packets as they move between points on the solar
surface through the solar interior. In order to interpret travel times
we derive, employing the Born approximation to the wave equation, a
linear relationship between travel time variations and perturbations to
a solar model; the results are essentially the "banana-doughnut" kernels
familiar from geophysics. We show preliminary inversion results for
large-scale structure inside the sun using these sensitivity kernels.
Title: Challenges in High-Degree Helioseismology
Authors: Rhodes, E. J.; Reiter, J.; Schou, J.; Kosovichev, A. G.;
Scherrer, P. H.
Bibcode: 2001AGUSM..SP21C06R
Altcode:
Some of the most exciting results that the field of helioseismology
has provided in recent years have come from numerical inversions of
different properties of the solar p-mode oscillations. Such inversions
have been primarily of three types: 1) structural inversions which
have employed tables of the frequencies of various p-modes and
their associated uncertainties to infer different thermodynamic
properties of the solar interior as functions of radius and latitude,
2) rotational inversions which have employed tables of the frequency
splittings of the modes of different azimuthal order to measure the
internal angular velocity as functions of radius and latitude, and 3)
horizontal flow inversions which have employed sets of frequencies
of the rings that are observed in three-dimensional power spectra to
infer sub-photospheric horizontal flow vectors as functions of depth,
latitude and longitude. Unfortunately, the vast majority of such
inversions have only included frequencies or frequency splittings
of the low- and the intermediate-degree oscillations. Furthermore,
the horizontal flow inversions have been somewhat limited by the
difficulties in accurately fitting the rings of the higher-degree
power spectra. These limitations have prevented helioseismologists from
accurately inferring the sound speed, density, adiabatic gradient, and
helium abundance in the outermost three to four percent (by radius)
of the solar interior. In addition, the absence of high-l frequency
splittings from most past rotational inversions has limited the
accuracy with which we have been able to estimate the angular velocity
of the solar surface layers. These limitations have mainly come about
because for l>= 200 the individual modal peaks blend together into
broad ridges of power. Fitting such ridges requires knowledge of the
amount of power which leaks into the sidelobes that are adjacent to
the true spectral peaks. Such leakage information requires detailed
knowledge of the spatial behavior of each different intrument, of
the ratio of horizontal and vertical components of the solar p-mode
eigenfunctions, and of the temporal window function of each dataset. In
this presentation we will demonstrate the high-l frequencies which we
have obtained from a new fitting technique which employs m-averaged
power spectra, temporal window functions, and spatial leakage matrices
to fit each mode or ridge with a total of seven peaks. We will also
demonstrate that we have obtained evidence from the fitting of GONG
power spectra that the true ratios of the eigenfunction components
match the theoretical predictions of these ratios. Finally, we will
also demonstrate that cross-correlations of the peaks and ridges in
the 2l+1 individual spectra at each l result in systematic jumps in the
frequency-splitting coefficients for l>=200 due to the blending of
the peaks into ridges. We will point out that, unless some method can
be found which overcomes these detrimental effects of peak-blending,
we will not be able to provide measures of the latitudinal behavior
of the solar angular velocity close to the photosphere which will
be independent of the horizontal flow mesurements obtained with the
so-called ``ring and trumpet'' technique.
Title: Resonant Vibrational Instabilities in Magnetized Stellar
Atmospheres
Authors: Birch, A. C.; Kosovichev, A. G.; Spiegel, E. A.; Tao, L.
Bibcode: 2001SoPh..199..291B
Altcode:
We perform linear stability analysis on stratified, plane-parallel
atmospheres in uniform vertical magnetic fields. We assume perfect
electrical conductivity and we model non-adiabatic effects with Newton's
law of radiative cooling. Numerical computations of the dispersion
diagrams in all cases result in patterns of avoided crossings and
mergers in the real part of the frequency. We focus on the case
of a polytrope with a prevalent, relatively weak, magnetic field
with overstable modes. The growth rates reveal prominent features
near avoided crossings in the diagnostic diagram, as has been seen
in related problems (Banerjee, Hasan, and Christensen-Dalsgaard,
1997). These features arise in the presence of resonant oscillatory
bifurcations in non-self adjoint eigenvalue problems. The onset of
such bifurcations is signaled by the appearance of avoided crossings
and mode mergers. We discuss the possible role of the linear stability
results in understanding solar spicules.
Title: Magnetic Energy Release and Transients in the Solar Flare of
2000 July 14
Authors: Kosovichev, A. G.; Zharkova, V. V.
Bibcode: 2001ApJ...550L.105K
Altcode:
High-resolution observations of a large solar flare on 2000 July 14
(``Bastille Day Flare'') from the Michelson Doppler Imager instrument
on the SOHO spacecraft reveal rapid variations of the magnetic field in
the lower solar atmosphere during the flare. Some of these variations
were irreversible, occurred in the vicinity of magnetic neutral lines,
and likely were related to magnetic energy release in the flare. A
surprising result is that these variations happened very rapidly on the
scale of 10-15 minutes in a large area of ~50 Mm2 at the
beginning of the flare. Other, more localized and impulsive magnetic
field variations somewhat similar to ``magnetic transients'' observed by
Zirin and coworkers were accompanied by impulses in continuum intensity
and Doppler velocity. These impulses have dynamic characteristics
similar to Ellerman's ``bombs'' and Severny's ``mustaches'' and were
probably caused by high-energy particles bombarding the solar surface.
Title: Probing Magnetic Structures in the Solar Interior by
Helioseismic Tomography
Authors: Kosovichev, A. G.; Duvall, T. L., Jr.; Scherrer, P. H.
Bibcode: 2001ASPC..248..169K
Altcode: 2001mfah.conf..169K
No abstract at ADS
Title: Heliotomography of the outer layers of the Sun
Authors: Kosovichev, A. G.; Duvall, T. L., Jr.; Birch, A. C.; Gizon,
L.; Scherrer, P. H.; Zhao, Junwei
Bibcode: 2001ESASP.464..701K
Altcode: 2001soho...10..701K
Heliotomography offers important diagnostics of the solar interior
by providing three-dimensional maps of the sound speed and flows in
the upper convection zone. These diagnostics are based on inversion
of travel times of acoustic waves which propagate between different
points on the solar surface through the interior. The most significant
variations in the thermodynamic structure found by this method
are associated with sunspots and complexes of solar activity. The
inversion results provide evidence for areas of higher sound speed
beneath sunspot regions located at depths of 4 - 20 Mm, which may be
due to accumulated heat or magnetic field concentrations. The results
reveal structures and flows associated with active regions and sunspots
at various stages of their evolution, and provide important constraints
for theories of solar dynamics and activity.
Title: Large-Scale Patterns of Solar Magnetic Field and Activity
Cycles
Authors: Benevolenskaya, E. E.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 2001ASPC..248..135B
Altcode: 2001mfah.conf..135B
No abstract at ADS
Title: Local-area helioseismology by SOT on-board Solar-B
Authors: Sekii, T.; Shibahashi, H.; Kosovichev, A. G.; Duvall, T. L.,
Jr.; Berger, T. E.; Bush, R.; Scherrer, P. H.
Bibcode: 2001ESASP.464..327S
Altcode: 2001soho...10..327S
Solar-B satellite, a successor to Yohkoh, will be launched
in 2005. Placed in a sun-synchronous orbit, it will carry out
multi-wavelength observation in optical, EUV and X-ray ranges. One of
the instruments on Solar-B, Solar Optical Telescope (SOT), a Japan/US
collaboration, aims at measuring the magnetic field and the Doppler
velocity field in the solar photosphere. Although it is not specifically
designed for helioseismic observations, the high-resolution Dopplergram
produced by SOT is potentially a very powerful tool for detailed
seismic investigation of subsurface magnetic and thermal structures
and associated mass flows. If successful, these measurements will be
an important contribution to the main goal of the Solar-B project:
understanding the origin and dynamics of the basic magnetic structures
and their effects on the solar corona. We discuss the prospect and
challenges of local-area helioseismology by SOT.
Title: Towards a Wave Theory Interpretation of Time-Distance
Helioseismology Data
Authors: Birch, A. C.; Kosovichev, A. G.
Bibcode: 2001IAUS..203..180B
Altcode:
Time-distance helioseismology, which measures the time for acoustic
waves to travel between points on the solar surface, has been used to
study small-scale three-dimensional features in the sun, for example
active regions, as well as large-scale features, for example meridional
flow, that are not accessible by standard global helioseismology. The
interpretation of travel times has typically been done in the
ray approximation. The interaction of acoustic waves with features
smaller than their wavelength, for example in active regions or in the
tachocline, is not expected to be well represented by ray theory. In
order to develop a wave interpretation of time-distance data we employ
the first Born approximation, which takes into account finite-wavelength
effects and allows a single scattering between the source and receiver
of the acoustic wave. We show the sensitivity of travel times to flows
and structure perturbations and compare the results with ray theory.
Title: The Born approximation in time-distance helioseismology
Authors: Birch, A. C.; Kosovichev, A. G.
Bibcode: 2001ESASP.464..187B
Altcode: 2001soho...10..187B
Time-distance helioseismology, which measures the time for acoustic
waves to travel between points on the solar surface, has been used to
study small-scale three-dimensional features in the sun, for example
active regions, as well as large-scale features, for example meridional
flow, that are not accessible by standard global helioseismology. The
interpretation of travel times has typically been done in the ray
approximation. The interaction of acoustic waves with features smaller
than their wavelength, such as in active regions or in the tachocline,
is not expected to be well represented by ray theory. In order to
develop a wave interpretation of time-distance data we employ the
first Born approximation, which takes into account finite-wavelength
effects and allows a single scattering between the source and receiver
of the acoustic wave. We show that in the case of spherically symmetric
perturbations the Born approximation can be easily related to normal
mode perturbation theory. The Born approximation agrees with ray theory
when applied to large scale perturbations, and performs better than
ray theory when applied to perturbations with small spatial scale. We
show, via an example OLA inversion of artificial data, that reasonable
averaging kernels can be built from Born approximation kernels.
Title: Active Longitudinal Structures of the Sun from MDI and EIT
Observations
Authors: Benevolenskaya, E. E.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 2001IAUS..203..251B
Altcode:
Using data from the EIT and MDI instruments on SOHO and from Kitt
Peak Observatory we have studied the non-axisymmetrical structure
and dynamics of solar activity at different levels of the solar
atmosphere. The data were reduced to synoptic maps of the photospheric
magnetic field and coronal structures in the EUV lines: 171Å, 195Å,
284Å, and 304Å. In addition, the coronal temperature maps were
obtained using the ration of the 171Å and 195Å lines. The results
reveal long-living longitudinal structures in the photosphere and
corona during the transition from Cycle 22 to 23 and the rising
phase of Cycle 23. We have found the Hale magnetic field polarity
reversal first occured at the active longitudes. Thus, the stable
longitudinal structures play an important role in the mechanism of
the solar cycle. These structures are also revealed in the large-scale
structure of the corona. We study the relation between the magnetic and
coronal longitudinal structures, and their role in formation of coronal
holes. We discuss the relations between rotation of the longitudinal
structures in the photosphere and corona and compare with the rotation
rate of the solar interior using helioseismic data. This work was
carried out in the collaboration with J. T. Hoeksema, A. G. Kosovichev
and P. H. Scherrer of Stanford University.
Title: Solar Active Longitudes and Their Rotations Using SOHO-MDI Data
(CD-ROM Directory: contribs/benevo2)
Authors: Benevolenskaya, E. E.; Hoeksema, J. T.; Kosovichev, A. G.;
Scherrer, P. H.
Bibcode: 2001ASPC..223..583B
Altcode: 2001csss...11..583B
No abstract at ADS
Title: Low-degree p-mode parameters: the solar-cycle dependence
Authors: Toutain, T.; Kosovichev, A. G.
Bibcode: 2001ESASP.464..123T
Altcode: 2001soho...10..123T
Using the LOI-proxy data of the MDI instrument we have studied the
solar-cycle dependence of the low-degree (l=0-3) p modes between May
1996 and August 2000. In addition to the usual frequency shift we have
also found that mode linewidths increase by about 15% from the solar
minimum to the solar maximum. Within the accuracy we have, the other
parameters: rotational splitting and asymmetry do not exhibit any
dependence with the solar cycle, except the a2 coefficient
which seems to increase as well with activity. Thought the mode energy
does not also vary much, this analysis indicates an unexplained maximum
of energy in the first half of year 1998.
Title: New Developments in Local Area Helioseismology
Authors: Duvall, T. L., Jr.; Kosovichev, A. G.
Bibcode: 2001IAUS..203..159D
Altcode:
Several techniques are used to study local areas in helioseismology,
including time-distance helioseismology, acoustic imaging/holography,
and ring diagram analysis. These techniques can be used to study flows,
magnetic fields, and temperature inhomogeneities. The "local" area
studied can be as small as a supergranule, or as large as the entire
convection zone in the case of meridional circulation as studied
by Giles and colleagues. Active regions have been studied with some
interesting results, with complicated flow patterns below sunspots
and detectable sound speed inhomogeneitities in the 10 Mm below the
spots. Another interesting result is the detection of sunspots on
the back side of the Sun by Lindsey and Braun using the holography
technique. A confirmation of their result using the time-distance
technique will be discussed.
Title: Helioseismic Tomography of Solar Active Regions (CD-ROM
Directory: contribs/kosovich)
Authors: Kosovichev, A. G.
Bibcode: 2001ASPC..223...99K
Altcode: 2001csss...11...99K
No abstract at ADS
Title: The physics of the solar core deduced from GOLF and MDI
acoustic modes
Authors: Turck-Chièze, S.; Kosovichev, A. G.; Couvidat, S.; García,
R. A.; Nghiem, P.; Pérez Hernández, F.; Turcotte, S.
Bibcode: 2001ESASP.464..485T
Altcode: 2001soho...10..485T
We use the recent results on solar acoustic modes coming from GOLF
and MDI, to demonstrate that we improve the quality of the inversion
of the sound speed and the density by avoiding several biases coming
from the turbulent surface. The present accuracy allows a quantitative
discussion on the physics of the solar nuclear core. We now exclude
several phenomena which are incompatible with the present observations:
central turbulent mixing or large modification of the pp chain nuclear
reaction rates. We propose a solution to the difference between the
Sun and solar models only in terms of turbulence at the base of the
convective zone, an increase of the weak interaction p-p reaction
rate by 2% in the framework of intermediate Mitler screening, and
an underestimate of CNO composition of no more than 3%. Even if this
solution is probably not unique, it allows a prediction of neutrino
fluxes induced by helioseismology. We also note that nowadays,
helioseismology puts just a few constraints on the reaction rates of
the CNO cycle, and let place for lower high energy neutrino predictions.
Title: Helioseismic and magnetic waves as signatures of energy
transport mechanisms in solar flares
Authors: Zharkova, V. V.; Kosovichev, A. G.
Bibcode: 2001ESASP.464..259Z
Altcode: 2001soho...10..259Z
Recently reported helioseismic waves caused by a X-class solar flare
propagating in the solar interior and observed in a form of ripples
at the photosphere as well as magnetic field variations in the lower
atmosphere associated with another X-class flares revived an interest
to the problem of energy release and transport in solar flares. A
comparison of simulations with the seismic observations revealed that
a starting time of the wave coincides with the onset of the X-ray
flare while the wave's amplitude corresponds to the energy momentum
higher by half of order of the magnitude than the one observed. In
the current paper electron beams are considered as alternative agents
delivering energy directly to the photosphere. Depth variations of
a momentum and energy deposition by beam electrons with the energy
power law precipitating from the corona into a flaring atmosphere
are calculated using the time-dependent kinetic solutions. A momentum
brought by weak beams to lower chromospheric levels was found not to
exceed of 1018 - 1019 whereas moderately intense
and hard beams can deliver the momentum of (5 - 10)×1022
g cm/s. Energy deposition functions are also shown to increase with
depths by about an order of magnitude at the lower chromosphere if
Ohmic losses are included in addition to pure Coulomb collisions.
Title: g-mode detection: Where do we stand?
Authors: Appourchaux, T.; Andersen, B.; Berthomieu, G.; Chaplin, W.;
Elsworth, Y.; Finsterle, W.; Frölich, C.; Gough, D. O.; Hoeksema,
T.; Isaak, G.; Kosovichev, A.; Provost, J.; Scherrer, P.; Sekii, T.;
Toutain, T.
Bibcode: 2001ESASP.464..467A
Altcode: 2001soho...10..467A
We review the recent developments in determining the upper limits to
g-mode amplitudes obtained by SOHO instruments, GONG and BiSON. We
address how this limit can be improved by way of new helioseismic
instruments and/or new collaborations, hopefully providing in the not
too distant future unambiguous g-mode detection.
Title: Solar Interior: Rotation
Authors: Kosovichev, A.
Bibcode: 2000eaa..bookE2010K
Altcode:
The Sun rotates differentially, i.e. the solar rotation rate varies with
both latitude and radius. The differential rotation is particularly
prominent in the convection zone where equatorial zones rotate almost
30% faster than near-polar regions. There are also strong variations
of the rotation rate with radius near the surface and at the bottom
of the convection zone (rotational shear layers). I...
Title: Signatures of the Rise of Cycle 23
Authors: Dziembowski, W. A.; Goode, P. R.; Kosovichev, A. G.; Schou, J.
Bibcode: 2000ApJ...537.1026D
Altcode:
During the rise of Cycle 23, we have found a sizable, systematic
evolution of the Solar and Heliospheric Observatory/Michelson Doppler
Imager solar oscillation frequencies implying significant changes in the
spherically symmetric structure of the Sun's outer layers as well as in
its asphericity up to a P18 Legendre distortion. We conducted
a search for corresponding asymmetries in Ca II K data from Big Bear
Solar Observatory. We found tight temporal and angular correlations of
the respective asphericities up through P10. This result
emphasizes the role of the magnetic field in producing the frequency
changes. We carried out inversions of the frequency differences and
the splitting coefficients assuming that the source of the evolving
changes is a varying stochastic magnetic field. With respect to the
most recent activity minimum, we detected a significant perturbation in
the spherical part at a depth of 25-100 Mm, which may be interpreted
as being a result of a magnetic perturbation, <B2>,
of about (60KG)2 and/or a relative temperature perturbation
of about 1.2×10-4. Larger, although less statistically
significant, perturbations of the interior structure were found in
the aspherical distortion.
Title: Observational Upper Limits to Low-Degree Solar g-Modes
Authors: Appourchaux, T.; Fröhlich, C.; Andersen, B.; Berthomieu, G.;
Chaplin, W. J.; Elsworth, Y.; Finsterle, W.; Gough, D. O.; Hoeksema,
J. T.; Isaak, G. R.; Kosovichev, A. G.; Provost, J.; Scherrer, P. H.;
Sekii, T.; Toutain, T.
Bibcode: 2000ApJ...538..401A
Altcode:
Observations made by the Michelson Doppler Imager (MDI) and Variability
of solar IRradiance and Gravity Oscillations (VIRGO) on the Solar and
Heliospheric Observatory (SOHO) and by the ground-based Birmingham
Solar Oscillations Network (BiSON) and Global Oscillations Network
Group (GONG) have been used in a concerted effort to search for solar
gravity oscillations. All spectra are dominated by solar noise in the
frequency region from 100 to 1000 μHz, where g-modes are expected to be
found. Several methods have been used in an effort to extract any g-mode
signal present. These include (1) the correlation of data-both full-disk
and imaged (with different spatial-mask properties)-collected over
different time intervals from the same instrument, (2) the correlation
of near-contemporaneous data from different instruments, and (3) the
extraction-through the application of complex filtering techniques-of
the coherent part of data collected at different heights in the solar
atmosphere. The detection limit is set by the loss of coherence
caused by the temporal evolution and the motion (e.g., rotation)
of superficial structures. Although we cannot identify any g-mode
signature, we have nevertheless set a firm upper limit to the amplitudes
of the modes: at 200 μHz, they are below 10 mm s-1 in
velocity, and below 0.5 parts per million in intensity. The velocity
limit corresponds very approximately to a peak-to-peak vertical
displacement of δR/Rsolar=2.3×10-8 at the
solar surface. These levels which are much lower than prior claims,
are consistent with theoretical predictions.
Title: Identification of Solar Acoustic Modes of Low Angular Degree
and Low Radial Order
Authors: Bertello, L.; Varadi, F.; Ulrich, R. K.; Henney, C. J.;
Kosovichev, A. G.; García, R. A.; Turck-Chièze, S.
Bibcode: 2000ApJ...537L.143B
Altcode:
We present evidence for the detection of low radial order (n<10)
acoustic modes of low angular degree, l=0-2, in the 759 day long
Global Oscillations at Low Frequency and Michelson Doppler Imager time
series. We used Random-Lag Singular Cross-Spectrum Analysis, which
searches for simultaneous oscillatory components in two or more time
series. We have determined 11 modes in the range n=3-9, of which eight
modes confirm the previous measurements by Toutain et al. and three
modes of l=0 and n=3, 5, and 6 are reliably measured for the first
time. The errors of frequency determination are also significantly
reduced for several previously identified modes. New sound speed
inversion results suggest that the effect of inhomogeneous initial
composition of the Sun should be included in the standard solar model.
Title: Comparison of the 1998 April 29 M6.8 and 1998 November 5
M8.4 Flares
Authors: Wang, Haimin; Goode, Philip R.; Denker, Carsten; Yang, Guo;
Yurchishin, Vasyl; Nitta, Nariaki; Gurman, Joseph B.; St. Cyr, Chris;
Kosovichev, Alexander G.
Bibcode: 2000ApJ...536..971W
Altcode:
We combined, and analyzed in detail, the Hα and magnetograph data
from Big Bear Solar Observatory (BBSO), full-disk magnetograms from
the Michelson Doppler Imager (MDI) on board Solar and Heliospheric
Observatory (SOHO), coronagraph data from the Large Angle Spectrometric
Coronagraph (LASCO) of SOHO, Fe XII 195 Å data from the Extreme
ultraviolet Imaging Telescope (EIT) of SOHO, and Yohkoh soft X-ray
telescope (SXT) data of the M6.8 flare of 1998 April 29 in National
Oceanic and Atmospheric Administration (NOAA) region 8375 and the
M8.4 flare of 1998 November 5 in NOAA region 8384. These two flares
have remarkable similarities:1. Partial halo coronal mass ejections
(CMEs) were observed for both events. For the 1998 April 29 event,
even though the flare occurred in the southeast of the disk center,
the ejected material moved predominantly across the equator, and the
central part of the CME occurred in the northeast limb. The direction
in which the cusp points in the postflare SXT images determines the
dominant direction of the CMEs.2. Coronal dimming was clearly observed
in EIT Fe XII 195 Å for both but was not observed in Yohkoh SXT for
either event. Dimming started 2 hr before the onset of the flares,
indicating large-scale coronal restructuring before both flares.3. No
global or local photospheric magnetic field change was detected from
either event; in particular, no magnetic field change was found in the
dimming areas.4. Both events lasted several hours and, thus, could be
classified as long duration events (LDEs). However, they are different
in the following important aspects. For the 1998 April 29 event,
the flare and the CME are associated with an erupting filament in
which the two initial ribbons were well connected and then gradually
separated. SXT preflare images show the classical S-shape sheared
configuration (sigmoid structure). For the 1998 November 5 event, two
initial ribbons were well separated, and the SXT preflare image shows
the interaction of at least two loops. In addition, no filament eruption
was observed. We conclude that even though these two events resulted
in similar coronal consequences, they are due to two distinct physical
processes: eruption of sheared loops and interaction of two loops.
Title: Comparison of Frequencies and Rotational Splittings of Solar
Acoustic Modes of Low Angular Degree from Simultaneous MDI and
GOLF Observations
Authors: Bertello, L.; Henney, C. J.; Ulrich, R. K.; Varadi, F.;
Kosovichev, A. G.; Scherrer, P. H.; Roca Cortés, T.; Thiery, S.;
Boumier, P.; Gabriel, A. H.; Turck-Chièze, S.
Bibcode: 2000ApJ...535.1066B
Altcode:
During the years 1996 through 1998 the Michelson Doppler Imager (MDI)
and the Global Oscillations at Low Frequency (GOLF) experiments on the
Solar and Heliospheric Observatory (SOHO) mission have provided unique
and nearly uninterrupted sequences of helioseismic observations. This
paper describes the analysis carried out on power spectra from 759
days of calibrated disk-averaged velocity signals provided by these two
experiments. The period investigated in this work is from 1996 May 25
to 1998 June 22. We report the results of frequency determination of
low-degree (l<=3) acoustic modes in the frequency range between 1.4
mHz and 3.7 mHz. Rotational splittings are also measured for nonradial
modes up to 3.0 mHz. The power spectrum estimation of the signals
is performed using classical Fourier analysis and the line-profile
parameters of the modes are determined by means of a maximum likelihood
method. All parameters have been estimated using both symmetrical and
asymmetrical line profile-fitting formula. The line asymmetry parameter
of all modes with frequency higher than 2.0 mHz is systematically
negative and independent of l. This result is consistent with the
fact that both MDI and GOLF data sets investigated in this paper are
predominantly velocity signals, in agreement with previous results. A
comparison of the results between the symmetric and asymmetric fits
shows that there is a systematic shift in the frequencies for modes
above 2.0 mHz. Below this frequency, the line width of the modes
is very small and the time base of the data does not provide enough
statistics to reveal an asymmetry. In general, the results show that
frequency and rotational splitting values obtained from both the
MDI and GOLF signals are in excellent agreement, and no significant
differences exist between the two data sets within the accuracy of the
measurements. Our results are consistent with a uniform rotation of
the solar core at the rate of about 435 nHz and show only very small
deviations of the core structure from the standard solar model.
Title: Diagnostics of Solar Magnetic Fields by Time-Distance
Helioseismology
Authors: Zhao, J.; Duvall, T. L., Jr.; Kosovichev, A. G.
Bibcode: 2000SPD....31.0120Z
Altcode: 2000BAAS...32..804Z
Sunspot seismology has been developed in recent years, and the
time-distance analysis plays an important role in it. Most of the
current inferences for interior structures were made by measuring
perturbations of the acoustic wave speed which is due to both
temperature and magnetic field variations. An important problem
of the time-distance seismology is to disentangle the effects of
temperature and magnetic field. The standard technique for the travel
time measurements is to divide annuli for given wave travel distances
into four sectors to get both the wave speed and flow velocity under the
surface. Here, we consider the inhomogeneity caused by the magnetic
field of sunspots. By dividing the annuli into eight sectors and
analyzing the travel time of each octants, we can obtain the direction
and the magnitude of the magnetic field in sunspot regions. Scattering
and absorption of incoming waves in each different direction may also
play an important role in these measurements.
Title: Active Longitudes in Solar Corona
Authors: Benevolenskaya, E. E.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 2000SPD....31.0226B
Altcode: 2000BAAS...32..815B
We present the results of the investigation of the large-scale
structure of the solar corona during the transition period between
solar cycles 22 and 23 and at the beginning of the current cycle 23
using the SOHO/EIT EUV data obtained in 171 Angstroms, 195 Angstroms,
284 Angstroms and 304 Angstroms lines. For this analysis the data
were transformed into synoptic maps for each of the spectral lines,
and for the 195 Angstroms/171 Angstroms line ratio which is an
index of the coronal temperature. The synoptic maps reveal stable
longitudinal structures in the coronal intensities and temperature,
which are related to large-scale magnetic field structures. We discuss
the relation between the coronal and photospheric magnetic structures
obtained from the SOHO/MDI data, and compare the rotation rates of
these structures with the rotation profile of the solar interior in
order to determine the possible origin of the coronal structures.
Title: Heliotomography: what happens just below the surface?
Authors: Kosovichev, A. G.; Duvall, T. L., Jr.
Bibcode: 2000SPD....31.0601K
Altcode: 2000BAAS...32..838K
Heliotomography (or time-distance helioseismology) is a relatively
new tool for diagnostics of internal structures and dynamics of
the Sun. It is based on inversion of travel times of acoustic
wave packets propagating through the solar interior and bouncing
back to the surface. The travel times provide information about the
variations of temperature, magnetic fields and flow velocities along
the wave paths. These properties of the solar interior are inferred
from the travel times by tomographic inversions. Heliotomography has
provided a three-dimensional view of the interior, not accessible by
traditional helioseismology based on mode frequencies. This method has
been applied to study both large-scale flows (meridional circulation,
North-South asymmetry of solar rotation) and small-scale phenomena
(supergranulation, sunspots, emerging magnetic flux). The results
reveal very dynamical and complicated structures below the surface,
associated with convection and magnetic fields, and shed new light
on the formation and evolution of active regions and sunspots. We
discuss the current limits for the temporal and spatial resolution and
recent achievements. Most inversion results provide the results to
a depth of 20 Mm. It has been demonstrated that with this method we
can measure the solar flows to the base of the convection zone which
is 200 Mm deep. However, resolving deep and small-scale features is
very challenging, and requires concentrated efforts for developing
both the measurement techniques and theoretical interpretations. We
review the recent progress in developing a wave-theory approach to
heliotomographic inversions, and perspectives for the diagnostics of
the physical processes below the Sun's surface.
Title: Observations of Magnetic Energy Release in Solar Flares
Authors: Kosovichev, A. G.; Zharkova, V. V.
Bibcode: 2000SPD....31.0259K
Altcode: 2000BAAS...32..821K
We report on direct observations of magnetic energy release in
solar flares using Michelson Doppler Imager (MDI) instrument on SOHO
spacecraft. Continuous monitoring a solar active region in May 1998
with high-quality magnetograms taken every minute has allowed us to
detect sudden decreases of the magnetic energy of the active region
during two moderate-class flares. The most rapid energy release occurred
near the maxima of the flare soft X-ray emission recorded by the GOES
satellite. After the flares the magnetic energy of the active region is
restored to nearly preflare values. We have also detected irreversible
changes in the magnetic energy distribution for this and other active
regions during solar flares.
Title: Optimal Masks for Low-Degree Solar Acoustic Modes
Authors: Toutain, T.; Kosovichev, A. G.
Bibcode: 2000ApJ...534L.211T
Altcode: 2000astro.ph..4153T
We suggest a solution to an important problem in observational
helioseismology of the separation of lines of solar acoustic (p) modes
of low angular degree in oscillation power spectra by constructing
optimal masks for Doppler images of the Sun. Accurate measurements
of oscillation frequencies of low-degree modes are essential for the
determination of the structure and rotation of the solar core. However,
these measurements for a particular mode are often affected by leakage
of other p-modes arising when the Doppler images are projected on to
spherical harmonic masks. The leakage results in overlapping peaks
corresponding to different oscillation modes in the power spectra. In
this Letter, we present a method for calculating optimal masks for a
given (target) mode by minimizing the signals of other modes appearing
in its vicinity. We apply this method to time series of 2 yr obtained
from the Michelson Doppler Imager instrument on board the Solar and
Heliospheric Observatory space mission and demonstrate its ability to
reduce efficiently the mode leakage.
Title: Sunspots: Frontside and Backside Measurements with
Time-Distance Helioseismology
Authors: Duvall, T. L., Jr.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 2000SPD....31.0505D
Altcode: 2000BAAS...32..837D
In time-distance helioseismology, travel times measured between
different surface locations are used to infer subsurface flows,
temperature inhomogeneities and magnetic fields. It has been suggested
that most of the travel time reduction near sunspots may be due to the
lowered reflection layer associated with the Wilson depression. This
will be examined by looking at rays that travel below the sunspot but do
not begin or end in the spot. A time-distance method of imaging sunspots
on the backside will be compared with that of Lindsey and Braun.
Title: Sunspots: frontside and backside measurements with
time-distance helioseismology.
Authors: Duvall, T. L., Jr.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 2000BAAS...32Q.837D
Altcode:
No abstract at ADS
Title: Heliotomography: what happens just below the surface?
Authors: Kosovichev, A. G.; Duvall, T. L., Jr.
Bibcode: 2000BAAS...32..837K
Altcode:
No abstract at ADS
Title: A Wave-Theory Approach to Time-Distance Helioseismology
Authors: Birch, A. C.; Kosovichev, A. G.
Bibcode: 2000SPD....31.0107B
Altcode: 2000BAAS...32R.802B
Time-distance helioseismology, which measures the time for acoustic
waves to travel between points on the solar surface, has been
used to study small-scale three-dimensional features in the sun,
for example active regions, as well as large-scale features, for
example meridional flow, that are not accessible by standard global
helioseismology. The interpretation of travel times has typically
been done in the ray approximation. The interaction of acoustic waves
with features smaller than their wavelength, for example in active
regions or in the tachocline, is not expected to be well represented
by ray theory. In order to develop a wave-based interpretation of
time-distance data we employ the first Born approximation, which takes
into account finite-wavelength effects and allows a single scattering
between the source and receiver of the acoustic wave. We calculate the
sensitivity functions, the solar equivalent of the 'banana-doughnut'
kernels from terrestrial seismology, for the wave travel times and
study the dependence of travel times on perturbations to a solar
model. The wave travel times are compared with ray theory. This work
was supported by NASA grant NAG-3077.
Title: Observations of magnetic energy release in solar flares.
Authors: Kosovichev, A. G.; Zharkova, V. V.
Bibcode: 2000BAAS...32Q.821K
Altcode:
No abstract at ADS
Title: The Magnetic Connectivity of Moss Regions
Authors: Zhao, X. P.; Hoeksema, J. T.; Kosovichev, A. G.; Bush, R.;
Scherrer, P. H.
Bibcode: 2000SoPh..193..219Z
Altcode:
A novel emission feature resembling moss was first identified in
high-resolution TRACE Fe ix/x 171 Å images by Berger et al. (1999). The
moss emission is characterized by dynamic arc-second scale, bright
elements surrounding dark inclusions in images of solar active
regions. Patches of moss elements, called moss regions, have a scale
of 20-30 Mm. Moss regions occur only above some of magnetic plages
that underlie soft X-ray coronal loops. Using the potential field
extrapolation of the photospheric magnetic field into the corona, we
find that the magnetic field lines in moss-associated magnetic plages
connect with adjacent plages with opposite polarity; however, all field
lines from mossless plages end in surrounding `quiet regions'. This
result is consistent with the idea that the TRACE moss is the emission
from the upper transition region due to heating of low-lying plasma
by field-aligned thermal conduction from overlying hot plasma (Berger
et al., 1999).
Title: Helioseismic diagnostics of solar convection and activity. Part
1, 2. Proceedings. SOHO-9 Workshop, Stanford, CA (USA), 12 - 15
Jul 1999.
Authors: Švestka, Z.; Harvey, J. W.; Kosovichev, A. G.; Duvall,
T. L., Jr.
Bibcode: 2000SoPh..192....1S
Altcode:
The following topics were dealt with: theories of solar convection,
rotation and activity, helioseismic tomography, acoustic imaging and
holography, ring-diagram analysis, magnetic fields and oscillations,
solar cycle variations of the internal structure and rotation, solar
convective structures and oscillations.
Title: Travel Time Sensitivity Kernels
Authors: Birch, A. C.; Kosovichev, A. G.
Bibcode: 2000SoPh..192..193B
Altcode:
We derive, following the standard first Born approximation approach
used in the geophysics literature, an expression for the travel time
perturbation caused by a perturbation to sound speed. In our simple
model we employ a point source at one point and calculate the time
taken for a wave packet created at the source to move to a second
point. In the first Born approximation the travel time delay caused
by a perturbation to the background model can be expressed as the
integral over the whole sun of some function, called the travel time
sensitivity kernel, multiplied by the perturbation. The sensitivity
kernels are zero along the geometrical ray connecting the two points
and have maximum weight in a tube around the ray; they are the solar
equivalent of `the banana-doughnut' kernels discussed in the geophysics
literature. Calculating sensitivity kernels that are more accurate
than those derived from ray theory is important for the future of
inversions done with time-distance helioseismology data as they will
allow greater confidence in the results as well as increased resolution.
Title: Time-Distance Inversion Methods and Results - (Invited Review)
Authors: Kosovichev, A. G.; Duvall, T. L. _Jr., Jr.; Scherrer, P. H.
Bibcode: 2000SoPh..192..159K
Altcode:
The current interpretations of the travel-time measurements in quiet
and active regions on the Sun are discussed. These interpretations
are based on various approximations to the 3-D wave equation such as
the Fermat principle for acoustic rays and the Born approximation. The
ray approximation and its modifications have provided the first view
of the 3-D structures and flows in the solar interior. However, more
accurate and computationally efficient approximations describing the
relation between the wave travel times and the internal properties
are required to study the structures and flows in detail. Inversion
of the large three-dimensional datasets is efficiently carried
out by regularized iterative methods. Some results of time-distance
inversions for emerging active regions, sunspots, meridional flows and
supergranulation are presented. An active region which emerged on the
solar disk in January 1998, was studied from SOHO/MDI for eight days,
both before and after its emergence at the surface. The results show
a complicated structure of the emerging region in the interior, and
suggest that the emerging flux ropes travel very quickly through the
depth range of our observations. The estimated speed of emergence is
about 1.3 km s−1. Tomographic images of a large sunspot
reveal sunspot `fingers' - long narrow structures at a depth of about
4 Mm, which connect the sunspot with surrounding pores of the same
polarity.
Title: Numerical Simulations of Oscillation Modes of the Solar
Convection Zone
Authors: Georgobiani, D.; Kosovichev, A. G.; Nigam, R.; Nordlund,
Å.; Stein, R. F.
Bibcode: 2000ApJ...530L.139G
Altcode: 1999astro.ph.12485G
We use the three-dimensional hydrodynamic code of Stein & Nordlund
to realistically simulate the upper layers of the solar convection zone
in order to study physical characteristics of solar oscillations. Our
first result is that the properties of oscillation modes in the
simulation closely match the observed properties. Recent observations
from the Solar and Heliospheric Observatory (SOHO)/Michelson Doppler
Imager (MDI) and Global Oscillations Network Group have confirmed the
asymmetry of solar oscillation line profiles, initially discovered
by Duvall et al. In this Letter, we compare the line profiles in
the power spectra of the Doppler velocity and continuum intensity
oscillations from the SOHO/MDI observations with the simulation. We
also compare the phase differences between the velocity and intensity
data. We have found that the simulated line profiles are asymmetric
and have the same asymmetry reversal between velocity and intensity
as observed. The phase difference between the velocity and intensity
signals is negative at low frequencies, and phase jumps in the vicinity
of modes are also observed. Thus, our numerical model reproduces the
basic observed properties of solar oscillations and allows us to study
the physical properties which are not observed.
Title: Helioseismic Waves and Magnetic Field Variations Induced by
Solar Flares as Probes of Energy Transport Mechanisms
Authors: Zharkova, V. V.; Kosovichev, A. G.
Bibcode: 2000ASPC..206...77Z
Altcode: 2000hesp.conf...77Z
No abstract at ADS
Title: Observation of Shock Waves Associated with Coronal Mass
Ejections from SOHO/LASCO
Authors: Stepanova, T. V.; Kosovichev, A. G.
Bibcode: 2000AdSpR..25.1855S
Altcode:
Theoretical models of coronal mass ejections (CME) predict formation
of shock waves in front of eruptive magnetic structures, loops and
arches. These shock waves have been detected in the solar wind at large
distances from the Sun. However, their observation at short distances in
the solar corona has been difficult. We have attempted to identify the
shocks in the LASCO data by comparing the radial intensity profiles in
limb CMEs with a theoretical model (Stepanova and Kosovichev, 1993). We
demonstrate that the shock scan be identified at least in some cases of
CME of the simple loop-like geometry. The LASCO observations indicate
a turbulent structure for the flows associated with the shocks. We
have determined the velocity of such shocks and expanding loops and
compared with our model. In most cases, the observed expansion speed
was rather slow possibly meaning that no significant disturbances on
the solar surface were involved in initiating the eruptions
Title: Solar tomography
Authors: Kosovichev, A. G.; Duvall, T. L., Jr.
Bibcode: 1999CSci...77.1467K
Altcode:
No abstract at ADS
Title: New and Old Magnetic Fluxes at the Beginning of Solar Cycle 23
Authors: Benevolenskaya, E. E.; Hoeksema, J. T.; Kosovichev, A. G.;
Scherrer, P. H.
Bibcode: 1999ESASP.448...69B
Altcode: 1999ESPM....9...69B; 1999mfsp.conf...69B
No abstract at ADS
Title: Variations of Photospheric Magnetic Field Associated with
Flares and CMEs
Authors: Kosovichev, A. G.; Zharkova, V. V.
Bibcode: 1999SoPh..190..459K
Altcode:
Using high-cadence magnetograms from the SOHO/MDI we have investigated
variations of the photospheric magnetic field during solar flares
and CMEs. In the case of a strong X-class flare of May 2, 1998, we
have detected variations of magnetic field in a form of a rapidly
propagating magnetic wave. During the impulsive phase of the flare we
have observed a sudden decrease of the magnetic energy in the flare
region. This provides direct evidence of magnetic energy release in
solar flares. We discuss the physics of the magnetic field variations,
and their relations to the Moreton Hα waves and the coronal waves
observed by the EIT.
Title: Structure and Dynamics of Interconnecting Loops and Coronal
Holes in Active Longitudes
Authors: Benevolenskaya, Elena E.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 1999SoPh..190..145B
Altcode:
Using SOHO/MDI and SOHO/EIT data we study properties and dynamics
of interconnected active regions, and the relations between the
photospheric magnetic fields and coronal structures in active longitudes
during the beginning of solar cycle 23. The emergence of new magnetic
flux results in appearance of new interconnecting loops. The existence
of stable coronal structures strongly depends on the photospheric
magnetic fluxes and their variations. We present some initial results
for a complex of solar activity observed in April 1997, and discuss
the role of reconnection in the formation of the interconnected loops
and coronal holes.
Title: Detection of Magnetic Wave Associated with Solar Flares and CME
Authors: Zharkova, V. V.; Kosovichev, A. G.
Bibcode: 1999ESASP.446..755Z
Altcode: 1999soho....8..755Z
Using high-cadence magnetograms from the SOHO/MDI we have investigated
variations of the photospheric magnetic field during solar flares and
CME. For most investigated events there was no significant change in
the magnetic fields. However, in the case of a strong X-class flare
we have detected a rapidly propagating magnetic wave. We discuss the
physics of this new phenomenon, and its relation to the Moreton H-alpha
waves and the coronal waves observed by the EIT.
Title: Frequencies and splittings of low-degree acoustic modes:
a comparison between MDI and GOLF observations
Authors: Bertello, L.; Henney, C. J.; Ulrich, R. K.; Varadi, F.;
Kosovichev, A. G.; Roca Cortes, T.; Garcia, R. A.; Scherrer, P. H.
Bibcode: 1999AAS...19410805B
Altcode:
During the years 1996 through 1998 the MDI and GOLF experiments on the
SOHO mission have provided unique and nearly uninterrupted sequences of
helioseismic observations. This paper describes the analysis carried out
on power spectra from 759 days of calibrated velocity signals provided
by these two experiments. The time series investigated in this work are
from 25 May, 1996 to 22 June, 1998. We report the results of frequency
and splitting determination of low-degree (l < 4) acoustic modes
in the frequency range between 1.5 mHz and 4.0 mHz. The power spectrum
estimation of the signals is performed using classical Fourier analysis
and the line-profile parameters of the modes are determined by means of
a maximum likelihood method. All parameters have been estimated using
both symmetrical and asymmetrical line profile-fitting formula. The
line asymmetry parameter of all modes with frequency higher than 2.0
mHz is systematically negative and independent from l. This result is
consistent with the fact that both MDI and GOLF data sets investigated
in this paper are predominantly velocity signals. A comparison of the
results between symmetric fit and asymmetric one shows that there
is a systematic shift in the eigenfrequencies for modes above 2.0
mHz. The results show that eigenfrequency and rotational splitting
values obtained from both the MDI and GOLF signals are in excellent
agreement, and no significant differences exist between the two data
sets within the indetermination of the measurement.
Title: Frequencies and splittings of low-degree acoustic modes:
a comparison between MDI and GOLF observations.
Authors: Bertello, L.; Henney, C. J.; Ulrich, R. K.; Varadi, F.;
Kosovichev, A. G.; Roca Cortes, T.; Garcia, R. A.; Scherrer, P. H.
Bibcode: 1999BAAS...31.1242B
Altcode:
No abstract at ADS
Title: Oscillations in Active Regions - Diagnostics and Seismology
Authors: Kosovichev, A. G.
Bibcode: 1999ASPC..184..151K
Altcode:
Oscillations in solar active regions provide a unique tool for
probing the structure and dynamics of the active regions in the
solar interior. The oscillations are excited stochastically by
turbulent convection and also by strong localized events, such as
flares. Oscillation power maps of stochastically excited waves show
a deficit of acoustic power in the frequency range 2-5 mHz, and
enhanced power at higher frequencies. These power variations are
stronger in regions with stronger magnetic field, and, probably,
are due to transformation of lower-frequency acoustic waves into
higher-frequency MHD waves. An impulsive excitation event was observed
as a result of a solar flare on July 9, 1996, from SOHO/MDI. The
amplitude of the seismic waves on the surface was sufficiently high
to determine the characteristics of the energy and momentum impact,
and also to construct time-distance diagrams of the seismic response
and to detect the deviation from the axial symmetry. This deviation
probably resulted from the interaction with sunspots. Thus, in
principle, the flare seismic waves can be used to probe the structure
of active regions by time-distance helioseismology. The current
methods of seismic diagnostics, acoustic imaging and time-distance
tomography, are based on accumulating the seismic signal by averaging
stochastically excited waves over a period of several hours. These
methods have provided interesting information about the structure and
evolution of active regions in the Sun's interior. As an example, I
present the results of acoustic imaging of sunspots, and helioseismic
tomography for large-scale imaging of active complexes of January 1991,
and for an active region which emerged near the center of the solar
disk in July 1996 and which studied from SOHO/MDI for nine days, both
before and after its emergence at the surface. The initial results
show complicated structures of the emerging regions in the interior,
and suggest that the emerging flux ropes travel very quickly through
the depth range of these observations.
Title: Inversion methods in helioseismology and solar tomography.
Authors: Kosovichev, A. G.
Bibcode: 1999JCoAM.109....1K
Altcode:
Basic methods by which the internal structure and dynamics of the
Sun can be inferred from observed frequencies of solar oscillations
and acoustic travel times are discussed. The methods for inverting
the oscillation frequencies are based on a variational formulation of
the adiabatic eigenvalue problem for a star. The inversion technique
formulated in terms of linear integral constraints provides estimates
of localized averages of properties of the solar structure, such as
density and sound speed, helium abundance in the convection zone for
a given equation of state, and, in addition, the estimates for the
internal rotation rate. The method of inverting acoustic travel times
employs a geometrical ray approximation and provides 3D images of
solar convective cells, active regions and sunspots. The information
about the global and local structures and flow velocities in the solar
interior is important for understanding solar evolution and mechanisms
of solar activity. The high-resolution helioseismology projects from
space provide a tremendous amount of data, the interpretation of which
is increasingly challenging and requires the development of efficient
inversion methods and algorithms.
Title: The Interaction of New and Old Magnetic Fluxes at the Beginning
of Solar Cycle 23
Authors: Benevolenskaya, E. E.; Hoeksema, J. T.; Kosovichev, A. G.;
Scherrer, P. H.
Bibcode: 1999ApJ...517L.163B
Altcode: 1999astro.ph..3404B
The 11 yr cycle of solar activity follows Hale's law by reversing
the magnetic polarity of leading and following sunspots in bipolar
regions during the minima of activity. In the 1996-1997 solar minimum,
most solar activity emerged in narrow longitudinal zones--``active
longitudes'' but over a range in latitude. Investigating the
distribution of solar magnetic flux, we have found that the Hale
sunspot polarity reversal first occurred in these active zones. We have
estimated the rotation rates of the magnetic flux in the active zones
before and after the polarity reversal. Comparing these rotation rates
with the internal rotation inferred by helioseismology, we suggest
that both ``old'' and ``new'' magnetic fluxes were probably generated
in a low-latitude zone near the base of the solar convection zone. The
reversal of active region polarity observed in certain longitudes at the
beginning of a new solar cycle suggests that the phenomenon of active
longitudes may give fundamental information about the mechanism of
the solar cycle. The nonrandom distribution of old-cycle and new-cycle
fluxes presents a challenge for dynamo theories, most of which assume
a uniform longitudinal distribution of solar magnetic fields.
Title: Imaging of Emerging Magnetic Flux by Time-Distance
Helioseismology
Authors: Kosovichev, A. G.; Duvall, T. L., Jr.; Scherrer, P. H.
Bibcode: 1999AAS...194.5901K
Altcode: 1999BAAS...31..917K
We have used measurements of acoustic travel time in the convection
zone to infer local perturbations of the sound speed and 3D flow
velocities associated with emerging active regions in July 1996 and
January 1998. Both regions were observed with the MDI instrument on
SOHO before and after emergence continuously for 9 days. The first
active region emerged in a long-lived complex of activity and produced
a strong X-class flare. The second active region was a high-latitude
region of the new solar cycle. The time-distance inversion results show
complicated dynamics of the magnetic flux in the convection zone, and
indicate that the emerging flux travels faster in the convection zone
than predicted by theory. We discuss the differences in the dynamics
of these active regions.
Title: Solar Internal Rotation as Measured by the SOHO SOI/MDI
Full-Disk Program
Authors: Rhodes, E. J., Jr.; Reiter, J.; Kosovichev, A. G.; Schou,
J.; Scherrer, P. H.
Bibcode: 1999AAS...194.5602R
Altcode: 1999BAAS...31..911R
We present estimates of the solar internal angular velocity obtained
from p-mode frequency splittings computed from observations of the
SOHO SOI/MDI Experiment's Full-Disk Program. Specifically, a time
series of full-disk Dopplergrams which was obtained during the 61-day
long 1996 Dynamics Run of the SOI/MDI Experiment were converted into
time series of spherical harmonic coeffificients for degrees ranging
up to 1000. These time series of spherical harmonic coefficients
were then converted into 1001 sets of zonal, sectoral, and tesseral
power spectra. Estimates of the rotationally-induced p-mode frequency
splittings for every degree between 1 and 1000 were then obtained from
these sets of power spectra through a cross-correlation of the 2l+1
spectra within each set. Because this cross-correlation was carried
out between the frequency limits of 1800 to 4800 mu Hz for each set
of spectra, the resulting splitting coefficients were effectively
averaged over the radial order, n, at each degree. Due to the blending
of individual p-mode spectral peaks for degrees above 200, the raw
frequency splitting coefficients for all degrees between 200 and 1000
had to be corrected before an inversion could be performed. We will
describe the method we adopted for correcting the raw splittings and
we will present both the raw and corrected splitting coefficients. We
will also present a two-dimensional inversion of the corrected
coefficients. Finally, we will also demonstrate how the inclusion of
the high-degree splittings allows us to obtain better estimates of the
solar internal angular velocity in the shallow sub-photospheric part
of the solar convection zone than have been possible in past studies
which included only the splittings of the low- and intermediate-degree
p-modes.
Title: The SOI-MDI Dynamics Program: Observing the Solar Cycle
Authors: Bush, R. I.; Beck, J. G.; Bogart, R. S.; Hoeksema, J. T.;
Kosovichev, A. G.; Scherrer, P. H.; Schou, J.; Sommers, J.; Duvall,
T. L.
Bibcode: 1999AAS...194.9205B
Altcode: 1999BAAS...31..987B
The Michelson Doppler Imager instrument on the SOHO spacecraft has been
observing the Sun over the last three years. The MDI Dynamics Program
provides nearly continuous full disk Doppler measurements of the solar
photosphere with 4 arc-second resolution for periods of 60 to 90 days
each year. Three of these Dynamics periods have been completed: 23 May
to 24 July 1996, 13 April to 14 July 1997, 9 January to 10 April 1998. A
fourth Dynamics observing period began on 13 March and is scheduled to
continue through mid July. These observations provide a unique view
of the evolution of the Sun in the early part of the solar cycle,
both from interior flows deduced by helioseimic analysis and changes
in large scale surface motion. Details of the Dynamics programs will
be presented along with an overview of current results. This research
is supported by the SOI-MDI NASA grant NAG5-3077 at Stanford University.
Title: SONAR - Solar Near-surface Active Region Rendering
Authors: Scherrer, P. H.; Hoeksema, J. T.; Kosovichev, A. G.; Duvall,
T. L.; Schrijver, K. J.; Title, A. M.
Bibcode: 1999AAS...194.7606S
Altcode: 1999BAAS...31Q.957S
The processes in the top 20,000-km of the Sun's convection zone govern
the growth and decay of active regions and provide the magnetic flux and
energy for the active phenomena of the upper solar atmosphere. The MDI
experiment on SOHO has demonstrated that this region is now accessible
to study by means of local helioseismology. However, SOHO provides
neither the temporal nor spatial resolution and coverage necessary
to exploit these techniques to study the eruption and evolution of
active region magnetic structures. The SONAR mission with moderate
resolution full disk Doppler and vector magnetic field observations,
and atmospheric magnetic connectivity observations via EUV imaging
can provide the necessary data. The science motivation and general
instrumentation requirements for the mission are presented.
Title: Three-dimensional simulations of solar oscillations: line
profiles and asymmetries
Authors: Georgobiani, D. G.; Nigam, R.; Kosovichev, A. G.; Stein,
R. F.; Nordlund, A.
Bibcode: 1999AAS...194.5605G
Altcode: 1999BAAS...31..912G
In order to study spectral characteristics of the solar oscillations,
we use the Stein-Nordlund 3d hydrodynamic code to generate lond
temporal sequencies of realistically simulated upper layers of the
solar convective zone. The simulation domain ranges from 0.5 Mm above
the surface of tau =1 to 2.5 Mm below this surface, and is 6 Mm by
6 Mm wide. We have generated 24 hours of solar time. We calculate
power spectra of the vertical velocity and temperature at different
heights and the emergent intensity at the surface. Here, we present the
profiles of velocity, intensity and temperature for both radial (l = 0)
and first nonradial (l = 700) mode. We compare line profiles from the
simulation with the power spectra of the Doppler velocity and continuum
intensity from the SOHO/MDI observations. Both simulated and observed
profiles demonstrate similar types of asymmetry, and the asymmetry
reversal between the local quantities like velocity and temperature, and
emergent intensity profiles is also present in the simulated data. The
preliminary results are promising as they allow us to establish a
connection between the observational data and realistic simulations,
and enable us to understand better the physics of solar oscillations.
Title: New Views of Active Regions
Authors: Duvall, T. L., Jr.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 1999AAS...194.4203D
Altcode: 1999BAAS...31..882D
3-d acoustic tomography of the region below the photosphere is providing
qualitatively new insights about solar active regions. This tomography
is based on the measurement of travel times between different surface
locations and is sensitive to subsurface flows and to wave-speed
inhomogeneities caused by temperature and magnetic field variations. A
flow cell has been seen below sunspots similar to the model of Parker
(Ap.J. 230,905-913,1979) in which there is a horizontal inflow near
the top of the convection zone, a downflow directly below the sunspot
and a horizontal outflow below. This flow may be what stabilizes the
sunspot. A wave-speed reduction is seen in the 2 Mm below the surface
sunspot and a wave-speed enhancement is seen below to at least 10 Mm
depth for a reasonably large spot. At 10 Mm depth, a wave-speed increase
of 3% could be caused by a 10 kG magnetic field or a temperature
excess of 6%. At present we cannot distinguish between temperature
and magnetic field effects on the wave speed, but we will present a
comparison between the wave speed as predicted from a sunspot model
and that measured with the tomography. This research is supported by
NASA contract NAG5-3077 at Stanford University.
Title: Instability of Nonadiabatic Oscillations in a Magnetized
Polytrope
Authors: Birch, A. C.; Kosovichev, A. G.; Spiegel, E. A.; Tao, L.
Bibcode: 1999AAS...194.9307B
Altcode: 1999BAAS...31S.989B
We apply linear stability analysis to a stratified plane parallel
perfectly conducting atmosphere with a vertical magnetic field and
Newton's law of cooling. For the case of a polytropically stratified
atmosphere we find that there are overstable modes. The instability is
due to the background temperature gradient combined with the cooling. We
show perturbation results for the case of the cooling time much longer
or much shorter than the oscillation period. Numerical calculations of
the dispersion relations are shown for the weak and strong magnetic
field cases, for long and short cooling times, and for polytrope
and isothermal atmospheres. Generically the k-omega diagrams show a
complicated pattern of avoided crossings. The damping or growth rates
in general show features where branches undergo avoided crossings. The
results, in particular the existence of overstable modes in the
polytrope atmosphere, may be significant to the investigation of MHD
phenomena in solar and stellar atmospheres, for example solar spicules.
Title: A New Technique for Inversion of Helioseismic Data
Authors: Larsen, R. M.; Kosovichev, A. G.; Schou, J.
Bibcode: 1999AAS...194.5604L
Altcode: 1999BAAS...31..912L
Inversions of rotational frequency splittings derived from helioseismic
data obtained by the MDI instrument and the GONG network have given a
detailed picture of the differential rotation in the convection zone
(Schou et al. 1998). However, features associated with sharp gradients
of the rotation rate such as jets, near surface shear layers (torsional
oscillations) as well as the transition layer to the radiative interior
(the "tachocline") are usually not well resolved. This is due to the
smoothing applied by traditional inversion methods such as Regularized
Least Squares (RLS) and Optimally Localized Averages (OLA). In this work
we show how a generalized version of the method proposed by Thompson
(1990) can used be to study these features by inverting directly
for the radial or latitudinal derivative of the rotation rate. This
research is supported by the SOI-MDI NASA grant NAG5-3077 at Stanford
University. References: G.I. Marchuk, Methods of Numerical Mathematics,
New York, Springer-Verlag, 1975. Schou, J. et al., 1998, Astrophys J.,
505, 390. Thompson, M. J., 1990, Sol. Phys., 125, 1.
Title: The Magnetic Structure of the Sun at the Beginning of Solar
Cycle 23
Authors: Benevolenskaya, E. E.; Hoeksema, J. T.; Kosovichev, A. G.;
Scherrer, P. H.
Bibcode: 1999AAS...194.9201B
Altcode: 1999BAAS...31R.986B
The 11-year cycle of solar activity follows Hale's law by reversing the
magnetic polarity of leading and following sunspots in bipolar regions
during the minima of activity. In the 1996-97 solar minimum, most solar
activity emerged in narrow longitudinal zones - `active longitudes'
but over a range in latitude. Investigating the distribution of solar
magnetic flux, we have found that the Hale sunspot polarity reversal
first occurred in these active zones. We have estimated the rotation
rates of the magnetic flux in the active zones before and after the
polarity reversal. Comparing these rotation rates with the internal
rotation inferred by helioseismology, we suggest that both `old' and
`new' magnetic fluxes were probably generated in a low-latitude zone
near the base of the solar convection zone. The reversal of active
region polarity observed in certain longitudes at the beginning of
a new solar cycle suggests that the phenomenon of active longitudes
may give fundamental information about the mechanism of the solar
cycle. The non-random distribution of old-cycle and new-cycle fluxes
presents a challenge for dynamo theories, most of which assume a
uniform longitudinal distribution of solar magnetic fields. We have
used accurate measurements of solar oscillation frequencies from the
GONG and SOHO/MDI to infer the latitudinal dependence of the solar
structure associated with magnetic fields beneath the surface. The
results show significant variations of the aspherical structure of
the Sun at the beginning of the new cycle. These variations correlate
with the latitudinal distribution of the surface magnetic flux. We
discuss possible variations at the base of the convection zone and
their relation to the dynamo mechanism.
Title: Subsurface Observations of Sunspots and Solar Supergranulation
Authors: Duvall, T. L., Jr.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 1999AAS...194.5606D
Altcode: 1999BAAS...31..912D
3-d acoustic tomography of the region below the photosphere is providing
new insights into sunspots and the apparently convective flow observed
at the surface called supergranulation. The tomography is based on
the measurement of travel times between different surface locations
and is sensitive to subsurface flows and to wave-speed inhomogeneities
caused by temperature and magnetic field variations. This study uses
dopplergrams from the MDI instrument on the SOHO spacecraft. A flow
cell has been seen below sunspots similar to the model of Parker
(Ap.J. 230,905-913,1979) in which there is a horizontal inflow near
the top of the convection zone, a downflow directly below the sunspot
and a horizontal outflow below. The depth of the supergranulation
flow will be discussed. This research is supported by NASA contract
NAG5-3077 at Stanford University.
Title: The Global View of the Solar Interior
Authors: Kosovichev, A. G.
Bibcode: 1999AAS...194.4201K
Altcode: 1999BAAS...31Q.882K
The Michelson Doppler Imager (MDI) instrument on SOHO, GONG and other
space and ground-based helioseismic experiments provide a unique
opportunity for continuous monitoring of the internal structure
and dynamics of the Sun. The new data reveal spatial and temporal
complexity of internal phenomena such as mixing beneath the base
of the convection zone, the subsurface zonal and meridional flows,
sound-speed asphericity, polar vortex and others. The exploration of
these phenomena by helioseismology is important for understanding the
physics of our nearest star. I discuss progress in the diagnostics
of the solar interior, based on 2 years of MDI and 4 years of GONG
observations. Accurately measured frequencies of solar oscillation
modes have provided detailed information about rotation and structure
in the radiative core, the transition region between the convective and
radiative zones and the upper convective boundary layer. Substantial
progress has been made in achieving better latitudinal and radial
resolution in structure and rotation inversions. In addition, new
information about the turbulent convection and internal magnetic
field is obtained from f-mode data. Further progress in helioseismology
crucially depends on understanding the physics of solar oscillations, in
particular, the properties of modal lines and ridges in the oscillation
power spectra.
Title: Source of Solar Acoustic Modes
Authors: Nigam, R.; Kosovichev, A. G.
Bibcode: 1999ApJ...514L..53N
Altcode:
Solar acoustic modes are found to be excited in a thin superadiabatic
layer of turbulent convection (about 75+/-50 km below the photosphere)
beneath the Sun's surface. Comparing the theoretical power spectra
of both velocity and pressure oscillations of medium angular degree
with that obtained from the Michelson Doppler Imager instrument on
board the Solar and Heliospheric Observatory, we find that a composite
source consisting of a monopole, which corresponds to mass or entropy
fluctuations, and a quadrupole, which consists of the Reynolds stress,
excites these oscillations. The dominant source is of a monopole
type since it provides the best match to the observed velocity and
intensity oscillation power spectra. For the above source to match the
observed asymmetry in intensity, a part of the background is found to be
correlated with the pressure perturbation. The sign of the correlation
is found to be negative, which suggests that there is photospheric
darkening prior to the occurrence of the localized acoustic event,
in agreement with the previous finding of P. R. Goode and coworkers.
Title: Computing 3 D Dynamo Models with Observed Solar Data
Authors: Franklin, Joel; Kosovichev, Alexander
Bibcode: 1999soho....9E..57F
Altcode:
We look at finite difference approaches to the three dimensional
solar dynamo problem using differential rotation and diffusivity data
from observation. As a model problem to test the finite difference
formulation and code, we attempt to numerically solve the axisymmetric,
2 D induction equations with alpha-effect.
Title: Phase and Amplitude Difference between Velocity and Intensity
Helioseismic Spectra
Authors: Nigam, R.; Kosovichev, A. G.
Bibcode: 1999ApJ...510L.149N
Altcode:
An explanation for the phase and amplitude difference between velocity
and intensity oscillations of the Sun is provided. The phase difference
along the modal lines in the power spectra was originally observed
by Deubner and coworkers in 1989. From a simple adiabatic theory of
solar oscillations, one expects this phase difference to be 90° for
modes below the acoustic cutoff frequency (bound states) and zero for
modes above the acoustic cutoff frequency (scattered states). But,
surprisingly, from observations, the bound states show a phase
difference that is below 90° along modal lines, and the scattered
states also show a nonzero phase difference. We compute the phase
difference between the velocity and intensity oscillations using
medium angular degree data obtained from the Michelson Doppler Imager
instrument on board the Solar and Heliospheric Observatory and confirm
Deubner's result. We conclude that the unusual phase characteristics
of the solar oscillations can be attributed to the fact that a part of
the background is correlated to the source responsible for exciting
the waves. The idea of the correlated background also explains why
the high-frequency modes above the acoustic cutoff frequency are
stronger in intensity than in the velocity power spectrum relative to
the uncorrelated background, while at frequencies below the acoustic
cutoff the velocity power relative to the uncorrelated background
is stronger compared to the intensity. In addition, this explains
the relative shift of the maxima in the velocity and intensity
high-frequency power spectra.
Title: Helioseismic Diagnostics of Solar Convection and Activity
Authors: Duvall, T. L., Jr.; Kosovichev, A. G.
Bibcode: 1999soho....9E....D
Altcode:
No abstract at ADS
Title: Time-distance helioseismology
Authors: Kosovichev, A. G.; Duvall, T. L.; Scherrer, P. H.
Bibcode: 1999AdSpR..24..163K
Altcode:
The time-distance helioseismology (or helioseismic tomography) is a new
promising method for probing 3-D structures and flows beneath the solar
surface, which is potentially important for studying the birth of active
regions in the sun's interior and for understanding the relation between
the internal dynamics of active regions and chromospheric and coronal
activity. In this method, the time for waves to travel along subsurface
ray paths is determined from the temporal cross correlation of signals
at two separated surface points. By measuring the times for many
pairs of points from Dopplergrams covering the visible hemisphere, a
tremendous quantity of information about the state of the solar interior
is derived. As an example, we present the results for supergranular
flows and for an active region which emerged near the center of the
solar disk in July 1996, and was studied from SOHO/MDI for nine days,
both before and after its emergence at the surface. Initial results
show a complicated structure of the emerging region in the interior,
and suggest that the emerging flux ropes travel very quickly through
the depth range of our observations.
Title: The Effect of Line-of-Sight Projection on Travel Time
Measurements
Authors: Birch, A. C.; Kosovichev, A. G.
Bibcode: 1999soho....9E..44B
Altcode:
We calculate the theoretical cross-correlation between line-of-sight
velocity signals derived from a spherically symmetric model
sun. The vertical and horizontal velocity eigenfunctions combined
with line-of-sight projection are included in the model. We show
that the travel time between two points depends not only on their
separation but also on their positions on the disk, as a result of
the correlation between the projections of the horizontal and radial
velocities. Variations of up to a minute in the travel time can occur
at separations on order of the solar radius. This apparent travel time
perturbation may be important in time-distance investigations of the
structure of the base of the convection zone and other problems. This
research is supported by NASA grant NAG5-3077 at Stanford University.
Title: Solar P-Mode Spectrum Asymmetries: Testing Theories With
Numerical Simulations
Authors: Georgobiani, Dali; Nigam, Rakesh; Kosovichev, Alexander G.;
Stein, Robert F.
Bibcode: 1999soho....9E..58G
Altcode:
We use a 36 hour sequence of 3-D hydrodynamic simulations of solar
convection to study the line profiles of the acoustic modes and their
asymmetries. We construct power spectra of the emergent intensity
and the vertical velocity at a fixed height of 200 km above the t = 1
surface, as well as their phase differences. We compare the synthetic
results with those obtained from the SOHO/MDI observations. The
simulations and observations show similar direction of asymmetry
and reversal of asymmetry between the velocity and intensity. Our
preliminary results confirm the theoretical model of Nigam (Nigam et
al. 1998). To make the simulation results more realistic, the intensity
and velocity will in future be obtained from the synthetic NiI 6768
line used in the observations.
Title: Helioseismic Studies of the Solar Tachocline
Authors: Larsen, R. M.; Kosovichev, A. G.; Schou, J.
Bibcode: 1999soho....9E..69L
Altcode:
Accurate determinations of the depth and width of the solar tachocline
provide important observational constraints on theoretical models of
solar convection and the solar dynamo, which drives the Sun's magnetic
field. We present the tachocline parameters obtained from inversions
of frequency splittings from MDI, GONG and LOWL and compare this with
previously published results. Finally we invert frequency splittings
for individual GONG months and a number of 72 days sets from MDI, in
order to see if any variation can be seen in the tachocline parameters
with the solar cycle. Briefly, our method consists in inverting
the frequency splittings directly for the radial derivative of the
solar rotation rate using a full 2D SOLA technique. Subsequently the
radial derivative of a parametric model of the rotation profile in
the tachocline is convolved with the averaging kernels from the SOLA
inversion, and the parameters in the models are adjusted to obtain a
chi-squared fit of the model to the inversion result. This research
is supported by NASA grant NAG5-3077 at Stanford University.
Title: Time-Distance Inversion Methods and Results
Authors: Kosovichev, A. G.
Bibcode: 1999soho....9E..20K
Altcode:
The current interpretations of the travel-time measurements in quiet
and active regions on the Sun will be discussed. These interpretations
are based on various approximations to the 3D wave equation such as the
Fermat principle for acoustic rays and the Born approximation. The ray
approximation and its modifications have provided the first view of the
3-D structures and flows in the solar interior. However, more accurate
and computationally efficient approximations describing the relation
between the wave travel times and the internal properties are required
to study the structures and flows in detail. Inversion of the large
three-dimensional datasets can be efficiently carried out by regularized
iterative methods. The results of time-distance inversions for emerging
active regions, sunspots and supergranulation will be presented. This
research is supported by NASA contract NAG5-3077 at Stanford University.
Title: Latitudinal Variation of Solar Subsurface Rotation
Authors: Kosovichev, A. G.; Birch, A. C.
Bibcode: 1998AAS...19310004K
Altcode: 1998BAAS...30Q1397K
Analysis of p-mode frequency splittings as measured by the Solar
Oscillations Investigation-Michelson Doppler Imager (SOI-MDI) on board
the Solar and Heliospheric Observatory (SOHO) and the ground-based
Global Oscillations Network Group (GONG) experiment reveals that the
symmetric component of the solar rotation rate, radially averaged
over the upper 28 Mm of the convection zone and closer than 15 deg
to the poles, is roughly 310 nHz (corresponding to a period of 37.3
days), which is slower than the 320 nHz (corresponding to a period
of 36.2 days) estimated from surface measurements. The slow polar
rotation is sometimes interpreted as a polar vortex and is important
for understanding solar dynamics. In addition, zonal flows, previously
inferred from the SOI-MDI f-mode splittings, are seen with both SOI-MDI
and GONG p-mode splittings. The GONG data provide strong support for
the findings of Schou and colleagues. We discuss possible physical
mechanisms for the slow rotation of the polar regions.
Title: A Nonlinear Model for Solar Spicules
Authors: Birch, A. C.; Spiegel, E. A.; Tao, L.; Kosovichev, A. G.
Bibcode: 1998AAS...19310001B
Altcode: 1998BAAS...30.1397B
We develop a weakly nonlinear theory for the growth of optically thin
perturbations to a plane-parallel chromosphere-like layer described
by the MHD equations in an attempt to explain the dynamics of solar
spicules. Damping of the perturbations is by Newton's law of cooling
and the magnetic field is assumed to be vertical. The theory leads to
a nonlinear PDE that describes the time evolution of perturbations to
the layer. Numerical results are presented.
Title: Solar Cycle Onset Seen in SOHO Michelson Doppler Imager
Seismic Data
Authors: Dziembowski, W. A.; Goode, P. R.; di Mauro, M. P.; Kosovichev,
A. G.; Schou, J.
Bibcode: 1998ApJ...509..456D
Altcode:
We have analyzed time changes in centroid frequencies and multiplet
frequency splittings of solar oscillations determined with the Michelson
Doppler Imager instrument (MDI) on SOHO. The data were divided into
five consecutive 72 day sets covering the period from 1996 May 1
through 1997 April 25. We have detected a significant trend in the
a4 and a6 frequency splitting coefficients,
which reflects a decrease in the P4 distortion (described by
the fourth-degree Legendre polynomial of colatitude) and an increase
in the P6 distortion. The rise of the latter distortion
seems to coincide precisely with the rise in the number of new cycle
sunspots. Such sharp and detailed clues to activity onset are new and do
not exist in splitting data from the rising phase of the last cycle. The
relative differences among the solar radii inferred from the f-mode
frequencies from the five sets (at most 6 × 10-6 or 4 km)
are formally significant, reaching a minimum during the observed period.
Title: The Source of Solar Oscillations
Authors: Nigam, R.; Kosovichev, A. G.
Bibcode: 1998AAS...19310002N
Altcode: 1998BAAS...30.1397N
In this study the role of line asymmetry and phase difference between
velocity and intensity helioseismic spectra for understanding the
excitation of solar oscillations is discussed. The solar intensity
and velocity oscillations are usually observed from variations in
an absorption line. These variations consist of two parts: solar
oscillation modes and granulation noise. Because the oscillation
modes are excited by granulation, we argue that the granulation signal
(noise) is partially correlated with the oscillations. The data from
the Michelson Doppler Imager (MDI) instrument on board the Solar and
Heliospheric Observatory (SOHO) have clearly revealed a reversal of
asymmetry between velocity and intensity power spectra. We have shown
that the cause of reversal in asymmetry between velocity and intensity
power spectra is due to the presence of the correlated noise in the
intensity data. This noise is also responsible for the high-frequency
shift in the two spectra at and above the acoustic cutoff frequency. Our
theory also explains the deviation of the observed phase difference
between velocity and intensity from that predicted by simple adiabatic
theory of solar oscillations. The observed phase, jumps in the vicinity
of an eigenfrequency, but theory does not explain such jumps. We
studied different types of excitation sources at various depths and
found that monopole and quadrupole acoustic sources when placed in the
superadiabatic layer (at a depth of 75 km below the photosphere) match
the observations. For these source types, the sign of the correlation is
negative corresponding to photospheric darkening. Finally, an asymmetric
fitting formula is used to determine the eigenfrequencies of solar
oscillations by fitting both the velocity and intensity power spectra.
Title: Asymmetry and Frequencies of Low-Degree p-Modes and the
Structure of the Sun's Core
Authors: Toutain, T.; Appourchaux, T.; Fröhlich, C.; Kosovichev,
A. G.; Nigam, R.; Scherrer, P. H.
Bibcode: 1998ApJ...506L.147T
Altcode:
An accurate determination of the frequencies of low-degree solar
p-modes is an important task of helioseismology. Using 679 days of
solar oscillation data observed in Doppler velocity and continuum
intensity from two Solar and Heliospheric Observatory instruments
(the Michelson Doppler Imager and the SunPhotoMeter), we show that
fitting the spectra with Lorentzian profiles leads to systematic
differences between intensity and velocity frequencies as large as
0.1 μHz for angular degrees l=0, 1, and 2 because of the opposite
asymmetry between intensity and velocity. We use a physics-based
asymmetrical line shape to fit p-mode lines, and we demonstrate
that their asymmetry is statistically significant and that frequency
differences are considerably reduced. These measurements provide more
accurate estimates of the solar eigenfrequencies. We discuss inferences
of the structure of the solar core.
Title: Helioseismic Studies of Differential Rotation in the Solar
Envelope by the Solar Oscillations Investigation Using the Michelson
Doppler Imager
Authors: Schou, J.; Antia, H. M.; Basu, S.; Bogart, R. S.; Bush,
R. I.; Chitre, S. M.; Christensen-Dalsgaard, J.; Di Mauro, M. P.;
Dziembowski, W. A.; Eff-Darwich, A.; Gough, D. O.; Haber, D. A.;
Hoeksema, J. T.; Howe, R.; Korzennik, S. G.; Kosovichev, A. G.;
Larsen, R. M.; Pijpers, F. P.; Scherrer, P. H.; Sekii, T.; Tarbell,
T. D.; Title, A. M.; Thompson, M. J.; Toomre, J.
Bibcode: 1998ApJ...505..390S
Altcode:
The splitting of the frequencies of the global resonant acoustic modes
of the Sun by large-scale flows and rotation permits study of the
variation of angular velocity Ω with both radius and latitude within
the turbulent convection zone and the deeper radiative interior. The
nearly uninterrupted Doppler imaging observations, provided by the
Solar Oscillations Investigation (SOI) using the Michelson Doppler
Imager (MDI) on the Solar and Heliospheric Observatory (SOHO) spacecraft
positioned at the L1 Lagrangian point in continuous sunlight,
yield oscillation power spectra with very high signal-to-noise ratios
that allow frequency splittings to be determined with exceptional
accuracy. This paper reports on joint helioseismic analyses of
solar rotation in the convection zone and in the outer part of the
radiative core. Inversions have been obtained for a medium-l mode set
(involving modes of angular degree l extending to about 250) obtained
from the first 144 day interval of SOI-MDI observations in 1996. Drawing
inferences about the solar internal rotation from the splitting data
is a subtle process. By applying more than one inversion technique
to the data, we get some indication of what are the more robust
and less robust features of our inversion solutions. Here we have
used seven different inversion methods. To test the reliability and
sensitivity of these methods, we have performed a set of controlled
experiments utilizing artificial data. This gives us some confidence
in the inferences we can draw from the real solar data. The inversions
of SOI-MDI data have confirmed that the decrease of Ω with latitude
seen at the surface extends with little radial variation through much
of the convection zone, at the base of which is an adjustment layer,
called the tachocline, leading to nearly uniform rotation deeper
in the radiative interior. A prominent rotational shearing layer in
which Ω increases just below the surface is discernible at low to
mid latitudes. Using the new data, we have also been able to study the
solar rotation closer to the poles than has been achieved in previous
investigations. The data have revealed that the angular velocity
is distinctly lower at high latitudes than the values previously
extrapolated from measurements at lower latitudes based on surface
Doppler observations and helioseismology. Furthermore, we have found
some evidence near latitudes of 75° of a submerged polar jet which
is rotating more rapidly than its immediate surroundings. Superposed
on the relatively smooth latitudinal variation in Ω are alternating
zonal bands of slightly faster and slower rotation, each extending
some 10° to 15° in latitude. These relatively weak banded flows
have been followed by inversion to a depth of about 5% of the solar
radius and appear to coincide with the evolving pattern of ``torsional
oscillations'' reported from earlier surface Doppler studies.
Title: Random Damping and Frequency Reduction of the Solar F Mode
Authors: Duvall, T. L., Jr.; Kosovichev, A. G.; Murawski, K.
Bibcode: 1998ApJ...505L..55D
Altcode:
We present observations showing that the frequency of the high-degree
f-mode is significantly lower than the frequency given by the simple
dispersion relation, ω2=gk , and that the line width grows
with the wavenumber k. We attempt to explain that this behavior is
the result of the interaction with granulation, which we model as a
random flow. Because the f-mode spends more time propagating against
the flow than with the flow, its effective speed and, consequently,
frequency are reduced. Additionally, an eddy viscosity introduces the
negative imaginary part of frequency. This negative imaginary part
represents the damping of the coherent field due to scattering. The
line width is proportional to the magnitude of the imaginary part
of the frequency. We apply an analytical perturbation technique and
numerical methods to estimate the line width and the frequency shift,
and we show that the results are consistent with the properties of the
f-mode obtained from the high-resolution Michelson Doppler Imager data
from the Solar and Heliospheric Observatory.
Title: Measuring the Sun's Eigenfrequencies from Velocity and
Intensity Helioseismic Spectra: Asymmetrical Line Profile-fitting
Formula
Authors: Nigam, R.; Kosovichev, A. G.
Bibcode: 1998ApJ...505L..51N
Altcode:
Solar eigenfrequencies are generally determined by fitting a Lorentzian
to the spectral lines in the power spectrum. This assumes that the
spectral line is symmetric. Recent observations from the Michelson
Doppler Imager (MDI) on board the Solar and Heliospheric Observatory
have indicated that the power spectra of p-modes show varying amounts
of asymmetry. Line asymmetry is an intrinsic property of solar
oscillations and depends on the properties of the excitation source
and the background noise correlated with the oscillations. Neglecting
asymmetry leads to systematic errors in the determination of frequencies
and thus affects the results of inversions. In this Letter, we use a
simple physical model to derive a new fitting formula that incorporates
the effects of asymmetry. It is then tested on artificial and real
solar MDI data. A comparison of the results of a symmetric fit with
those of an asymmetric one shows that there is a systematic shift in
the eigenfrequencies. Our formula will yield more accurate estimates
of the solar eigenfrequencies, which is important for improving the
accuracy of helioseismic inversions.
Title: Latitudinal Variation of Solar Subsurface Rotation Inferred
from p-Mode Frequency Splittings Measured with SOI-MDI and GONG
Authors: Birch, A. C.; Kosovichev, A. G.
Bibcode: 1998ApJ...503L.187B
Altcode:
Analysis of p-mode frequency splittings as measured by the Solar
Oscillations Investigation-Michelson Doppler Imager (SOI-MDI) on
board the Solar and Heliospheric Observatory and the ground-based
Global Oscillations Network Group (GONG) experiment reveal that
the symmetric component of the solar rotation rate in the upper 4%
by radius and closer than 20° to the poles is approximately 10 nHz
slower than would be expected from a three-term fit, of the form
traditionally used to express the main components of latitudinal
differential rotation, to the inferred subsurface rotation rate. The
slow polar rotation is important for understanding solar dynamics. In
addition, zonal flows, previously inferred from the SOI-MDI f-mode
splittings by Kosovichev & Schou, are seen with both SOI-MDI and
GONG p-mode frequency splittings. These results were obtained with a
one-dimensional latitudinal inversion technique that provides better
angular resolution near the poles, at the expense of decreased radial
resolution, than the standard two-dimensional inversions. Both of
these findings confirm and extend the findings of Schou and colleagues
(1998) from SOI-MDI data. The agreement found between the different
helioseismic experiments, SOI-MDI and GONG, gives confidence in the
current inferences of the differential rotation in the Sun's subsurface
layers. The physics of the slow polar rotation is not understood yet.
Title: The Adiabatic Exponent in the Solar Core
Authors: Elliott, J. R.; Kosovichev, A. G.
Bibcode: 1998ApJ...500L.199E
Altcode:
We present helioseismic inversions of solar p-mode frequencies
obtained by the Michelson Doppler Imager instrument of the Solar
Oscillation Investigation project on board SOHO, which indicate a
lower central value for the adiabatic exponent Γ1 in the
Sun than in current models. We show how the inclusion of relativistic
effects in the equation of state removes the discrepancy and allows
almost perfect agreement in Γ1 between the models and
the Sun. This increases our confidence both in the reliability of the
equation-of-state calculation and the determination of solar p-mode
frequencies.
Title: X-ray flare sparks quake inside Sun
Authors: Kosovichev, A. G.; Zharkova, V. V.
Bibcode: 1998Natur.393..317K
Altcode:
Solar flares involve a release of the Sun's magnetic energy as
X-radiation, particle beams and high-speed plasma flows. But
we have discovered, using data from the Solar and Heliospheric
Observatory (SOHO), that these flares also affect the Sun's interior,
generating seismic waves similar to earthquakes. For example, a
three-kilometre-high seismic wave was caused by a moderate X-ray flare
that occurred on 9 July 1996 and propagated at about 50 kilometres
per second to a distance 120,000 kilometres from the flare site.
Title: Initial High-Degree p-Mode Frequencies and Rotational Frequency
Splittings from the SOHO SOI/MDI Experiment
Authors: Rhodes, E. J., Jr.; Reiter, J.; Kosovichev, A. G.; Schou,
J.; Scherrer, P. H.
Bibcode: 1998AAS...192.1901R
Altcode: 1998BAAS...30..845R
We present the first high-degree p-mode frequencies and
rotationally-induced frequency splittings obtained from the Full-Disk
Program of the SOHO Solar Oscillation Investigation/Michelson
Doppler Imager experiment. The frequencies and splittings which we
present here were computed from power spectra obtained during the
1996 SOI/MDI Dynamics Run. Specifically, a 60.75-day time series of
full-disk Dopplergrams was converted into sets of zonal, tesseral,
and sectoral power spectra covering the degree range of 0 through
1000. Estimates of the n-averaged frequency splittings were computed
for the frequency range of 1800 to 4800 microhertz at each degree and
these averaged splitting coefficients were then employed to compute an
average power spectrum for that degree. From these 1001 average power
spectra estimates were made of the frequencies, frequency uncertainties,
widths, peak power densities, and background power densities of a total
of 13664 separate peaks in the set of 1001 average power spectra. A
total of 2554 of these peaks were isolated enough in their respective
spectra to be fit as single p-modes. However, for the remaining 11110
peaks (mostly those above l =200), the individual p-mode peaks and
their spatial sidelobes were located so close together in frequency
that they appeared as ridges rather than as isolated modal peaks in the
average power spectra. For these p-mode ridges we obtained so-called
``ridge-fit'' parameter estimates. Observed asymmetries in the p-mode
ridge shapes altered the fitted peak frequencies from their ``true''
values and required that we correct the raw ridge-fit frequencies. Forty
sets of these power spectra were also processed to yield estimates of
the rotational splitting coefficients for individual p-mode ridges
for every 25th degree between l =25 and 1000. For l between 25
and 175 we will compare these Full-Disk program splittings with the
previously-published splittings from the 1996 SOI/MDI Medium-l Program
(Kosovichev et al., Solar Physics, 170, 43-61,1997).
Title: Asymmetry in Velocity and Intensity Helioseismic Spectra:
A Solution to a Long-standing Puzzle
Authors: Nigam, R.; Kosovichev, A. G.; Scherrer, P. H.; Schou, J.
Bibcode: 1998ApJ...495L.115N
Altcode:
We give an explanation for the opposite sense of asymmetry of the
solar acoustic mode lines in velocity and intensity oscillation
power spectra, thereby solving the half-decade-old puzzle of Duvall
and coworkers. The solution came after comparing the velocity and
intensity oscillation data of medium angular degree l obtained from the
Michelson Doppler Imager instrument on board the Solar and Heliospheric
Observatory with the theoretical power spectra. We conclude that the
solar noise in the velocity and intensity spectra is made up of two
components: one is correlated to the source that is responsible for
driving the solar p-modes, and the other is an additive uncorrelated
background. The correlated component of the noise affects the line
profiles. The asymmetry of the intensity spectrum is reversed because
the correlated component is of a sufficiently large level, while the
asymmetry of the velocity spectrum remains unreversed because the
correlated component is smaller. This also explains the high-frequency
shift between velocity and intensity at and above the acoustic cutoff
frequency. A composite source consisting of a monopole term (mass term)
and a dipole term (force due to Reynolds stress) is found to explain
the observed spectra when it is located in the zone of superadiabatic
convection at a depth of 75+/-50 km below the photosphere.
Title: Asymmetry and Fitting of Velocity and Intensity Power Spectra
from SOHO/MDI
Authors: Nigam, R.; Kosovichev, A. G.
Bibcode: 1998ESASP.418..945N
Altcode: 1998soho....6..945N
The line profiles of solar modes show marked asymmetry at frequencies
less than the acoustic cut-off frequency. Observations from the
Michelson Doppler Imager instrument on board the Solar and Heliospheric
Observatory have revealed a reversal of asymmetry between velocity
and intensity power spectra of medium angular degree. We have
argued that the cause of reversal in asymmetry between velocity and
intensity power spectra is due to the presence of correlated noise,
whose level happens to be more in the intensity data, hence reverses
its asymmetry (Nigam et al., 1998). The correlated noise is also
responsible for the high-frequency shift in the two spectra at and
above the acoustic cut-off frequency. It is found that the asymmetry
depends on the type and depth of the source that excites the solar
acoustic modes. By studying line asymmetry an insight into the
physics of excitation of solar oscillations can be gained. Finally,
a fitting formula incorporating line asymmetry is developed. This
is used to simultaneously fit the two spectra. For the theoretical
spectra, the fits yield the same fitted frequency, which is close
to the eigenfrequency computed from the solar model. The frequency
corrections will have an impact on the inversions.
Title: Observation of seismic effects of solar flares from the SOHO
Michelson Doppler Imager
Authors: Kosovichev, A. G.; Zharkova, V. V.
Bibcode: 1998IAUS..185..191K
Altcode:
Solar flares are the most strong seismic localized disturbances on
the solar surface. During the impulsive phase a high-energy electron
beam heats the chromosphere, resulting in explosive evaporation of
chromospheric plasma at supersonic velocities. This upward motion is
balanced by recoil of the lower part of the chromosphere downward into
the Sun that excites propagating waves in the solar interior. We report
on first observations of the seismic effect of solar flares from the
Michelson Doppler Imager (MDI) Dynamics data, and compare the results
with theoretical models. Observations of seismic response to solar
flares provide important information about the flare mechanism and
about the subphotospheric structure of active regions.
Title: The Comparison of Simultaneous SOI/MDI and Mt. Wilson 60-foot
Tower Power Spectra and p-Mode Parameters
Authors: Rhodes, E. J., Jr.; Reiter, J.; Kosovichev, A. G.; Schou,
J.; Scherrer, P. H.; Rose, P. J.; Irish, S.; Jones, A. R.
Bibcode: 1998ESASP.418..311R
Altcode: 1998soho....6..311R
We present the results of the first detailed comparison between the
1996 SOHO SOI/MDI Dynamics Run program of full-disk Ni I Dopplergram
observations and a simultaneous time series of ground-based
observations obtained in the Na D lines at the 60-Foot Solar Tower
of the Mt. Wilson Observatory (MWO). Specifically, we will compare
sets of simultaneously-observed SOHO/MDI and MWO power spectra and
the high-degree p-mode frequencies, frequency splittings, widths, and
power densities which we obtained by fitting these two sets of power
spectra. Beginning on May 23, 1996, the SOI/MDI experiment began its
first high duty cycle run of 1024x1024 pixel images. this was the
1996 Dynamics Run. On all but three of the days of this 60.75-day
time series a second time series of simultaneous 1024x1024 pixel
full-disk Dopplergrams was obtained at MWO. From these simultaneous MWO
observations we have computed 601 sets of zonal, tesseral, and sectoral
power spectra which covered the degree range of 0 to 600. These sets
of power spectra were then analyzed in two different ways to yield
both frequencies and frequency splittings. First, estimates of the
frequency splittings were computed for the frequency range of 1800 to
4800 microhertz at each degree and these n-averaged splittings were
employed to compute an average power spectrum for that degree. Estimates
were then made of the frequencies, frequency uncertainties, widths, peak
power densities, and background power densities of the set of peaks in
these 601 average power spectra. As is described in our companion paper
on the MDI ridge-fit frequency measurements (Rhodes et al.,1998), we
also had to correct our raw MWO ridge-fit frequencies for the effects
the merger of individual p-mode peaks and sidelobes into ridges. We
could also directly compare the frequency dependence of the observed
power density in both the MDI and MWO power spectra. We have found
that there is a systematic difference such that the power density in
the chromospheric-level power spectra from MWO tends to be below that
of the photosheric-level MDI spectra at low frequencies and to rise
above the MDI power density as the frequency increases. All but the l
= 0 MWO power spectra were also processed to yield estimates of the
rotational splitting coefficients for individual p-mode ridges for
every degree between l = 4 and 600. We will compare these splittings
with both the previously-published splittings from the 1996 SOI/MDI
Medium-l Program (Kosovichev et al., 1997) and with the MDI high-degree
splittings presented in our companion MDI paper.
Title: Line Asymmetry of VIRGO and MDI Low-Degree p Modes
Authors: Toutain, T.; Appourchaux, T.; Frohlich, C.; Kosovichev, A.;
Rakesh, N.; Scherrer, P.
Bibcode: 1998ESASP.418..973T
Altcode: 1998soho....6..973T
Using continuous time series of 610 days of velocity (MDI, LOI-proxy)
and intensity (VIRGO, SPM and LOI) we show that Lorentzian profiles as
a model of low-degree p-mode line leads to systematic differences in
the determination of intensity and velocity mode frequencies. These
differences, as large as 0.1 muHz for degrees l = 0, 1, 2 and 3,
are frequency-dependent. The use of a physics-based asymmetrical line
shape (Rakesh et al., 1998) to fit the same lines has allowed us to
significantly reduce differences in the frequency determination. P-mode
lines in velocity exhibit a significant negative asymmetry (excess
of power in the left wing) whereas p-modes lines in intensity have a
positive asymmetry (excess of power in the right wing). The magnitude
and sign of this asymmetry are directly related to the location of
the source of p-mode excitation and to the correlation between mode
and solar noise.
Title: Comparison of SOHO-SOI/MDI and GONG Spectra
Authors: Komm, R. W.; Anderson, E.; Hill, F.; Howe, R.; Kosovichev,
A. G.; Scherrer, P. H.; Schou, J.; Fodor, I.; Stark, P.
Bibcode: 1998ESASP.418..253K
Altcode: 1998soho....6..253K
We compare solar p-mode parameters, such as central frequency, width,
and amplitude, derived from GONG and SOHO-SOI/MDI Medium-l Program
time series obtained during the same time period. With the excellent
data available now from GONG and SOHO-SOI/MDI, there exist data
sets long enough to make such a comparison useful. For this study,
we have chosen time series of three ell values (ell = 30, 65, and 100)
corresponding to GONG month 16 (Oct 28 -- Dec 2, 1996). For each time
series, we calculated multitaper power spectra using generalized
sine tapers to reduce the influence of the gap structure, which is
different for the two data sets. Then, we applied the GONG peakfitting
algorithm to the spectra to derive mode parameters and selected `good'
fits common to both MDI and GONG spectra, according to three selection
criteria. Preliminary results show that mode frequencies determined
from MDI spectra are essentially the same as the frequencies from
GONG spectra and that the difference is, in general, well within one
formal error bar. The background slope at frequencies above 5mHz is
different between MDI and GONG spectra depending on ell. At present,
we are analyzing 3-month time series of ell = 0 to ell = 150. We intend
to present the results of the on-going comparison.
Title: Damping and Frequency Shift of the Solar f-mode Due to the
Interaction with Turbulent Convection
Authors: Murawski, K.; Duvall, T. L., Jr.; Kosovichev, A. G.
Bibcode: 1998ESASP.418..825M
Altcode: 1998soho....6..825M
Observations indicate that the frequency of the high-degree f-mode
is substantially smaller that the frequency given by the simple
dispersion relation, ω2 = gk, and that the line width
grows with the wavenumber k. We attempt to explain this behaviour as
a result of interaction with granulation which we model as a random
flow. Because of buffeting from the random flow the f-mode wave
speed and consequently frequency are reduced. Additionally, a random
flow makes an appearance of the negative imaginary part of frequency,
ωi. This negative complex part represents the damping of the
mean field, i. e. the generation of random field at the expense of the
mean field energy. The line width is proportional to the magnitude of
the imaginary part of the frequency. We apply an analytical perturbation
technique and numerical methods to estimate the imaginary part and
the frequency shift, and show that the results are consistent with
the properties of the f-mode obtained from the high-resolution MDI data.
Title: Initial SOI/MDI High-Degree Frequencies and Frequency
Splittings
Authors: Rhodes, E. J., Jr.; Reiter, J.; Kosovichev, A. G.; Schou,
J.; Scherrer, P. H.
Bibcode: 1998ESASP.418...73R
Altcode: 1998soho....6...73R
We present the first high-degree p-mode frequencies and frequency
splittings obtained from the Full-Disk Program of the SOHO Solar
Oscillation Investigation/Michelson Doppler Imager experiment. The
frequencies and splittings which we present here were computed
from power spectra obtained during the 1996 SOI/MDI Dynamics
Run. Specifically, a time series of full-disk Dopplergrams, which
began on May 23, 1996, and which covered 87480 minutes (60.75 days),
was converted into sets of zonal, tesseral, and sectoral power spectra
covering the degree range of 0 through 1000. These sets of power spectra
were then analyzed in two different ways to yield both frequencies and
frequency splittings. First, estimates of the frequency splittings
were computed for the frequency range of 1800 to 4800 microhertz at
each degree and these n-averaged splittings were employed to compute an
average power spectrum for that degree. Estimates were then made of the
frequencies, frequency uncertainties, widths, peak power densities,
and background power densities of a total of 13664 separate peaks
in these 1001 average power spectra. A total of 2554 of these peaks
were isolated enough in their respective spectra to be fit as single
p-modes. However, for the remaining 11110 peaks (mostly those above l
= 200), the individual p-mode peaks and their spatial sidelobes were
located so close together in frequency that they appeared as ridges
rather than as isolated modal peaks in the average power spectra. For
these cases we were forced to employ a wider fitting range for our
frequency-estimation code and in so doing we obtained so-called
``ridge-fit'' parameter estimates. Due to a degree-dependence in
the measured velocity power density, the observed p-mode ridges were
asymmetric in shape. These asymmetries in the p-mode ridge shapes,
which are in addition to the intinsic asymmetries caused by the
excitation mechanism of the p-modes themselves, alter the fitted peak
frequencies from their ``true'' values. For this reason, we had to
develop a mechanism which we then used to correct the raw ridge-fit
frequencies. Forty sets of these power spectra were also processed to
yield estimates of the rotational splitting coefficients for individual
p-mode ridges for every 25th degree between l = 25 and 1000. For l
between 25 and 175 we will compare these Full-Disk program splittings
with the previously-published splittings from the 1996 SOI/MDI Medium-l
Program (Kosovichev et al., Solar Physics, 170, 43-61,1997).
Title: Splittings of MDI Low-Degree p Modes
Authors: Toutain, T.; Kosovichev, A. G.
Bibcode: 1998ESASP.418..349T
Altcode: 1998soho....6..349T
Applying an optimal mask technique (Toutain et al, 1997) to an
uninterruped > time series of 610 days of MDI/LOI-proxy Doppler
velocity data we produce cleaned m- ν diagrams for degrees l = 1,
2, 3. We derive p-mode parameter assuming a diagonal leakage matrix
but taking into account the correlation of noise between different
m-spectra (Schou,J., 1992 and Appourchaux et al., 1997). This fitting
technique allows an unbiased determination of the p-mode parameters
(Appourchaux et al., 1997). We fit the a1 coefficient using
a Lorentzian profile and an asymmetrical profile as described by Rakesh
et al, 1998. The inversion results will be presented at the meeting.
Title: Line asymmetry and excitation mechanism of solar oscillations
Authors: Nigam, R.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 1998IAUS..185..195N
Altcode:
The width and asymmetry of lines in the power spectrum of solar
oscillations, obtained from the Michelson Doppler Imager (MDI) data,
on board the Solar and Heliospheric Observatory (SOHO), are used to
study the physics of excitation and damping of the oscillations. A
theoretical model for solar oscillations is developed. In this model,
the asymmetry is an effect of interference between the trapped waves
from the source that pass through the region of wave propagation in the
Sun's interior. From this the power spectrum is computed for different
values of the source location and for various values of the angular
degree l. It is seen that there is marked line asymmetry below the
acoustic cut-off frequency, which corresponds to the asymmetry of
bound states in quantum mechanics. The asymmetry is reduced above
the acoustic cut-off frequency, which corresponds to the asymmetry
of scattered states, which is a result of interference between an
outward direct wave from the source and corresponding inward untrapped
waves. The asymmetry is found to depend strongly on the source location
and on the value of l. We discuss the properties of the solar acoustic
source inferred from the MDI data.
Title: Solar Asymmetries from SOHO/MDI Splitting Data
Authors: Goode, P. R.; Dziembowski, W. A.; DiMauro, M. P.; Kosovichev,
A. G.; Schou, J.
Bibcode: 1998ESASP.418..887G
Altcode: 1998soho....6..887G
No abstract at ADS
Title: Latitudinal Dependence of the Solar Rotation Rate in the
Upper Convection Zone
Authors: Birch, A. C.; Kosovichev, A. G.
Bibcode: 1998ESASP.418..679B
Altcode: 1998soho....6..679B
Analysis of p-mode frequency splittings as measured by the Solar
Oscillations Investigation-Michelson Doppler Imager (SOI-MDI) on board
the Solar and Heliospheric Observatory (SOHO) and the ground-based
Global Oscillations Network Group (GONG) experiment reveals that the
symmetric component of the solar rotation rate, radially averaged
over the upper 28 Mm of the convection zone and closer than 15-circ
to the poles, is roughly 310 nHz (corresponding to a period of 37.3
days), which is slower than the 320 nHz (corresponding to a period
of 36.2 days) estimated from surface measurements (e.g. Snodgrass,
1984). The slow polar rotation is sometimes interpreted as a polar
vortex and is important for understanding solar dynamics. In addition,
zonal flows, previously inferred from the SOI-MDI f-mode splittings
(Kosovichev and Schou, 1997), are seen with both SOI-MDI and GONG p-mode
splittings. The GONG data provide strong support for the findings of
Schou and colleagues (Schou et al., 1998). We discuss possible physical
mechanisms for the slow rotation of the polar regions.
Title: Improved SOLA Inversions of MDI Data
Authors: Larsen, R. M.; Christensen-Dalsgaard, J.; Kosovichev, A. G.;
Schou, J.
Bibcode: 1998ESASP.418..813L
Altcode: 1998soho....6..813L
We present a new version of 2d-SOLA, where the target functions have
been modified to match the behavior of the mode kernels near the
rotation axis and to minimize near-surface contributions. Inversion
of artificial data show that these modifications significantly improve
the effective resolution near the pole, which allows us to assess the
reliability of the high-latitude features seen by other inversion
methods. Most importantly, our new inversions seem to confirm the
detection of a submerged polar jet previously seen in the 2d-RLS
inversions reported by Schou et al. 1998. A test of the robustness of
the improved method is carried out by inverting artificial data from
the MDI Hare and Hounds exercise. We analyze the averaging kernels and
error propagation of the method, and also describe the error-correlation
between different points in the solution, the latter being a potential
source of spurious features in the solutions as pointed out by Howe
and Thompson, 1996. So far, helioseismic datasets given in the form of
a-coefficients have been inverted under the assumption that the errors
in different a-coefficients are uncorrelated. The MDI peak-bagging
procedure, however, does produce estimates of the error-correlation
between a-coefficients within the same multiplet. Here we investigate
the effect of including this knowledge in the inversions.
Title: Seismic Response to Solar Flares Observed SOHO/MDI
Authors: Zharkova, V. V.; Kosovichev, A. G.
Bibcode: 1998ESASP.418..661Z
Altcode: 1998soho....6..661Z
First observations with the SOHO Michelson Doppler Imager (MDI) of
seismic effects in the solar atmosphere in response to solar flares
are investigated in the complex environment using X-ray (Yohkoh),
white light (LASCO) and magnetographic observations. Solar flares are
found to produce the outgoing circular waves which can be detected on
MDI dopplergrams visually. The time-distance diagrams for the first
three azimuthal components of the flare seismogram are constructed from
1 minute velocity differences. These observations were compared with
the theoretical models of a seismic response to solar flares using the
thick target model with electron beam injection. Some discrepancies
were found as for the energy momentum of flare's shock waves being
able to produce a noticeable seismic response, so for the observed
travel time of seismic waves in comparison with the theoretical
predictions. Possible mechanisms of such discrepancies are discussed.
Title: Tests of Convective Frequency Effects with SOI/MDI High-Degree
Data
Authors: Rosenthal, C. S.; Christensen-Dalsgaard, J.; Kosovichev,
A. G.; Nordlund, A. A.; Reiter, J.; Rhodes, E. J., Jr.; Schou, J.;
Stein, R. F.; Trampedach, R.
Bibcode: 1998ESASP.418..521R
Altcode: 1998astro.ph..7066R; 1998soho....6..521R
Advances in hydrodynamical simulations have provided new insight into
the effects of convection on the frequencies of solar oscillations. As
more accurate observations become available, this may lead to an
improved understanding of the dynamics of convection and the interaction
between convection and pulsation (Rosenthal et al. 1999). Recent
high-resolution observations from the SOI/MDI instrument on the
SOHO spacecraft have provided the so-far most-detailed observations
of high-degree modes of solar oscillations, which are particularly
sensitive to the near-surface properties of the Sun. Here we present
preliminary results of a comparison between these observations and
frequencies computed for models based on realistic simulations of
near-surface convection. Such comparisons may be expected to help
in identifying the causes for the remaining differences between the
observed frequencies and those of solar models.
Title: Spherical and aspherical structure of the sun: First year of
SOHO/MDI observations
Authors: Kosovichev, A. G.; Schou, J.; Scherrer, P. H.; Goode, P. H.;
Dziembowski, W. A.; Rhodes, E. J., Jr.; SOI Structure Inversion Team
Bibcode: 1998IAUS..185..157K
Altcode:
We report the results of one year of continuous observations of the
Sun's internal structure using data from the Medium-l Program of
the Michelson Doppler Imager (MDI) on board SOHO. The data provide
continuous coverage of p modes of angular degree l from 0 to 250,
and the f mode from l=100 to 250. The striking stability of solar
Dopplergrams measured by MDI, without an intervening atmosphere,
substantially decreases the noise in the solar oscillations power
spectrum compared with ground-based observations. This permits
detection of lower amplitude oscillations, extending the range and
precision of measured normal mode frequencies. We present new inversion
results for the radial and latitudinal seismic solar structures with
particular attention to zonal asphericity inferred with the high
angular resolution from the data. Using f-mode frequency splitting
we estimate the large-scale structure of the subsurface magnetic
fields. The variations of the solar structure observed during the
first year of MDI observations are also discussed.
Title: Solar Asymmetries from SOHO/MDI Splitting Data
Authors: Dziembowski, W. A.; Goode, P. R.; Di Mauro, M. P.; Kosovichev,
A. G.; Schou, J.
Bibcode: 1998ESASP.418..887D
Altcode: 1998soho....6..887D
Systematic changes in p-mode frequencies through the solar cycle
have been discovered during the previous high activity phase. Most
significant changes were found in the even-a coefficients of
the fine structure in the oscillation spectra (Kuhn, 1988;
and Libbrecht and Woodard, 1990). We analyzed time changes in
frequencies determined with the SOHO/MDI instrument. The data were
divided into five 72-day sets covering (1) 5/1/96-7/11/96, (2)
7/12/96-9/21/96, (3) 9/22/96-12/2/96, (4) 12/3/96-2/12/97, and (5)
2/13/97-4/25/97. The splitting coefficients ak are defined by
nuvlosell,n,m-bar nuell,n = sum{k = 1}
ak {cal P}kell(m), where {cal P} are
are orthogonal polynomials (see Ritzwoller and Lavely 1991 and Schou,
et al. 1994). We analyzed behavior of the even order coefficients,
a2k, which arise from the respective, P2k
(cos θ), distortion of the Sun's structure. We found a significant
trend in behavior of the a4 and a6 coefficients,
which reflects a decrease of the P4 and an increase of the
P6 distortions. This trend is the same as seen in the BBSO
data (Libbrecht and Woodard, 1990) between 1986 and 1988 i.e. at the
onset of the previous activity phase. The trend in a2 is
not so apparent. The centroid frequencies, bar nuell,n, as
already reported by Kosovichev et al. (1998), exhibit small nonmonotonic
variations. The relative differences in solar radius inferred from the
f-mode frequencies in the five sets (at most 5 times 10-6)
are formally significant, but again there is no trend.
Title: Observational Upper Limits for Low-Degree Solar g-modes
Authors: Fröhlich, C.; Finsterle, W.; Andersen, B.; Appourchaux, T.;
Chaplin, W. J.; Elsworth, Y.; D. O. Gough; Hoeksema, J. T.; Isaak,
G. R.; Kosovichev, A. G.; Provost, J.; Scherrer, P. H.; Sekii, T.;
Toutain, T.
Bibcode: 1998ESASP.418...67F
Altcode: 1998soho....6...67F
No abstract at ADS
Title: Observation of low-degree modes from SOHO/MDI using optimal
masks
Authors: Toutain, T.; Kosovichev, A. G.
Bibcode: 1998IAUS..185..179T
Altcode:
The overlap of peaks of low-degree p-mode split multiplets in
oscillation power spectra significantly affects the measurements of
the rotational frequency splitting, and, thus, contributes to the
uncertainty of the rotation rate of the solar core. We have developed
optimal masks to isolate individual components of the multiplets, and
discuss the advantages of the optimal-mask method over the integrated
disk observations and projection onto spherical harmonics. The method
has been applied to the Michelson Doppler Imager Low-l data, and the
results of the measurements of the mode frequencies, linewidth, line
asymmetry and amplitude are reported.
Title: Relativistic Effects in the Solar Equation of State
Authors: Elliott, J. R.; Kosovichev, A. G.
Bibcode: 1998ESASP.418..453E
Altcode: 1998soho....6..453E
We present helioseismic inversions of solar p-mode frequencies
obtained by the Michelson Doppler Imager (MDI) instrument of the Solar
Oscillation Investigation (SOI) project on board SOHO, indicating that
a lower central value for the adiabatic exponent Γ1 in the
sun than in current models. We show how the inclusion of relativistic
effects in the equation of state removes the discrepancy and allows
almost perfect agreement in Γ1 between the models and
the sun. This increases our confidence both in the reliability of the
equation of state calculation and the determination of solar p-mode
frequencies.
Title: Solar rotation and large-scale flows determined by
time-distance helioseismology MDI
Authors: Giles, P. M.; Duvall, T. L., Jr.; Kosovichev, A. G.
Bibcode: 1998IAUS..185..149G
Altcode:
We use differences of reciprocal travel time of acoustic waves
propagating along parallels and meridians to determine variations of
solar rotation and large-scale flows beneath the Sun's surface. This
new technique can provide an effective measure of the antisymmetric
component of the differential rotation and meridional flows that are not
determined by the traditional helioseismic techniques based on inversion
of frequency splitting of normal modes. We investigate resolution and
precision of the time-distance measurements using the Michelson Doppler
Imager (MDI) Medium-l data. The first results show a clear picture of
the latitudinal differential rotation and its variation with depth. We
discuss preliminary results of the meridional flow measurements.
Title: Some Numerical Simulations of Oscillations in the Solar
Atmosphere
Authors: Andreev, A. S.; Kosovichev, A. G.
Bibcode: 1998ESASP.418...87A
Altcode: 1998soho....6...87A
The new results, obtained by simulations of atmospheric oscillations are
presented. The details of quasi-periodic shock waves, which are produced
by photospheric and subphotospheric oscillations and spread up into the
upper chromosphere and corona, as well as their interaction with the
transition region between chromosphere and corona are considered. The
results show agreement with cooperative studies of the phenomena,
using observations from MDI and SOHO coronal instruments.
Title: Numerical model for coronal shock waves formation in two-fluid
approximation
Authors: Kosovichev, A. G.; Stepanova, T. V.
Bibcode: 1998PAICz..88..167K
Altcode:
No abstract at ADS
Title: Some results of numerical modeling of solar atmospheric
oscillations.
Authors: Andreev, A. S.; Kosovichev, A. G.
Bibcode: 1998BCrAO..94..123A
Altcode:
The nonlinear response of the solar atmosphere to a single
subphotospheric wave disturbance, one part of the energy of which is
spent on exciting internal p-modes and the other generates traveling
oscillations in the chromosphere and corona, is examined. The latter,
reaching the upper chromosphere and corona, are transformed into
quasi-periodic shock waves as a result of the so-called wave wake
effect. Results of one-dimensional calculations of this process on
the basis of observations made during the MDI and SOHO projects are
presented.
Title: Comparative Studies of Low-Order and Low-Degree Solar p Modes
Authors: Appourchaux, T.; Andersen, B.; Chaplin, W.; Elsworth, Y.;
Finsterle, W.; Frohlich, C.; Gough, D.; Hoeksema, J. T.; Isaak, G.;
Kosovichev, A.; Provost, J.; Scherrer, P.; Sekii, T.; Toutain, T.
Bibcode: 1998ESASP.418...95A
Altcode: 1998soho....6...95A
The amplitudes of solar p-modes decrease steeply with decreasing
radial order below about 17. The background solar signal (solar noise)
in general increases steadily with decreasing frequency. For the
irradiance and radiance measurements with VIRGO or SOI/MDI on SOHO this
combination makes it difficult to detect low degree modes below about
1.8 mHz. The solar noise as observed in velocity with SOI/MDI or the
ground based BISON network is significantly lower in this region than
in intensity measurements. This allows low degree modes to be observed
close to 1 mHz. We present results of detection and charaterization
of the lowest order observable p-modes both in velocity and intensity
measurements. Where applicable the properties of the modes observed
with the two methods are compared.
Title: Acoustic tomography of solar convective flows and structures
Authors: Kosovichev, A. G.; Duvall, T. L., Jr.
Bibcode: 1997ASSL..225..241K
Altcode: 1997scor.proc..241K; 2018arXiv180603273K
We present a new method for helioseismic diagnostics of the
three-dimensional structure of sound speed, magnetic fields and flow
velocities in the convection zone by inversion of acoustic travel-time
data. The data are measurements of the time for acoustic waves to
travel between points on the solar surface and surrounding annuli
obtained from continuous observations at the South Pole in 1991 and
from high-resolution observations from the Solar and Heliospheric
Observatory (SOHO) in 1996. The travel time of the waves depends
primarily on the sound speed perturbations and the velocity of flow
along the ray paths. The effects of the sound speed perturbations
and flows can be separated by measuring the travel time of waves
propagating in opposite directions along the same ray paths. Magnetic
fields result in anisotropy of the wave speed. A 3D inversion method
based on Fermat's Principle and a regularized least-squares technique
have been applied to infer the properties of convection in the quiet
Sun and in active regions.
Title: Probing the Internal Structure of the Sun with the SOHO
Michelson Doppler Imager
Authors: Kosovichev, A. G.; Nigam, R.; Scherrer, P. H.; Schou, J.;
Reiter, J.; Rhodes, E. J., Jr.; Toutain, T.
Bibcode: 1997AAS...191.7311K
Altcode: 1997BAAS...29R1322K
The inference of the thermodynamic structure of the Sun from the
observed properties of the solar normal modes of oscillation is a
principal goal of helioseismology. We report the results of the first
year of continuous observations of the Sun's internal structure using
data from the Medium-l Program of the Michelson Doppler Imager (MDI)
on board ESA/NASA spacecraft SOHO. The data provide continuous coverage
of the acoustic (p) modes of angular degree l from 0 to 250, and the
fundamental (f) mode of the Sun from l=100 to 250. During two 2-month
intervals, the high-degree modes, up to l=1000, have been observed. The
great stability of solar Dopplergrams measured by MDI permits detection
of lower amplitude oscillations, extending the range and precision of
measured normal mode frequencies, and thus substantially increasing
the resolution and precision of helioseismic inversions. We present
new inversion results for the radial and latitudinal seismic solar
structures with particular attention to the transition region between
the radiative and convection zones and to the energy-generating core. We
discuss evidence for convective overshoot at the base of the convection
zone, and the significance of deviations in the core structure from
the standard evolutionary model. Comparing the f-mode frequencies
with the corresponding frequencies of the standard solar models, we
argue that the apparent photospheric solar radius (695.99 Mm) used to
calibrate the models should be reduced by approximately 0.3 Mm. The
discrepancy between the `seismic' and apparent photospheric radii is
not explained by the known systematic errors in the helioseismic and
photospheric measurements. If confirmed, this discrepancy represents
a new interesting challenge to theories of solar convection and solar
modeling. Using f-mode frequency splitting we estimate the large-scale
structure of the subsurface magnetic fields. The variations of the solar
oscillation frequencies during the first year of MDI observations are
also discussed.
Title: Rotation and Zonal Flows in the Solar Envelope from the
SOHO/MDI Observations
Authors: Scherrer, P. H.; Schou, J.; Bogart, R. S.; Bush, R. I.;
Hoeksema, J. T.; Kosovichev, A. G.; Antia, H. M.; Chitre, S. M.;
Christensen-Dalsgaard, J.; Larsen, R. M.; Pijpers, F. P.; Eff-Darwich,
A.; Korzennik, S. G.; Gough, D. O.; Sekii, T.; Howe, R.; Tarbell,
T.; Title, A. M.; Thompson, M. J.; Toomre, J.
Bibcode: 1997AAS...191.7310S
Altcode: 1997BAAS...29.1322S
We report on the latest inferences concerning solar differential
rotation that have been drawn from the helioseismic data that are now
available from the Solar Oscillations Investigation (SOI) using the
Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory
(SOHO). That spacecraft is positioned in a halo orbit near the Sun-Earth
Lagrangian point L_1, in order to obtain continuous Doppler-imaged
observations of the sun with high spatial fidelity. Doppler velocity,
intensity and magnetic field images are recorded, based on modulations
of the 676.8 nm Ni I solar absorption line. The high spatial resolution
of MDI thereby permits the study of many millions of global resonant
modes of solar oscillation. Determination and subsequent inversion
of the frequencies of these modes, including the degeneracy-splitting
by the rotation of the sun, enables us to infer how the sun's angular
velocity varies throughout much of the interior. The current MDI data
are providing substantial refinements to the helioseismic deductions
that can be made about differential rotation both within the convection
zone and in its transition to the radiative interior. The shearing
layer evident in the angular velocity Omega just below the solar
surface is becoming better defined, as is the adjustment layer or
tachocline near the base of the convection zone. The MDI data are also
revealing a prominent decrease in Omega at high latitudes from the
rotation rate expressed by a simple three-term expansion in latitude
that was originally deduced from surface Doppler measurements. Further,
there are indications that a submerged polar vortex involving somewhat
faster Omega than its surroundings exists at about 75(deg) in latitudes.
Title: Determination of the Sun's Seismic Radius from the SOHO
Michelson Doppler Imager
Authors: Schou, J.; Kosovichev, A. G.; Goode, P. R.; Dziembowski, W. A.
Bibcode: 1997ApJ...489L.197S
Altcode:
Dopplergrams from the Michelson Doppler Imager (MDI) instrument on board
the SOHO spacecraft have been used to accurately measure frequencies of
the Sun's fundamental (f) mode in the medium angular degree range, l =
88--250. The comparison of these frequencies with the corresponding
frequencies of the standard solar models suggests that the apparent
photospheric solar radius (695.99 Mm) used to calibrate the models
should be reduced by approximately 0.3 Mm. The precise value of the
seismologically determined solar radius depends on the description
of the subsurface layer of superadiabatic convection. The discrepancy
between the "seismic" and apparent photospheric radii is not explained
by the known systematic errors in the helioseismic and photospheric
measurements. If confirmed, this discrepancy represents an interesting
new challenge to theories of solar convection and solar modeling.
Title: Measurements of Frequencies of Solar Oscillations from the
MDI Medium-l Program
Authors: Rhodes, E. J., Jr.; Kosovichev, A. G.; Schou, J.; Scherrer,
P. H.; Reiter, J.
Bibcode: 1997SoPh..175..287R
Altcode:
Inversions of solar internal structure employ both the frequencies
and the associated uncertainties of the solar oscillation modes
as input parameters. In this paper we investigate how systematic
errors in these input parameters may affect the resulting inferences
of the sun's internal structure. Such systematic errors are likely
to arise from inaccuracies in the theoretical models which are used
to represent the spectral lines in the observational power spectra,
from line blending, from asymmetries in the profiles of these lines,
and from other factors. In order to study such systematic effects we
have employed two different duration observing runs (one of 60 days
and the second of 144 days) obtained with the Medium-l Program of the
Michelson Doppler Imager experiment onboard the SOHO spacecraft. This
observing program provides continuous observations of solar oscillation
modes having angular degrees, l, ranging from 0 to ∼ 300. For this
study intermediate- and high-degree p-mode oscillations having degrees
less than 251 were employed.
Title: Tri-Phonic Helioseismology: Comparison of Solar P Modes
Observed by the Helioseismology Instruments Aboard SOHO
Authors: Toutain, T.; Appourchaux, T.; Baudin, F.; Fröhlich, C.;
Gabriel, A.; Scherrer, P.; Andersen, B. N.; Bogart, R.; Bush, R.;
Finsterle, W.; García, R. A.; Grec, G.; Henney, C. J.; Hoeksema,
J. T.; Jiménez, A.; Kosovichev, A.; Roca Cortés, T.; Turck-Chièze,
S.; Ulrich, R.; Wehrli, C.
Bibcode: 1997SoPh..175..311T
Altcode:
The three helioseismology instruments aboard SOHO observe solar p modes
in velocity (GOLF and MDI) and in intensity (VIRGO and MDI). Time series
of two months duration are compared and confirm that the instruments
indeed observe the same Sun to a high degree of precision. Power
spectra of 108 days are compared showing systematic differences between
mode frequencies measured in intensity and in velocity. Data coverage
exceeds 97% for all the instruments during this interval. The weighted
mean differences (V-I) are −0.1 µHz for l=0, and −0.16 µHz for
l=1. The source of this systematic difference may be due to an asymmetry
effect that is stronger for modes seen in intensity. Wavelet analysis
is also used to compare the shape of the forcing functions. In these
data sets nearly all of the variations in mode amplitude are of solar
origin. Some implications for structure inversions are discussed.
Title: Studying the temporal behavior of the Evershed flow from
SOHO/MDI
Authors: Burnette, A. B.; Kosovichev, A. G.
Bibcode: 1997BAAS...29.1120B
Altcode:
No abstract at ADS
Title: Asymptotic Latitudinal Inversion of Frequency Splitting Data
Authors: Birch, A. C.; Kosovichev, A. G.
Bibcode: 1997BAAS...29Q1121B
Altcode:
No abstract at ADS
Title: Detection of Zonal Shear Flows beneath the Sun's Surface from
f-Mode Frequency Splitting
Authors: Kosovichev, A. G.; Schou, J.
Bibcode: 1997ApJ...482L.207K
Altcode:
We report on the first successful measure of the zonal variations of
the Sun's differential rotation (so-called torsional oscillations) by
helioseismology. Using new helioseismic data from the Michelson Doppler
Imager on board SOHO, we have detected zonal flow bands with velocity
variation of 5 m s-1 at a depth of 2-9 Mm beneath the
surface. The subsurface flow is inferred from rotational splitting of
frequencies of the fundamental mode of solar oscillations in the range
of angular degree l from 120 to 250, using a 144 day uninterrupted time
series of the Sun's Doppler velocities. The structure of the subsurface
shear flow resembles the pattern of the torsional oscillations observed
on the surface. Comparing with previous surface measurements, we found
evidence for migration of the flow bands towards the equator.
Title: Solar Meridional Circulation and Rotation Determined by
Time-Distance Helioseismology using MDI Data From SOHO
Authors: Giles, P. M.; Duvall, T. L., Jr.; Kosovichev, A. G.; Scherrer,
P. H.
Bibcode: 1997SPD....28.1002G
Altcode: 1997BAAS...29..914G
Using the technique of time-distance helioseismology, acoustic wave
travel times can be measured between pairs of points on the solar
surface. The travel time of the waves depends primarily on the wave
group velocity and on the component of flow velocity which is parallel
to the direction of wave propagation. By choosing pairs of points which
share a common longitude, it is possible to use these waves to probe
the meridional flow beneath the surface. Any flows present will cause
a difference between the northward and southward travel times along
the meridian. Varying the distance between points allows isolation
of waves which propagate to different depths beneath the surface,
and thus the flow velocity can be measured as a function of latitude
and depth. Similarly, by choosing pairs of points which share a common
latitude it is possible to measure the effects of solar rotation using
an analogous procedure. This technique could provide high resolution
in latitude and allows study of the northern and southern hemispheres
independently. Using velocity images taken by the Michelson Doppler
Imager during June 1996, we have detected meridional flows in the
uppermost layers of the sun. Measurements of this flow velocity
and of the rotation rate as functions of latitude and depth will be
presented. This research is supported by NASA contract NAG5-3077 at
Stanford University.
Title: Analysis of Velocity and Intensity Helioseismic Spectra
from SOHO/MDI
Authors: Nigam, R.; Kosovichev, A. G.; Scherrer, P. H.; Schou, J.
Bibcode: 1997SPD....28.0904N
Altcode: 1997BAAS...29..913N
We give an explanation for the cause of the asymmetry of spectral lines
of solar oscillation power spectrum. We also explain the cause of the
opposite sense of asymmetry in velocity and intensity oscillation power
spectra, thereby resolving a half-decade old puzzle. The motivation for
the investigation came after comparing the velocity and intensity data
obtained from the Michelson Doppler Imager (MDI) instrument on board the
Solar and Heliospheric Observatory (SOHO). The analysis is based on a
theoretical model of wave excitation with viscous damping in conjunction
with a spherically symmetric solar model. Neglecting asymmetry can
lead to systematic errors in the eigenfrequency measurements, which
in turn leads to errors in inversion. This research was supported by
NASA grant NAG5-3077 at Stanford University.
Title: Spherical and Aspherical Structure of the Sun
Authors: Kosovichev, A. G.; SOI Structure Inversion Team
Bibcode: 1997SPD....28.0902K
Altcode: 1997BAAS...29..913K
The striking stability of solar Dopplergrams measured by SOHO,
without an intervening atmosphere, substantially decreases the noise
in the solar oscillations power spectrum compared with ground-based
observations. This permits detection of lower amplitude oscillations,
extending the range and precision of measured normal mode frequencies
and frequency splitting for inferring the internal structure of the
Sun. We report on new inversion results for the radial and latitudinal
seismic solar structures with particular attention to zonal asphericity
inferred with the high angular resolution. The frequency splitting of
the fundamental mode is used to estimate the large-scale structure of
the subsurface magnetic fields. The sound-speed profile inferred from
the mean frequencies of mode multiplets gives evidence for significant
deviations from a standard solar model in the upper convective boundary
layer, in a thin layer just beneath the convection zone and in the
energy-generating core. This research is supported by the SOI-MDI NASA
contract NAG5-3077 at Stanford University.
Title: Performance of the Michelson Doppler Imager Instrument on SOHO
Authors: Scherrer, P.; Bogart, R.; Bush, R.; Duvall, T.; Hoeksema,
J. T.; Kosovichev, A.; Schou, J.; Morrison, M.; Tarbell, T.; Title, A.
Bibcode: 1997SPD....28.0207S
Altcode: 1997BAAS...29..894S
Launched on SOHO in December 1995, the MDI instrument took its 10
millionth filtergram in early April, 1997. The instrument and spacecraft
have performed admirably since commissioning, providing over a year
of virtually uninterrupted time series of velocity and intensity
measurements at moderate resolution, a continuous 60-day time series
of full disk 4" velocity and line depth maps, monthly 72+ hour time
series in various observables, a host of daily 8-hour campaigns, and
full-disk magnetograms every 96 minutes. Another uninterrupted 90-day
interval of nearly full data recovery is scheduled to be completed in
mid July. Various scientific results using MDI data are being presented
at this meeting. About a dozen terabytes of data sets have been created
and archived and normal pipeline processing is now completed soon after
retrieving the data, typically less than a month after the observations
are made. Most of the data products are generally available on the WWW,
see http://soi.stanford.edu. Selected data are available in near real
time. The SOI team welcomes collaborations. Routine and extraordinary
calibrations along with analysis of scientific data sets allow us to
make good estimates of the noise and understand many of the sources
of systematic errors in the instrument. In almost every respect the
instrument performs as well or better than expected before launch,
the primary limitations being photon noise on the short term and
fixed or slowly varying offsets on the long term. We have found that
the Michelsons are somewhat more sensitive to operational temperature
variations than was expected, adding some additional constraints on
our observing sequences.
Title: The Effects of Systematic Errors in the Estimation of p-Mode
Frequencies on the Inversion of Solar Internal Structure
Authors: Rhodes, E. J., Jr.; Appourchaux, T.; Bachmann, K.; Kosovichev,
A. G.; Scherrer, P. H.; Schou, J.; Reiter, J.
Bibcode: 1997SPD....28.0901R
Altcode: 1997BAAS...29..913R
The frequencies and associated uncertainties of the low-, intermdeiate-,
and high-degree p-mode oscillations are the input quantities for the
inversion programs which infer the thermodynamic structure of the solar
interior. In this review we will attempt to demonstrate the different
possible systematic errors that are currently present in our estimation
of both the modal frequencies and their uncertainties. We will also
demonstrate the effects of some of these errors upon the inferred
radial profile of the solar internal sound speed. Among the different
possible systematic errors which we will discuss are the effects of:
1)the asymmetric shapes of the peaks in observational power spectra,
2)the realization noise which is present in the case of the low-degree
modes, 3)the different frequency estimation methods used on different
types of power spectra (i.e., on either tesseral power spectra or
on m-averaged power spectra), 4) the differences in the frequencies
which are estimated from velocity- and intensity-based power spectra,
5) the blending of individual p-modes into so-called "ridges" of
observed power at both high degrees and high frequencies, and 6) the
spatial and temporal aliasing which occurs at both high degrees and
at high frequencies. We will demonstrate these different errors using
results obtained with the VIRGO and MDI experiments onboard the SOHO
spacecraft. We will also compare some of these space-based results
with the results of similar estimates obtained from co-temporaneous
ground-based observations, such as from the Mt. Wilson 60-Foot Solar
Tower. We will include the results from different structural inversions
carried out with different sets of input frequencies and uncertainties
in order to demonstrate the effects of these different systematic
errors upon the inverted internal sound speed profile.
Title: Structure and Rotation of the Solar Interior: Initial Results
from the MDI Medium-L Program
Authors: Kosovichev, A. G.; Schou, J.; Scherrer, P. H.; Bogart, R. S.;
Bush, R. I.; Hoeksema, J. T.; Aloise, J.; Bacon, L.; Burnette, A.; de
Forest, C.; Giles, P. M.; Leibrand, K.; Nigam, R.; Rubin, M.; Scott,
K.; Williams, S. D.; Basu, Sarbani; Christensen-Dalsgaard, J.; Dappen,
W.; Rhodes, E. J., Jr.; Duvall, T. L., Jr.; Howe, R.; Thompson, M. J.;
Gough, D. O.; Sekii, T.; Toomre, J.; Tarbell, T. D.; Title, A. M.;
Mathur, D.; Morrison, M.; Saba, J. L. R.; Wolfson, C. J.; Zayer, I.;
Milford, P. N.
Bibcode: 1997SoPh..170...43K
Altcode:
The medium-l program of the Michelson Doppler Imager instrument on board
SOHO provides continuous observations of oscillation modes of angular
degree, l, from 0 to ∽ 300. The data for the program are partly
processed on board because only about 3% of MDI observations can be
transmitted continuously to the ground. The on-board data processing,
the main component of which is Gaussian-weighted binning, has been
optimized to reduce the negative influence of spatial aliasing of the
high-degree oscillation modes. The data processing is completed in a
data analysis pipeline at the SOI Stanford Support Center to determine
the mean multiplet frequencies and splitting coefficients. The initial
results show that the noise in the medium-l oscillation power spectrum
is substantially lower than in ground-based measurements. This enables
us to detect lower amplitude modes and, thus, to extend the range of
measured mode frequencies. This is important for inferring the Sun's
internal structure and rotation. The MDI observations also reveal the
asymmetry of oscillation spectral lines. The line asymmetries agree
with the theory of mode excitation by acoustic sources localized in the
upper convective boundary layer. The sound-speed profile inferred from
the mean frequencies gives evidence for a sharp variation at the edge
of the energy-generating core. The results also confirm the previous
finding by the GONG (Gough et al., 1996) that, in a thin layer just
beneath the convection zone, helium appears to be less abundant than
predicted by theory. Inverting the multiplet frequency splittings from
MDI, we detect significant rotational shear in this thin layer. This
layer is likely to be the place where the solar dynamo operates. In
order to understand how the Sun works, it is extremely important to
observe the evolution of this transition layer throughout the 11-year
activity cycle.
Title: Internal structure and rotation of the Sun: First results
from MDI data
Authors: Kosovichev, A. G.; Schou, J.; Scherrer, P. H.; Bogart, R. S.;
Bush, R. I.; Hoeksema, J. T.; Aloise, J.; Bacon, L.; Burnette, A.;
De Forest, C.; Giles, P. M.; Leibrand, K.; Nigam, R.; Rubin, M.;
Scott, K.; Williams, S. D.; Basu, Sarbani; Christensen-Dalsgaard,
J.; Däppen, W.; Rhodes, E. J., Jr.; Duvall, T. L., Jr.; Howe, R.;
Thompson, M. J.; Gough, D. O.; Sekii, T.; Toomre, J.; Tarbell, T. D.;
Title, A. M.; Mathur, D.; Morrison, M.; Saba, J. L. R.; Wolfson,
C. J.; Zayer, I.; Milford, P. N.
Bibcode: 1997IAUS..181..203K
Altcode:
No abstract at ADS
Title: New Time-distance helioseismology results from the SOI/MDI
experiment
Authors: Duvall, T., Jr.; Kosovichev, A. G.; Scherrer, P. H.
Bibcode: 1997IAUS..181...83D
Altcode:
No abstract at ADS
Title: Time-Distance Helioseismology with the MDI Instrument:
Initial Results
Authors: Duvall, T. L., Jr.; Kosovichev, A. G.; Scherrer, P. H.;
Bogart, R. S.; Bush, R. I.; de Forest, C.; Hoeksema, J. T.; Schou,
J.; Saba, J. L. R.; Tarbell, T. D.; Title, A. M.; Wolfson, C. J.;
Milford, P. N.
Bibcode: 1997SoPh..170...63D
Altcode:
In time-distance helioseismology, the travel time of acoustic waves
is measured between various points on the solar surface. To some
approximation, the waves can be considered to follow ray paths that
depend only on a mean solar model, with the curvature of the ray
paths being caused by the increasing sound speed with depth below the
surface. The travel time is affected by various inhomogeneities along
the ray path, including flows, temperature inhomogeneities, and magnetic
fields. By measuring a large number of times between different locations
and using an inversion method, it is possible to construct 3-dimensional
maps of the subsurface inhomogeneities. The SOI/MDI experiment on SOHO
has several unique capabilities for time-distance helioseismology. The
great stability of the images observed without benefit of an intervening
atmosphere is quite striking. It has made it possible for us to detect
the travel time for separations of points as small as 2.4 Mm in the
high-resolution mode of MDI (0.6 arc sec pixel-1). This has
enabled the detection of the supergranulation flow. Coupled with the
inversion technique, we can now study the 3-dimensional evolution of
the flows near the solar surface.
Title: Frequencies of solar oscillations and the seismic structure
of the Sun from SOHO/MDI.
Authors: Rhodes, E. J., Jr.; Kosovichev, A. G.; Scherrer, P. H.;
Schou, J.; Reiter, J.
Bibcode: 1997AGAb...13..163R
Altcode:
No abstract at ADS
Title: Inferences of element abundances from helioseismic data
Authors: Kosovichev, Alexander G.
Bibcode: 1997AIPC..385..159K
Altcode: 1997recs.conf..159K
The abundance of helium in the Sun's interior is estimated by the
method of `model-oriented' helioseismic inversion of solar oscillation
frequencies. In this method, the equations of state and thermal
balance are used in addition to the hydrostatic equation. By inverting
the observed p-mode frequencies, direct evidence for gravitational
settling of helium has been obtained. However, the helium settling
cannot account for the anomalously low helium abundance of the solar
corona and wind. The ratio of the helium to hydrogen densities in the
convection zone inferred from the helioseismic data [1,2] using the
most recent equation of state [3] is 8.5%+/-0.2% suggesting that the
separation of helium and hydrogen predominantly occurs in the solar
atmosphere and corona.
Title: The seismic structure of the Sun from GONG
Authors: Anderson, E.; Antia, H. M.; Basu, S.; Chaboyer, B.; Chitre,
S. M.; Christensen-Dalsgaard, J.; Eff-Darwich, A.; Elliott, J. R.;
Giles, P. M.; Gough, D. O.; Guzik, J. A.; Harvey, J. W.; Hill,
F.; Leibacher, J. W.; Kosovichev, A. G.; Monteiro, M. J. P. F. G.;
Richard, O.; Sekii, T.; Shibahashi, H.; Takata, M.; Thompson, M. J.;
Toomre, J.; Vauclair, S.; Vorontsov, S. V.
Bibcode: 1997IAUS..181..151A
Altcode:
No abstract at ADS
Title: Observation of solar convection with the MDI instrument
on SOHO.
Authors: Kosovichev, A. G.; Scherrer, P. H.; Duvall, T. L., Jr.
Bibcode: 1996BAAS...28.1298K
Altcode:
No abstract at ADS
Title: New Views of the Sun's Interior from the SOHO/MDI Space
Experiment
Authors: Scherrer, P. H.; Bogart, R. S.; Bush, R. I.; Hoeksema, J. T.;
Kosovichev, A. G.; Nigam, R.; Schou, J.; Duvall, T. L., Jr.
Bibcode: 1996AAS...189.1803S
Altcode: 1996BAAS...28.1298S
The strking stability of solar Dopplergrams measured by the Michelson
Doppler Imager (MDI) instrument on the SOHO spacecraft, without an
intervening atmosphere, substantially decreases the noise in the solar
oscillations power spectrum compared with groundbased observations. This
permits detection of lower amplitude oscillations, extending the range
of measured normal mode frequencies. This is important for improving
resolution and precision of helioseismic inferences about the Sun's
internal structure and dynamics. The MDI observations also reveal the
asymmetries of oscillation spectral lines that until now have been
largely hidden in noise. The line asymmetries agree with a theory of
excitation of solar oscillations by acoustic sources localized in the
upper convective boundary layer. High-resolution MDI images make it
possible to measure the travel time of acoustic waves propagating
inside the Sun by comparing points on the surface as close as 2.4
Mm. This is sufficient to detect supergranulation flows beneath the
surface. Coupled with tomographic inversion techniques, we can now study
the 3-dimensional evolution of the flows near the photosphere. The
sound-speed profile inferred from normal modes frequencies shows a
sharp variation at the edge of the energy-generating core, something
not accounted for by the standard evolution theory. The analysis also
confirms recent GONG results suggesting that helium is less abundant
than theory predicts in a thin layer just beneath the convection
zone. Inversion of the multiplet frequency splittings shows significant
rotational shear in this thin layer. This shear flow probably generates
turbulence that mixes the plasma in the upper radiative zone. This layer
is likely to be the place where the solar dynamo operates. Continuous
observation of the evolution of this transition layer during the entire
11-year activity cycle will be extremely important for understanding
the mechanisms of solar activity.
Title: Helioseismic Constraints on the Gradient of Angular Velocity
at the Base of the Solar Convection Zone
Authors: Kosovichev, A. G.
Bibcode: 1996ApJ...469L..61K
Altcode:
The layer of transition from the nearly rigid rotation of the radiative
interior to the latitudinal differential rotation of the convection zone
plays a significant role in the internal dynamics of the Sun. Using
rotational splitting coefficients of the p-mode frequencies, obtained
during 1986--1990 at the Big Bear Solar Observatory, we have found that
the thickness of the transitional layer is 0.09 +/- 0.04 solar radii
(63 +/- 28 Mm), and that most of the transition occurs beneath the
adiabatically stratified part of the convection zone, as suggested by
the dynamo theories of the 22 yr solar activity cycle.
Title: Helioseismic measurements of elemental abundances in the
sun's interior
Authors: Kosovichev, A. G.
Bibcode: 1996BASI...24..355K
Altcode:
No abstract at ADS
Title: Study of solar high-frequency modes near the acoustic cut-off
frequency
Authors: Nigam, R.; Kosovichev, A. G.
Bibcode: 1996BASI...24..195N
Altcode:
No abstract at ADS
Title: Determination of 3D internal structure and flows by tomographic
inversion
Authors: Kosovichev, A. G.
Bibcode: 1996BASI...24..191K
Altcode:
No abstract at ADS
Title: The MDI Structure Program: Continuous Monitoring of the
Solar Interior
Authors: Bush, R. I.; Bogart, R. S.; Hoeksema, J. T.; Kosovichev,
A. G.; Scherrer, P. H.; Mathur, D.; Morrison, M.; Tarbell, T. D.
Bibcode: 1996AAS...188.3708B
Altcode: 1996BAAS...28..878B
The Structure Program for the Michelson Doppler Imager instrument
on SOHO is designed to provide valuable helioseismic observations
through a continuously operating 5 kbps telemetry channel. Only about
3% of the total MDI raw data can be transmitted without interruption
to the ground. Therefore, extensive data processing must be performed
by the MDI instrument in order produce data products that will fit
into the restricted telemetry and to meet the scientific observing
requirements. The Structure Program consists of five observing programs:
the Medium-l Velocity, Low-l (LOI) Velocity and Intensity, Limb Figure,
Flux Budget, and Magnetic Proxy. These programs generate spatial
and temporal averages of three of the MDI full disk observables:
the velocity, computed continuum intensity, and the computed line
depth. The performance of these filters will be described. The
goal of the Medium-l Program is to provide reliable measurements
of solar p-modes in the range of the angular degree, l, from 0 to
300, by generating a 23,000 bin spatial average of the full disk
velocity. The Low-l Program re-bins the velocity and continuum intensity
images into 180 bins, with the intent of observing long period global
oscillations. The other three Structure Observations are averaged over
24 minutes with a 12 minute sample time. The Limb Figure extracts a 14
arcsecond annulus at the Sun's limb in order to study long wavelength
oscillations in the observed limb. The Flux Budget and Magnetic Proxy
are 128 by 128 pixel re-bins of the continuum intensity and line depth
observables respectively, to study the solar luminosity and large
scale features. Extensive tests of the individual components of the
Structure Program have been performed during the commissioning phase of
the SOHO operation. Uninterrupted operation started in April 1996. This
research is supported by NASA contract NAG5-3077 at Stanford University.
Title: The Current State of Solar Modeling
Authors: Christensen-Dalsgaard, J.; Dappen, W.; Ajukov, S. V.;
Anderson, E. R.; Antia, H. M.; Basu, S.; Baturin, V. A.; Berthomieu,
G.; Chaboyer, B.; Chitre, S. M.; Cox, A. N.; Demarque, P.; Donatowicz,
J.; Dziembowski, W. A.; Gabriel, M.; Gough, D. O.; Guenther, D. B.;
Guzik, J. A.; Harvey, J. W.; Hill, F.; Houdek, G.; Iglesias, C. A.;
Kosovichev, A. G.; Leibacher, J. W.; Morel, P.; Proffitt, C. R.;
Provost, J.; Reiter, J.; Rhodes, E. J., Jr.; Rogers, F. J.; Roxburgh,
I. W.; Thompson, M. J.; Ulrich, R. K.
Bibcode: 1996Sci...272.1286C
Altcode:
Data from the Global Oscillation Network Group (GONG) project and
other helioseismic experiments provide a test for models of stellar
interiors and for the thermodynamic and radiative properties, on which
the models depend, of matter under the extreme conditions found in the
sun. Current models are in agreement with the helioseismic inferences,
which suggests, for example, that the disagreement between the predicted
and observed fluxes of neutrinos from the sun is not caused by errors in
the models. However, the GONG data reveal subtle errors in the models,
such as an excess in sound speed just beneath the convection zone. These
discrepancies indicate effects that have so far not been correctly
accounted for; for example, it is plausible that the sound-speed
differences reflect weak mixing in stellar interiors, of potential
importance to the overall evolution of stars and ultimately to estimates
of the age of the galaxy based on stellar evolution calculations.
Title: Detection of Subsurface Supergranulation Structure and Flows
from MDI High-Resolution Data using Time-Distance Techniques
Authors: Duvall, T. L., Jr.; Kosovichev, A. G.; Scherrer, P. H.;
Milford, P. N.
Bibcode: 1996AAS...188.4908D
Altcode: 1996BAAS...28Q.898D
The supergranulation is seen at the surface of the sun in the doppler
shift of spectrum lines as an apparent cellular convection pattern
with a scale of about 4% of the solar radius. This scale is about
30 times larger than the granulation, seen in white light. Why these
distinct scales would be present (and possibly a third intermediate
scale mesogranulation) is somewhat of a mystery. Also unknown is
the depth structure of the convection. We have used acoustic wave
measurements from the MDI experiment on SOHO to address these
questions. By crosscorrelating the signal at one location with
that on annuli centered on the location, it is possible to measure
times for waves to travel over known subsurface ray paths. With some
variations on this theme, it is possible to measure horizontal and
vertical flows and sound speed variations. Of course, the resulting
measurements refer to quantities integrated along these ray paths. An
inversion technique based on Fermat's principle has been developed
and used to map the flow velocities and sound speed variations as
a function of horizontal position and depth. The MDI experiment on
SOHO makes doppler shift maps with 1Kx1K points in two choices of
image scale, 2 and 0.6 arcsec/pixel. For the present study, we have
used the higher resolution mode to observe 8.5 hours of doppler maps
sampled once per minute. In order to average enough crosscorrelations
to see time-distance effects, the resultant time-distance maps are
reduced in resolution by a factor of 10 from the initial data. This
still yields about 7 samples across a single supergranulation cell,
or 49 over the area of a square cell. Our initial inversions based
on the ray theory suggest that the supergranulation flow extends at
least to 0.5% of the solar radius below the surface. This research is
supported by the SOI-MDI NASA contract NAG5-3077 at Stanford University.
Title: The Seismic Structure of the Sun
Authors: Gough, D. O.; Kosovichev, A. G.; Toomre, J.; Anderson,
E.; Antia, H. M.; Basu, S.; Chaboyer, B.; Chitre, S. M.;
Christensen-Dalsgaard, J.; Dziembowski, W. A.; Eff-Darwich, A.;
Elliott, J. R.; Giles, P. M.; Goode, P. R.; Guzik, J. A.; Harvey,
J. W.; Hill, F.; Leibacher, J. W.; Monteiro, M. J. P. F. G.; Richard,
O.; Sekii, T.; Shibahashi, H.; Takata, M.; Thompson, M. J.; Vauclair,
S.; Vorontsov, S. V.
Bibcode: 1996Sci...272.1296G
Altcode:
Global Oscillation Network Group data reveal that the internal
structure of the sun can be well represented by a calibrated standard
model. However, immediately beneath the convection zone and at the
edge of the energy-generating core, the sound-speed variation is
somewhat smoother in the sun than it is in the model. This could be a
consequence of chemical inhomogeneity that is too severe in the model,
perhaps owing to inaccurate modeling of gravitational settling or to
neglected macroscopic motion that may be present in the sun. Accurate
knowledge of the sun's structure enables inferences to be made about
the physics that controls the sun; for example, through the opacity,
the equation of state, or wave motion. Those inferences can then be
used elsewhere in astrophysics.
Title: Differential Rotation and Dynamics of the Solar Interior
Authors: Thompson, M. J.; Toomre, J.; Anderson, E. R.; Antia, H. M.;
Berthomieu, G.; Burtonclay, D.; Chitre, S. M.; Christensen-Dalsgaard,
J.; Corbard, T.; De Rosa, M.; Genovese, C. R.; Gough, D. O.; Haber,
D. A.; Harvey, J. W.; Hill, F.; Howe, R.; Korzennik, S. G.; Kosovichev,
A. G.; Leibacher, J. W.; Pijpers, F. P.; Provost, J.; Rhodes, E. J.,
Jr.; Schou, J.; Sekii, T.; Stark, P. B.; Wilson, P. R.
Bibcode: 1996Sci...272.1300T
Altcode:
Splitting of the sun's global oscillation frequencies by large-scale
flows can be used to investigate how rotation varies with radius
and latitude within the solar interior. The nearly uninterrupted
observations by the Global Oscillation Network Group (GONG) yield
oscillation power spectra with high duty cycles and high signal-to-noise
ratios. Frequency splittings derived from GONG observations confirm
that the variation of rotation rate with latitude seen at the surface
carries through much of the convection zone, at the base of which is
an adjustment layer leading to latitudinally independent rotation at
greater depths. A distinctive shear layer just below the surface is
discernible at low to mid-latitudes.
Title: The Internal Structure of the Sun
Authors: Gough, D. O.; Kosovichev, A. G.; GONG Structure Inversions
Team
Bibcode: 1996AAS...188.5303G
Altcode: 1996BAAS...28..903G
The principal first inferences that have been drawn from the GONG data
concerning the internal structure of the Sun will be reported. After
explaining briefly the procedures by which the inferences have been
drawn, deviations of the spherically averaged structure of the Sun
from that of standard solar models will be presented. Those deviations
occur predominantly near the base of the convection zone, and perhaps
in the energy-generating core. There is also evidence for a deviation
from spherical symmetry, principally near the surface. The possible
physical implications of our findings will be discussed.
Title: The Solar Acoustic Spectrum and Eigenmode Parameters
Authors: Hill, F.; Stark, P. B.; Stebbins, R. T.; Anderson, E. R.;
Antia, H. M.; Brown, T. M.; Duvall, T. L., Jr.; Haber, D. A.;
Harvey, J. W.; Hathaway, D. H.; Howe, R.; Hubbard, R. P.; Jones,
H. P.; Kennedy, J. R.; Korzennik, S. G.; Kosovichev, A. G.; Leibacher,
J. W.; Libbrecht, K. G.; Pintar, J. A.; Rhodes, E. J., Jr.; Schou, J.;
Thompson, M. J.; Tomczyk, S.; Toner, C. G.; Toussaint, R.; Williams,
W. E.
Bibcode: 1996Sci...272.1292H
Altcode:
The Global Oscillation Network Group (GONG) project estimates
the frequencies, amplitudes, and linewidths of more than 250,000
acoustic resonances of the sun from data sets lasting 36 days. The
frequency resolution of a single data set is 0.321 microhertz. For
frequencies averaged over the azimuthal order m, the median formal
error is 0.044 microhertz, and the associated median fractional error
is 1.6 x 10-5. For a 3-year data set, the fractional error
is expected to be 3 x 10-6. The GONG m-averaged frequency
measurements differ from other helioseismic data sets by 0.03 to 0.08
microhertz. The differences arise from a combination of systematic
errors, random errors, and possible changes in solar structure.
Title: Diagnostics of Shallow Convective Structures by Time-Distance
Helioseismology
Authors: Kosovichev, A. G.; Duvall, T. L., Jr.; Scherrer, P. H.
Bibcode: 1996AAS...188.3709K
Altcode: 1996BAAS...28R.878K
We present a new method of 3D helioseismic diagnostics to study
subphotospheric flow and thermal and magnetic structure associated with
turbulent convection. The main difference from the previous studies
by Duvall et al. (1996, Nature, 379, 235) and by Kosovichev (1996,
ApJL, 461, L55) is that the new method can be applied for measuring
solar properties in the shallow layer just beneath the surface. The
shallow layer of superadiabatic convection, which is only few thousand
kilometers deep, is the region of the greatest uncertainty in our
knowledge of the Sun's interior. Recent numerical simulations have
demonstrated substantial deviations of the structure of this layer from
the mixing-length theory commonly used in modeling stellar structure
and evolution. The uncertainty in the physics of turbulent convection
also affects helioseismic inferences about the deep interior. Our method
of 3D diagnostics is based on measuring and inverting anomalies of the
sound-wave travel time between two areas on the solar surface. Because
of the stochastic nature of solar waves, these two areas must
be sufficiently large to provide a good signal-to-noise ratio. In
practice, the travel time can be measured from the cross-correlation
function averaged over several thousand cross-correlations between
individual points on the surface. Therefore, it is essential to have
stable high-resolution series of Doppler images. Such data have been
obtained from the Michelson Doppler Imager instrument on SOHO. In this
paper, we present some details of the cross-correlation time-distance
analysis, and the technique to invert the travel-time measurements using
the optical ray approximation. The travel time of the waves depends
primarily on the wave group velocity and on the velocity of flow along
the ray paths. The effects of the wave speed structure and of flows are
separated by measuring the travel time of waves propagating in opposite
directions along the same ray paths. The effects of magnetic fields are
measured through anisotropy of the wave speed. We discuss the limits
for observing small-scale features beneath the surface. This research is
supported by the SOI-MDI NASA contract NAG5-3077 at Stanford University.
Title: Tomographic Imaging of the Sun's Interior
Authors: Kosovichev, A. G.
Bibcode: 1996ApJ...461L..55K
Altcode:
A new method is presented of determining the three-dimensional
sound-speed structure and flow velocities in the solar convection
zone by inversion of the acoustic travel-time data recently obtained
by Duvall and coworkers. The initial inversion results reveal
large-scale subsurface structures and flows related to the active
regions, and are important for understanding the physics of solar
activity and large-scale convection. The results provide evidence
of a zonal structure below the surface in the low-latitude area of
the magnetic activity. Strong converging downflows, up to 1.2 km s-1,
and a substantial excess of the sound speed are found beneath growing
active regions. In a decaying active region, there is evidence for
the lower than average sound speed and for upwelling of plasma.
Title: Helelioseismic constraints on the properties of the solar
core and on the solar neutrino fluxes.
Authors: Kosovichev, A.
Bibcode: 1996sube.conf..171K
Altcode:
No abstract at ADS
Title: The Solar Oscillations Investigation - Michelson Doppler Imager
Authors: Scherrer, P. H.; Bogart, R. S.; Bush, R. I.; Hoeksema, J. T.;
Kosovichev, A. G.; Schou, J.; Rosenberg, W.; Springer, L.; Tarbell,
T. D.; Title, A.; Wolfson, C. J.; Zayer, I.; MDI Engineering Team
Bibcode: 1995SoPh..162..129S
Altcode:
The Solar Oscillations Investigation (SOI) uses the Michelson Doppler
Imager (MDI) instrument to probe the interior of the Sun by measuring
the photospheric manifestations of solar oscillations. Characteristics
of the modes reveal the static and dynamic properties of the
convection zone and core. Knowledge of these properties will improve
our understanding of the solar cycle and of stellar evolution. Other
photospheric observations will contribute to our knowledge of the
solar magnetic field and surface motions. The investigation consists
of coordinated efforts by several teams pursuing specific scientific
objectives.
Title: Probing the Sun's Interior Structure and Flows By Tomographic
Inversion
Authors: Kosovichev, A. G.
Bibcode: 1995AAS...18710111K
Altcode: 1995BAAS...27.1427K
I present results of inversion of travel-time maps recently obtained
by Duvall, Jefferies and Harvey (1995, BAAS, v.25, 950). The maps
represent measurements of the time for acoustic waves to travel between
points on a solar surface and surrounding annuli. The measurements
are sensitive to perturbations of the sound speed and flows along the
ray pathes. A 3D inversion method based on Fermat's Principle and a
conjugate-gradient technique has been applied to infer the sound speed
and the velocity of flows from the observations obtained by Duvall
et al. at the South Pole Jan. 4-5 1991. The spatial resolution of the
inversion is 1.75 degree in both longitude and latitude, and 15 Mm in
depth. The results reveal large-scale subsurface structures and flows
related to the active regions, and are important for understanding
the physics of solar activity and large-scale convection.
Title: Non-linear effects at tidal capture of stars by a massive
black hole - II. Compressible affine models and tidal interaction
after capture
Authors: Diener, P.; Kosovichev, A. G.; Kotok, E. V.; Novikov, I. D.;
Pethick, C. J.
Bibcode: 1995MNRAS.275..498D
Altcode:
This paper continues our investigations published in Paper I. The
tidal interaction of stars with a massive black hole after tidal
capture is investigated in the framework of the ellipsoidal `affine'
stellar model proposed by Carter & Luminet. We investigate the
influence of the orientation of the ellipsoidal stellar model and
the influence of the phase and amplitude of the oscillations on the
subsequent passages. Finally a parametrization of the probability for
a star to lose (or gain) energy during subsequent passages is found
for a compressible model.
Title: Helioseismic measurement of element abundances in the solar
interior
Authors: Kosovichev, A. G.
Bibcode: 1995AdSpR..15g..95K
Altcode: 1995AdSpR..15...95K
Inversions of the current solar data give a value for the helium
abundance in the convection zone, which is significantly lower than
the value in standard solar models, thus suggesting that gravitational
settling of helium takes place in the Sun. This is consistent with the
inversion results in the radiative interior, where evidence is found
for large-scale material redistribution. There is also indication
of variations of the helium abundance in the energy-generating core,
which are not described by standard solar evolution theory.
Title: Coronal Manifestations of Oscillations: a Numerical Model
Authors: Andreev, A. S.; Kosovichev, A. G.
Bibcode: 1995ESASP.376b.471A
Altcode: 1995help.confP.471A; 1995soho....2..471A
No abstract at ADS
Title: Structure of the Solar Core: Inversion of Recent Low-Degree
Data
Authors: Kosovichev, A. G.
Bibcode: 1995ESASP.376b..21K
Altcode: 1995soho....2...21K; 1995help.confP..21K
The deviations of the property, u ≡ p/ρ, in the Sun's interior from
a standard solar model have been studied by inverting six different
sets of frequencies of low-degree modes, obtained from space- and
ground-based observations. The results from the IPHIR space experiment,
the IRIS network and from the LOI ground-based observations indicate
an increase of the deviation δu towards the center. The inversions
of the three datasets from BISON, corresponding to low, high and mean
levels of solar activity, show complicated changes of δu in the core
among the datasets, with the average tendency of decreasing towards
the center. The reason of the inconsistency between BISON and the
other datasets remains unknown. Nevertheless, property u (or the speed
of sound) in the central core inferred from each of the datasets is
higher than in a solar model with gravitational settling of helium,
which is currently the closest to the inversion results.
Title: An Attempt to Measure Latitudinal Variation of the Depth of
the Convection Zone
Authors: Gough, D. O.; Kosovichev, A. G.
Bibcode: 1995ESASP.376b..47G
Altcode: 1995help.confP..47G; 1995soho....2...47G
The location of the base of the convection zone coincides with the
sharp variation of the slope of the relative difference in the quantity
u = p/ρ between the Sun and a solar model, provided that the zone
of adiabatic convection is deeper in the model than in the Sun. The
authors have determined the difference δu/u as a function of radius at
various latitudes between the Sun and a spherically symmetrical solar
model by inverting the BBSO data (Libbrecht and Woodard, 1993). The
results offer evidence that the convection zone may be somewhat deeper
at the equator than it is at the poles. The variation of the depth,
however, does not exceed 0.2 Rsun.
Title: Seismic Response to Solar Flares: Theoretical Predictions
Authors: Kosovichev, A. G.; Zharkova, V. V.
Bibcode: 1995ESASP.376b.341K
Altcode: 1995help.confP.341K; 1995soho....2..341K
No abstract at ADS
Title: Constrained Estimates of Low-Degree Mode Frequencies and the
Determination of the Interior Structure of the Sun
Authors: Gough, D. O.; Kosovichev, A. G.; Toutain, T.
Bibcode: 1995SoPh..157....1G
Altcode:
Low-degreep-modes penetrate to the solar centre and provide direct
information about the core. However, the high observational accuracy
that is required to resolve the details of the structure of the core
is difficult to achieve because the oscillation power spectrum is
significantly distorted by stochastic forcing of the oscillations,
which appears as multiplicative noise. Here, an attempt is reported to
reduce uncertainties of spectral parameter estimation by incorporating
constraints imposed by smooth behaviour of some of the parameters
(e.g., linewidths, background noise, rotational splitting) over a
group of lines. Instead of estimating these parameters independently
for each line, we determine them as smooth functions of frequency. It
is expected that this procedure gives more accurate estimates of the
average frequencies of any multiplet in the power spectrum, to which
we have applied no constraints. We give some examples of the procedure
for whole-disk measurements by the IPHIR space experiment. It is shown
that the additional constraints do not result in significant changes
in the frequency estimates, except for one mode whose peak in the power
spectrum has the lowest signal-to-noise ratio. However, the uncertainty
in the frequency of that mode does not influence substantially the
results of the structure inversion in the core. Inversions of the IPHIR
datasets are compared with corresponding inversions of data from the
Birmingham Solar Oscillation Network (BISON). The IPHIR data indicate
a sharp increase towards the centre of the deviation of the squared
sound speed of the sun from that of a standard solar model, whereas the
BISON data show a decrease. The difference between the IPHIR and BISON
inversions is significant, preventing any definite conclusion about
the deviation of the structure of the solar core from that of the model.
Title: Solar Cycle Variations of the Internal Rotation: a Search
for Dynamo Waves
Authors: Kosovichev, A. G.
Bibcode: 1995SPD....26..405K
Altcode: 1995BAAS...27..955K
No abstract at ADS
Title: Testing the Statistical Significance of the Asymmetries of
p-Mode Line Profiles: Application to the IPHIR Data
Authors: Appourchaux, T.; Toutain, T.; Gough, D. O.; Kosovichev, A.
Bibcode: 1995ASPC...76..314A
Altcode: 1995gong.conf..314A
No abstract at ADS
Title: The Upper Convective Boundary Layer
Authors: Kosovichev, A. G.
Bibcode: 1995ESASP.376a.165K
Altcode: 1995soho....1..165K; 1995heli.conf..165K
Reviews the progress that has been made in our understanding of
the physics of the upper boundary layer of the Sun, its influence on
frequencies of five-minute oscillations, and its role in excitation of
the oscillations. The author also discusses approaches to seismological
diagnosis of the properties of the layer, important information about
which will be obtained from MDI high-resolution data.
Title: Constraints on Oblique Rotation of the Solar Core from
Low-Degree Modes
Authors: Gough, D. O.; Kosovichev, A. G.; Toutain, T.
Bibcode: 1995ASPC...76...55G
Altcode: 1995gong.conf...55G
No abstract at ADS
Title: Working Group 9 - Interior Structure and Inversions
Authors: Kosovichev, A. G.; Basu, S.; Christensen-Dalsgaard, J.;
Eff-Darwich, A.; Gough, D. O.; Iglesias, C. A.; Pérez-Hernández,
F.; Rogers, F.; Sekii, T.; Shibahashi, H.
Bibcode: 1995ESASP.376a.211K
Altcode: 1995heli.conf..211K
No abstract at ADS
Title: Prediction of g-Mode Frequencies
Authors: Kosovichev, A. G.; Gavryuseva, E. A.
Bibcode: 1995ASPC...76..180K
Altcode: 1995gong.conf..180K
No abstract at ADS
Title: Inversions of BBSO Rotational Splitting Data
Authors: Sekii, T.; Gough, D. O.; Kosovichev, A. G.
Bibcode: 1995ASPC...76...59S
Altcode: 1995gong.conf...59S
No abstract at ADS
Title: Frequencies of Low-Degree Modes and the Structure of the
Solar Core
Authors: Gough, D. O.; Kosovichev, A. G.; Toutain, T.
Bibcode: 1995ASPC...76..176G
Altcode: 1995gong.conf..176G
No abstract at ADS
Title: Seismic Effects of North-South Asymmetry of Sun's Rotation
Authors: Gough, D. O.; Kosovichev, A. G.
Bibcode: 1995ASPC...76...63G
Altcode: 1995gong.conf...63G
No abstract at ADS
Title: Determination of Interior Structure by Inversion
Authors: Kosovichev, A. G.
Bibcode: 1995ASPC...76...89K
Altcode: 1995gong.conf...89K
No abstract at ADS
Title: Numerical modeling of macrospicules and mass ejections in
the corona
Authors: Andreev, A. S.; Kosovichev, A. G.
Bibcode: 1994ESASP.373..179A
Altcode: 1994soho....3..179A
No abstract at ADS
Title: Modelling UV spectral lines from solar coronal transients
Authors: Kosovichev, A. G.; Spadaro, D.; Stepanova, T. V.; Ventura, R.
Bibcode: 1994ESASP.373..159K
Altcode: 1994soho....3..159K
No abstract at ADS
Title: Helioseismological measurements of the distribution of helium
inside the Sun
Authors: Kosovichev, A. G.
Bibcode: 1994AAS...18512301K
Altcode: 1994BAAS...26.1522K
Helioseismology provides a unique tool for investigating chemical
composition of the interior of the Sun. There are two basic ways of
obtaining the information. The first one is to calibrate theoretical
solar models by comparing either the observed oscillation frequencies
with the eigenfrequencies of the models, or primary seismic parameters
(e.g. the sound speed, the density and the adiabatic exponent) inverted
from the observed frequencies with the corresponding parameters of
the solar models. The second approach is to measure abundances by
direct (`secondary') inversions of the frequencies, incorporating
additional equations of the stellar structure into the helioseismic
inverse problem. The additional equation to estimate composition of
the convection zone is the equation of state which relates variations
of the adiabatic exponent in zones of ionization of elements to their
abundances. In the radiative interior where the most abundant elements
are almost totally ionized, the energy equations together with equations
of the energy generation rate and the opacity are used to relate primary
seismic parameters with abundances. Inversions of the current solar
data give a value for the helium abundance in the convection zone,
which is significantly lower than the value in standard solar models,
thus suggesting that gravitational settling of helium takes place
in the Sun. This is consistent with the inversion results in the
radiative interior, where evidence is found for large-scale material
redistribution. There is also indication of variations of the helium
abundance in the energy-generating core, which are not described by
standard solar evolution theory. It is anticipated that the new data
from the GONG and SOI will substantially improve accuracy of the
measurements of the helium abundance.
Title: Modeling of dynamic evolution of roconal loops
Authors: Stepanova, T. V.; Kosovichev, A. G.
Bibcode: 1994SSRv...70..171S
Altcode:
Parameters of expanding magnetic loops and arches and of mass flows
generated by them in the corona have been computed in a 1D two-fluid
approximation. Two possible trigger mechanisms of the coronal transients
have been considered: (i) sudden increase of the background magnetic
field strength, and (ii) heating and compression plasma inside these
magnetic structures. We discuss the formation of shock waves and their
dependence on dynamics and geometry of the magnetic structures.
Title: Numerical simulations of spicule driving mechanisms
Authors: Andreev, A. S.; Kosovichev, A. G.
Bibcode: 1994SSRv...70...53A
Altcode:
Spicules are known as one of the most prevalent small-scale
dynamic phenomena on the sun, which are likely to give considerable
contribution to coronal heating and mass supply. We discuss a model
of the spicules driven by a train of slow MHD shock waves propagating
along a vertical expanding magnetic flux tube. The shocks are initiated
due to compression of the tube by the increasing external pressure
in the lower chromosphere. Downflow of spicular material depends on
radiative cooling and other dissipative processes.
Title: On a mechanism of spicule formation by shock waves in
magnetic tubes
Authors: Andreev, A. S.; Kosovichev, A. G.
Bibcode: 1994AstL...20..323A
Altcode: 1994PAZh...20..383A
No abstract at ADS
Title: A new estimate of the solar core rotation from IPHIR.
Authors: Toutain, T.; Kosovichev, A. G.
Bibcode: 1994A&A...284..265T
Altcode:
The effect of a rigidly rotating core on frequency splitting is shown
to be approximately the same for all the observed low-degree modes. It
is suggested that, in analysing poorly resolved rotationally split
multiplets in oscillation power spectra, one should first determine
a single value for the splitting instead of estimating the splitting
for individual modes separately. By using this technique for fitting
lines with l=1, 2 of the IPHIR spectra, a mean sidereal splitting of
0.468+/-0.036 and of 0.427+/-0.046 μHz has been obtained for the green
and red channels of the IPHIR photometer, respectively. The average
value of these measurements, 0.452+/-0.020 μHz, suggests that the
central core of the sun rotates with the same period of approximately
26 days as the outer part of the radiative zone.
Title: Helioseismic evidence for mixing in the radiative interior
Authors: Kosovichev, A. G.
Bibcode: 1994LNP...432...47K
Altcode: 1994LNPM...11...47K
Results are presented of a determination of the hydrostatic parameters
of the solar structure, namely density, sound speed, and a parameter
of convectioe stability, by direct inversion of solar oscillation
frequencies. The analysed data sets include frequencies of acoustic
modes of intermediate degree (l = 4 - 140, observed by Libbrecht et
al. (1990, and those of low degree (l = 0 - 2), obtained from the IPHIR
space experiment (Toutain and Fröhlich 1992. The low-degree data sets
are of particular importance for resolving the structure of the solar
core. The inversion results show that the overall structure of the solar
interior is consistent with a non-standard solar model constructed by
Christensen-Dalsgaard et al. (1993 by increasing opacities beneath the
convection zone in accordance with Rogers and Iglesias (1992, and by
taking into account gravitational settling of helium. The inversions
give also evidence for an overshoot beneath the convection zone and
for a moderate localized mixing in the energy-generating core.
Title: Seismic measurements of the helium abundance and the depth
of stellar convection zones
Authors: Kosovichev, A. G.
Bibcode: 1993MNRAS.265.1053K
Altcode:
A new inversion technique is presented for measuring the depth of
stellar convection zones and the helium abundance from oscillation
frequencies of low-degree modes. It is based on a linearized variational
principle for stellar oscillations, reformulated in terms of a parameter
of convective stability (which is proportional to the radial gradient
of the specific entropy) and the helium abundance, by using the
equations of hydrostatic support of stellar structure and the equation
of state. In addition, it is assumed that the parameter of convective
stability is close to zero in the zones of adiabatic convection. It
is then possible to locate the lower boundary of the convection zone
in the envelope by applying a regularized least-squares fit to the
oscillation frequencies. It is demonstrated that the technique works
for low-degree data alone, such as can be obtained from full-disk
measurements. The technique has been applied to solar data obtained
from the IPHIR instrument on the Phobos spacecraft. The estimates of
the solar helium abundance and the depth of the convection zone so
obtained are discussed.
Title: The Influence of Low-Degree P-Mode Frequencies on the
Determination of the Structure of the Solar Interior
Authors: Gough, D. O.; Kosovichev, A. G.
Bibcode: 1993MNRAS.264..522G
Altcode:
Accurate measurements of the frequencies of low-degree acoustic
oscillations provide valuable information about the structure of the
solar core. We determine the radial resolution that can be achieved by
direct inversions of frequency data sets recently obtained by various
observers to find the hydrostatic parameters density, sound speed and
a parameter of convective stability. The outcome of those inversions
indicates that the outer part of the radiative zone of the Sun is
similar to that of a solar model that takes account of helium settling
against microscopic diffusion. From the two data sets with lowest
estimated errors (those by Toutain & Fröhlich and Anguera Gubau
et al.) there is some evidence for an error in the modelling of the
energy-generating core, which could be accounted for by local material
redistribution in the core. Another data set by Elsworth et al.,
however, is almost compatible with the core of the theoretical model.
Title: Interplanetary shocks generated by expanding magnetic loops
Authors: Stepanova, T. V.; Kosovichev, A. G.
Bibcode: 1993AdSpR..13f..51S
Altcode: 1993AdSpR..13...51S
We present a self-consistent numerical model of shock wave formation in
the heliosphere by an expanding magnetic loop. In the model a coronal
mass ejection is initiated by a loss of magnetohydrostatic equilibrium
of the loop as a result of an increase of underlying magnetic field
strength. The expanding magnetic loops produce propagating shock
waves. The plasma motions are described by a system of two-fluid
Navier-Stokes equations taking account of modified coefficients for
electron and ion heat conduction, ion viscosity and energy exchange
between ions and electrons. We obtain shock wave parameters in the
outer heliosphere vs initial perturbations of the magnetic loops, and
show that the shocks can be divided into two types, depending on their
intensity. In the case of relatively weak shocks a typical feature
is formation of a dense and cold layer (``piling-up'' of material)
near the upper boundary of the loop. In the case of strong shocks
large-scale turbulence and viscous heating in the relaxation zone
behind the front play an important role, and no appreciable piling-up
of plasma occurs. We demonstrate that expanding magnetic loops,
which are observed as magnetic clouds in the outer heliosphere, can
effectively drive transient shocks ahead.
Title: Helioseismic Test of the Standard Solar Model
Authors: Kosovichev, A. G.
Bibcode: 1993BAAS...25.1219K
Altcode:
No abstract at ADS
Title: Seismic evidence of modulation of the structure and angular
velocity of the Sun associated with the solar cycle
Authors: Gough, D. O.; Kosovichev, A. G.; Sekii, T.; Libbrecht, K. G.;
Woodard, M. F.
Bibcode: 1993ASPC...40...93G
Altcode: 1993IAUCo.137...93G; 1993ist..proc...93G
No abstract at ADS
Title: On the Influence of Treatment of Heavy Elements in the Equation
of State on the Resulting Values of the Adiabatic Exponent
Authors: Däppen, W.; Gough, D. O.; Kosovichev, A. G.; Rhodes,
E. J., Jr.
Bibcode: 1993ASPC...40..304D
Altcode: 1993ist..proc..304D
No abstract at ADS
Title: The High-Frequency P-Mode Spectrum
Authors: Milford, P. N.; Scherrer, P. H.; Frank, Z.; Kosovichev,
A. G.; Gough, D. O.
Bibcode: 1993ASPC...42...97M
Altcode: 1993gong.conf...97M
No abstract at ADS
Title: The Form of the Angular Velocity in the Solar Convection Zone
Authors: Gough, D. O.; Kosovichev, A. G.; Sekii, T.; Libbrecht, K. G.;
Woodard, M. F.
Bibcode: 1993ASPC...42..213G
Altcode: 1993gong.conf..213G
No abstract at ADS
Title: Initial asteroseismic inversions
Authors: Gough, D. O.; Kosovichev, A. G.
Bibcode: 1993ASPC...40..541G
Altcode: 1993IAUCo.137..541G; 1993ist..proc..541G
No abstract at ADS
Title: Seismic Analysis of Stellar P-Mode Spectra
Authors: Gough, D. O.; Kosovichev, A. G.
Bibcode: 1993ASPC...42..351G
Altcode: 1993gong.conf..351G
No abstract at ADS
Title: It is possible to determine whether a star is rotating about
a unique axis?
Authors: Gough, D. O.; Kosovichev, A. G.
Bibcode: 1993ASPC...40..566G
Altcode: 1993IAUCo.137..566G; 1993ist..proc..566G
No abstract at ADS
Title: Plans for MT.WILSON - Crimean Observatory High-Degree
Helioseismology Network
Authors: Rhodes, E. J., Jr.; Cacciani, A.; Dappen, W.; Didkovsky,
L. V.; Hill, F.; Korzennik, S. G.; Kosovichev, A. G.; Kotov, V. A.;
Scherrer, P. H.
Bibcode: 1993ASPC...42..477R
Altcode: 1993gong.conf..477R
No abstract at ADS
Title: Sources of uncertainty in direct seismological measurements
of the solar helium abundance
Authors: Kosovichev, A. G.; Christensen-Dalsgaard, J.; Daeppen, W.;
Dziembowski, W. A.; Gough, D. O.; Thompson, M. J.
Bibcode: 1992MNRAS.259..536K
Altcode:
The methods by which Dappen et al. (1988, 1990, 1991) and Dziembowski
et al. (1990, 1991, 1992) recently obtained discrepant estimates of
the helium abundance in the solar convection zone are compared. The
aim of the investigation reported in this paper is to identify the
main source of the discrepancy. Using as proxy data eigenfrequencies
of a set of modes of a theoretical solar model, computed with the
same physics as were the frequencies of a reference model with which
these data are compared, the two methods yield similar results. Thus
we ascertain that the principal source of the discrepancy is not in
the inversions themselves, which yield essentially a measure of the
variation of the adiabatic exponent gamma of the material in the He II
ionization zone. Instead it is in the approximations adopted in the
treatment of heavy elements in the equation of state used to relate
the variation of gamma to chemical composition. We obtain acceptably
consistent results when inverting solar data by two methods using the
same equation of state. We attempt to identify the likely residual
sources of uncertainty.
Title: A numerical model of interaction of coronal transients with
the solar wind.
Authors: Stepanova, Tatiana V.; Kosovichev, A. G.
Bibcode: 1992ESASP.348..209S
Altcode: 1992cscl.work..209S
The authors consider loss of hydrostatic equilibrium of a coronal
magnetic loop as a result of an increase of the magnetic field under
the loop and as a result of a flare-induced heating of plasma inside
the loop. The expanding magnetic loops act as a piston on coronal
plasma and produce shock waves propagating in the solar wind. The
shock waves are computed in a one-dimensional, two-fluid approximation,
taking into account the processes of turbulent dissipation. The authors
describe two regimes of the process: i) almost monotonic expansion of
a magnetic loop with a strong shock ahead of it, when magnetic forces
are dominant; ii) oscillatory expansion of a loop with multiple shocks
in the ambient plasma, if the magnetic forces can be balanced by the
gravity force when there is a significant mass input into the loop.
Title: Non-linear effects at tidal capture of stars by a massive
black hole. I - Incompressible affine model
Authors: Kosovichev, A. G.; Novikov, I. D.
Bibcode: 1992MNRAS.258..715K
Altcode:
Non-linear effects of the tidal interaction of a star with a massive
black hole are discussed on the basis of an ellipsoidal 'affine' stellar
model proposed by Carter and Luminet. The effects are considered for an
incompressible stellar model. We compute the amount of energy deposited
into the star from the orbital motion by tidal forces and determine
an effective Roche limit of tidal disruption for a parabolic orbit. A
comparison between the non-linear affine model and a linear theory of
small perturbations is made, and the limits of their applicability are
found. The dynamics of the tidal interactions at subsequent pericenter
passages after the tidal capture are considered, and it is shown that
the non-linear effects significantly reinforce the absorption of the
orbital energy by the star, and result in tidal disruptions far beyond
the Roche limit.
Title: Numerical model for coronal shock wave formation in two-fluid
approximation
Authors: Kosovichev, A. G.; Stepanova, T. V.
Bibcode: 1992sws..coll...61K
Altcode:
We present results of modeling of shock waves, generated by coronal
transients and expanding flare loops. We consider loss of hydrostatic
equilibrium of a magnetic loop as a result of an increase of the
magnetic field under the loop and as a result of a flare-induced
heating of plasma inside the loop. The expanding magnetic loops act
as a piston on coronal plasma and produce shock waves propagating
in the solar wind. The shock waves are computed in one-dimensional,
two-fluid approximation, by taking into account the processes of
turbulent dissipation. The corresponding gas-dynamic equations are
solved simultaneously with equations of motion of the magnetic tube
to provide a self-consistent picture of the shock formation by the
transients.
Title: Numerical simulation of shock waves in the heliosphere
Authors: Kosovichev, A. G.; Stepanova, T. V.
Bibcode: 1991AZh....68.1283K
Altcode:
Results of calculations of shock waves generated by coronal transients
and expanding flare loops are presented. The expanding magnetic
loops act as a piston on the coronal plasma and produce shock waves
propagating in the solar wind. The shock waves are computed in a 1D
two-fluid approximation, taking turbulent dissipation processes into
account. Shock wave parameters are obtained as functions of the initial
perturbations of the magnetic loops.
Title: Numerical Simulation of Shocks in the Heliosphere
Authors: Kosovichev, A. G.; Stepanova, T. V.
Bibcode: 1991SvA....35..646K
Altcode:
No abstract at ADS
Title: Construction of a Seismic Model of the Sun
Authors: Kosovichev, A. G.; Fedorova, A. V.
Bibcode: 1991SvA....35..507K
Altcode:
No abstract at ADS
Title: A New Inversion for the Hydrostatic Stratification of the Sun
Authors: Däppen, W.; Gough, D. O.; Kosovichev, A. G.; Thompson, M. J.
Bibcode: 1991LNP...388..111D
Altcode: 1991ctsm.conf..111D
Inversions for the spherically symmetric component of the hydrostatic
stratification of the Sun are presented. These employ the Backus-Gilbert
optimally localized averaging procedure applied to oscillation multiplet
frequencies in the range 1.5 - 3 mHz of modes with 4 l 140 determined
by Libbrecht et al. (1990) from observations carried out in 1986. We
also obtain an estimate of the helium abundance in the solar convective
envelope.
Title: Oscillations and Tidal Resonance Phenomena in the Beta-Lyrae
System
Authors: Kosovichev, A. G.; Skulskii, M. Y.
Bibcode: 1990SvAL...16..103K
Altcode:
No abstract at ADS
Title: Oscillation and tidal resonant phenomena in beta-Lyrae.
Authors: Kosovichev, A. G.; Skulsky, M. Yu.
Bibcode: 1990PAZh...16..240K
Altcode:
In the Beta Lyrae system, oscillations in H-alpha emission lines
and magnetic field variations with a period of 1.85 d have been
observed. It is suggested that these oscillations and variations are
related to the excitation of quadrupole fundamental eigenmodes in the
bright component of the binary system. This excitation is the result
of a resonant interaction between the modes and the tidal wave, due
to asynchronized orbital and rotational periods.
Title: Helioseismological Determination of Stratification and Dynamic
Processes in the Solar Core
Authors: Kosovichev, A. G.
Bibcode: 1990IAUS..142...56K
Altcode:
No abstract at ADS
Title: Structure of the Solar Core Inferred from Inversion of
Frequencies of Low-Degree p-Modes
Authors: Kosovichev, A. G.
Bibcode: 1990LNP...367..319K
Altcode: 1990psss.conf..319K
Results of estimations of density and a parameter of convective
stability in the central regions of the Sun from observed frequencies
of 5-min modes are presented.
Title: Using Helioseismic Data to Probe the Hydrogen Abundance in
the Solar Core
Authors: Gough, D. O.; Kosovichev, Alexander G.
Bibcode: 1990ASSL..159..327G
Altcode: 1990IAUCo.121..327G; 1990insu.conf..327G
No abstract at ADS
Title: An attempt to understand the stanford p-mode data.
Authors: Gough, Douglas O.; Kosovichev, A. G.
Bibcode: 1988ESASP.286..195G
Altcode: 1988ssls.rept..195G
The p-mode frequencies reported by Henning and Scherred (1986),
showing anomalous behavior at degree 5 and frequencies below 2
mHz were investigated. No plausible solar model is consistent with
them. A density inversion including the low-degree 5 min data of
Jimenez et al. (1988) implies that the density of the solar core
is 10 percent greater than it is in the standard solar model 1 of
Christensen-Dalsgaard (1982). Although that result is in keeping with
previous suggestions either that the Sun has a greater evolutionary
age than is usually supposed or that there is a cloud of weakly
interacting massive particles in the solar core and its environs,
the behavior of the sound speed in the core is not consistent with
either hypothesis. Both the inferred sound-speed variation, and a
secondary inversion for hydrogen abundance (relying on an assumption of
thermal balance), provide evidence for material redistribution in the
energy-generating core. A sound-speed inversion for the entire radiative
interior, using also frequencies of low and intermediate degree compiled
by Duvall et al. (1988), confirms the earlier finding that the sound
speed in the Sun exceeds that of a standard solar model by up to 1
percent in a region extending 30 percent of the solar radius and centred
at r = 0.4R. That is consistent with, though does not necessarily
imply, that the opacity in the outer layers of the radiative interior
at temperatures of up to 4 million K is underestimated by 20 percent.
Title: The effect of a nonspherical sound speed on the acoustic
frequency spectrum of the Sun.
Authors: Kosovichev, A. G.; Perdang, J.
Bibcode: 1988ESASP.286..539K
Altcode: 1988ssls.rept..539K
The authors study the effect on the solar acoustic frequency spectrum
of a localised deviation from spherical symmetry concentrated near the
outer convective zone of the sun. The local asymmetry is modelled by
an angular-dependent contribution to the sound speed. Calculations of
the acoustic frequencies in the range 1500 to 4000 μHz have been made
for a nonspherical deformation of relative amplitude ɛ varying from
0 to 0.1. For ɛ > 0.02 all acoustic frequencies of this range are
strongly and irregularly displaced (quantum chaos). For lower values of
ɛ only a small fraction of the frequencies are irregularly shifted. The
deviation of the frequency spectrum of the deformed solar model with
respect to the spectrum of a spherically symmetric model is measured by
a relative scatter σ(ɛ). The relative scatter σobs of the
current observed solar frequencies with respect to the frequencies of
a standard spherically symmetric solar model is found to correspond to
an amplitude of the disturbance ɛ = 0.025. The results are indicative
that even slight local asymmetries influence the precise positions
of the frequencies. They suggest the possibility of a diagnosis of
nonspherical effects in the structure of the sun.
Title: Determination of the solar speed by an asymptotic inversion
technique.
Authors: Kosovichev, A. G.
Bibcode: 1988ESASP.286..533K
Altcode: 1988ssls.rept..533K
A new asymptotic inversion technique is developed using a weighted
least-squares bicubic spline fit to the observational data. The accuracy
of the asymptotic approach is discussed, and it is shown that the
asymptotic theory is adequate to determine the sound speed in the
solar interior between 0.4 and 0.9 R. The inversion of the data of
Duval et al. (1988) shows that the sound speed between 0.4 and 0.7
R is about 1% greater than it is in the standard solar model 1 of
Christensen-Dalsgaard (1982); this result is in agreement with the
previous asymptotic inversion of Christensen-Dalsgaard et al. (1985)
and also that of Christensen-Dalsgaard et al. (1988). It is also
consistent with the inversions presented by Gough and Kosovichev (1988)
using a kernel function approach.
Title: An Asymptotic Solution of the Inverse Problem of
Helioseismology for the Internal Differential Rotation of the Sun
Authors: Kosovichev, A. G.; Parchevskii, K. V.
Bibcode: 1988SvAL...14..201K
Altcode:
No abstract at ADS
Title: The Structure of Thermal Waves in High-Temperature Solar-Flare
Plasma
Authors: Kosovichev, A. G.
Bibcode: 1988SvAL...14..243K
Altcode:
No abstract at ADS
Title: The structure of thermal waves in a high temperature flare
plasma
Authors: Kosovichev, A. G.
Bibcode: 1988PAZh...14..569K
Altcode:
A numerical simulation of thermal waves which have been observed
in the high-temperature plasma (Te ≡ 107K,
ne ≡ 1010cm-3) are carried
out. Heat flow saturation and ion heating have been taken into
account. It is shown that in the case of high energy input rate (with
the characteristic time τ0 ⪉ 1 s) the thermal wave is
a running temperature pulse with a steep front. Otherwise, in the
case of gradual energy input (τ0 ⪆ 3 s), the thermal
wave with a monotonous temperature profile is formed. The calculated
velocities of the thermal waves correspond to the observed values of
(900 - 1600) km/s at the energy input of 3×1030erg on the
timescale τ0 = (7 - 10) s.
Title: An asymptotic solution to the inverse problem of
helioseismology for determining the internal differential rotation
of the sun
Authors: Kosovichev, A. G.; Parchevskii, K. V.
Bibcode: 1988PAZh...14..473K
Altcode:
The dependence of angular velocity versus radius and latitude in the
solar interior can be obtained from the frequency splitting of 5 minute
oscillations by solving two-dimensional integral Abels equation, if
the observational data are presented as a function of (l+1/2)/σ and
(l+1/2)/m.
Title: The determination of the angular velocity of the sun's inner
rotation using helioseismological data
Authors: Kosovichev, A. G.
Bibcode: 1988PAZh...14..344K
Altcode:
Duvall and Harvey's (1984) and Brown and Morrow's (1987) data of the
rotational frequency splitting between solar acoustic ("5-minute")
modes were analyzed. The author determined the angular velocity near
the equatorial plane for 0.06 ≤ r/R ≤ 0.9 and radial variations
of the latitudinal differential rotation for 0.4 ≤ r/R ≤ 0.9. The
latitudinal differential rotation is found to be much smaller in
the radiative zone than in the convective zone. A method for the
observational data reduction is suggested providing a more accurate
determination of the solar rotation.
Title: The Internal Rotation of the Sun from Helioseismological Data
Authors: Kosovichev, A. G.
Bibcode: 1988SvAL...14..145K
Altcode:
No abstract at ADS
Title: Conditions of plasma layers formation with enhanced electric
current density and temperature in the Sun's atmosphere
Authors: Boiko, A. Ya.; Kosovichev, A. G.; Popov, Yu. P.; Sokolov,
V. S.
Bibcode: 1988BCrAO..79....9B
Altcode: 1990BCrAO..79....9B
A finite-difference numerical method is used to study nonlinear
development of MHD thermal instability (or superheating instability)
in one-dimensional approximation. The authors consider: (1) growth of
small perturbations in unstable quasistationary current layers, and
(2) the development of instability occurring due to interaction of a
shock wave with the magnetic field (in case of strong perturbation). It
is found that the intensive Joule heating in the perturbations leads
to self-supported high-temperature current layers (so-called T-layers
). Under conservation of total magnetic flux the instability leads to
more than hundredfold increase of temperature of plasma in a free-force
magnetic field. The authors suggest that this is the case in solar
flares. It has been found that typical thickness of the T-layers is
about 103 - 105cm, assuming an isotropic Coulomb
resistivity. Thus, the authors suggest that high resistivity of solar
plasma inferred from observations can be caused by turbulence resulting
from the MHD thermal instability.
Title: Nonlinear mode coupling in oscillating stars. III. Amplitude
limitingeffect of the rotation in the Delta Scuti stars.
Authors: Dziembowski, W.; Krolikowska, M.; Kosovichev, A.
Bibcode: 1988AcA....38...61D
Altcode:
The authors derive the amplitude equations describing the three-mode
coupling in the presence of rotation. The formalism is applied to
calculate the amplitudes of the low order acoustic modes at the onset
of the parametric instability to the growth of gravity modes. Numerical
calculations made for a ZAMS star model show that significant lowering
of these amplitudes occurs for Vrot > 20 km/s. This
may explain observed absence of high amplitude pulsators in rapidly
rotating stars.
Title: An Attempt to Determine the Structure of the Solar Core from
Observed G-Mode Frequencies
Authors: Kosovichev, A. G.
Bibcode: 1988IAUS..123..141K
Altcode:
Useful information about the stratification of the solar core (0 <
r < 0.3 R) can be obtained from inversions of g-mode frequencies
(a range of periods 100 - 200 min) using Tikhonov's standard form of
regularization theory.
Title: Inertial Modes Trapped in the Solar Convective Envelope
Authors: Dziembowski, W.; Kosovichev, A.; Kozlowski, M.
Bibcode: 1988IAUS..123..117D
Altcode:
No abstract at ADS
Title: Asymptotic Solution Of The Inverse Problem Of Helioseismology
For Determining The Internal Differential Rotation Of The Sun
Authors: Kosovichev, A. G.; Parchevsky, K. V.
Bibcode: 1988AZh....14..473K
Altcode:
No abstract at ADS
Title: On conditions of plasma layers formation with enhanced electric
current density and temperature in the atmosphere of the Sun.
Authors: Bojko, A. Ya.; Kosovichev, A. G.; Popov, Yu. P.; Sokolov,
V. S.
Bibcode: 1988IzKry..79...11B
Altcode:
A finite-difference numerical method is used to study the nonlinear
development of a MHD thermal instability (or superheating instability)
in one-dimensional approximation. The authors consider: (1) growth of
small perturbations in unstable quasi-stationary current layers, and (2)
the development of instability occurring due to interaction of a shock
wave with the magnetic field (in case of strong perturbation). It
is found that the intensive Joule heating in the perturbations
leads to self-supported high-temperature current layers (so called
T-layers). Under conservation of total magnetic flux the instability
leads to more than hundredfold increase of temperature of plasma in a
force-free magnetic field. The authors suggest that this is the case in
solar flares. It has been found that typical thickness of the T-layers
is about 103 - 105 cm, assuming an isotropic
Coulomb resistivity. Thus, it is suggested that high resistivity of
solar plasma inferred from observations can be caused by turbulence
resulting from the MHD thermal instability.
Title: Spatial structure of normal-mode solar oscillations
Authors: Gorkin, L. B.; Kosovichev, A. G.
Bibcode: 1988BCrAO..80..151G
Altcode: 1990BCrAO..80..151G
The physical properties and geometric structure of normal hydrodynamic
modes of three types - acoustic (p), internal gravity (g), and
inertial (r) - are described qualitatively. Regions in which
they are trapped inside the Sun and properties of the frequency
spectra are indicated. Distributions of the radial velocities of the
oscillations are plotted in projection onto the Sun's disk, and the
spatial frequencies that characterize the oscillation sensitivity
of observations of spectral-line Doppler shifts in the whole-disk
emission and the Doppler-signal difference between a central zone and
limb zones symmetric about the center of the disk are calculated.
Title: Determining the internal rotation of the Sun from the frequency
splitting of acoustic modes
Authors: Kosovichev, A. G.
Bibcode: 1988BCrAO..80..167K
Altcode: 1990BCrAO..80..167K
A method for determination of the Sun's angular velocity of rotation
as a function of radius and latitude has been developed on the basis of
the WKB approximation and Abel's inversion. Existing observational data
were used to find the angular velocities in the plane of the equator at
0.06 ≤ r/R ≤0.9 and the latitudinal differential rotation at 0.4
≤ r/R ≤0.9. It is established that the latitudinal differential
rotation decreases in the radiative zone and that a criterion of
hydrodynamic instability of the rotation is satisfied in the outer
part of the radiative zone.
Title: Heat-source energetics in high-temperature flare plasma
Authors: Andreev, A. S.; Kosovichev, A. G.
Bibcode: 1988BCrAO..78..153A
Altcode: 1989BCrAO..78..153A
The release and transfer of energy in the high-temperature
plasma (Te ≡ 107K, ne ≡
1010cm-3) during solar flares is modeled
numerically in a one-dimensional nonstationary approximation with
allowance for heat flux saturation effects and heating of the ion
component of the plasma. It is shown that when energy release is rapid
with a characteristic time τ0 ⪉ 1 sec, thermal energy
propagates in the form of a traveling temperature pulse with a steep
leading edge. In the case of gradual energy release (τ0
⪆ 3 sec), a thermal wave with a monotonic temperature distribution
is formed. The average propagation velocities of the wave are found
as functions of the amount of energy and the release rate. It is found
that the 900 - 1600 km sec-1 velocities measured are obtained
when an energy of (2.3 - 3.5)·1030erg is released during 6 -
10 sec. A qualitative comparison is made with observations of certain
characteristics of the plasma's X-ray emission.
Title: Spatial structures of normal modes of solar oscillations.
Authors: Gorkin, L. B.; Kosovichev, A. G.
Bibcode: 1988IzKry..80..157G
Altcode:
Physical properties and geometrical structures of three kinds of
hydrodynamic oscillations: acoustic (p), internal gravity (g) and
inertial (r) modes are discussed. Resonant cavities in the solar
interior and characteristics of frequency spectra of the modes are
described. Patterns of Doppler velocities, projected onto the solar
disk, are considered. Spatial filter functions, characterizing the
sensitivity of the whole disk observations and the differential velocity
measurements are calculated.
Title: Energetics of heat source in high-temperature plasma of
solar flares.
Authors: Andreev, A. S.; Kosovichev, A. G.
Bibcode: 1988IzKry..78..140A
Altcode:
A numerical simulation of energy input and transfer in high-temperature
plasma (Te ≡ 107K, ne ≡
1010cm-3) are carried out by using one-dimensional
non-stationary approximation. Heat flow saturation and ion heating have
been taken into account. It is shown that in the case of high input
rate (with the characteristic time τ0 ⪉ 1 s) thermal
energy is transferred by a running temperature pulse with a steep
front. Otherwise, in the case of gradual energy input (τ0
⪆ 3 s), the thermal wave with a monotonous temperature profile is
formed. The average velocity of thermal wave as a function of total
energy and input rate was calculated. The observed velocities 900 -
1600 km s-1 correspond to a total energy input (2.5 -
3.5)×1030erg during 7 - 10 s. A qualitative comparison
is made between the numerical results and the observed parameters of
X-ray radiation during solar flares.
Title: Nonlinear effects of acoustic oscillations in the Sun
Authors: Kosovichev, A. G.
Bibcode: 1987BCrAO..76..188K
Altcode: 1989BCrAO..76..188K
No abstract at ADS
Title: Low frequency oscillations in slowly rotating stars. I. General
properties.
Authors: Dziembowski, W.; Kosovichev, A.
Bibcode: 1987AcA....37..313D
Altcode:
The case of general nonuniform rotation is considered. Some of the
results, however, are applicable only to spherical rotation. Partial
differential equations for adiabatic oscillations are reduced to a
system of ordinary equations by means of a truncated spherical-harmonic
expansion. Asymptotic solutions are obtained and used in the discussion
of mode properties and classification. These solutions are also
employed in the numerical method of a quantitative treatment of the
problem. It is pointed out that approximations used in previous studies
of quasi-toroidal modes are not generally valid in whole stellar models,
and this refers also to the case of the uniform rotation.
Title: On the possibility of rapid rotation of the solar core.
Authors: Kotov, V. A.; Kosovichev, A. G.
Bibcode: 1987IzKry..77...72K
Altcode:
The authors discuss a conjecture that the central core of the Sun
rotates plausibly with very short period near 160m. The
rotational splitting of low degree p-modes oscillations was calculated
for the standard solar model, but with the inner core of various size
spinning with the 160m period. The results of calculations
agree with the actual splitting of l = 1, 2 and 3 modes inferred by
Duvall et al. (1984) from observations of 5m-oscillations,
if one assumes that the radius of a rapidly spinning core is less than
0.08 R_sun;. Such small core contains about 6% of the total solar
mass. The resulting gravitational quadrupole moment J2
of the Sun, ⪉4×10-6, does not contradict the results of
observational data on solar oblateness.
Title: Low Frequency Oscillations in Slowly Rotating Stars - Part
Two - Inertial Modes in the Solar Convective Envelope
Authors: Dziembowski, W.; Kosovichev, A.; Kozlowski, M.
Bibcode: 1987AcA....37..331D
Altcode:
Due to their efficient trapping in the outer layers the inertial
oscillations may reach observable amplitudes even if they have energies
as low as those of the individual "five minute" modes. The authors
present results of eigenfrequencies and eigenvectors calculations that
should facilitate spectroscopic search for such oscillations.
Title: Nonlinear effects of acoustic oscillations in the Sun.
Authors: Kosovichev, A. G.
Bibcode: 1987IzKry..76..179K
Altcode:
A finite-difference method for the adiabatic equations of gas dynamics
was used to study nonlinear interactions of radial acoustic modes
(p1_0 - p30) in the Sun. It is shown that
nonlinear effects generate a low frequency oscillation and high
frequency oscillations whose frequencies are the sums of the acoustic
mode frequencies. It has been found that although the period of
low frequency oscillation may be close to 160 min, the amplitudes
of acoustic modes must be 102 - 103 times the
observed ones independently of the number of interacting modes and their
phases. Thus the author tends to conclude that the 160-min oscillation
cannot be explained in terms of nonlinear effects of acoustic mode
oscillations in the Sun.
Title: Numeric calculations of thermal waves in the solar corona
Authors: Andreev, A. S.; Kosovichev, A. G.
Bibcode: 1987BCrAO..76..195A
Altcode: 1989BCrAO..76..195A
No abstract at ADS
Title: Numerical calculations of thermal waves in the solar corona.
Authors: Andreev, A. S.; Kosovichev, A. G.
Bibcode: 1987IzKry..76..186A
Altcode:
A numerical simulation of thermal waves observed in high-temperature
coronal plasma during solar flares has been carried out. A heat
flux saturation and energy transfer from electrons to ions are
shown to be two principal physical factors responsible fot thermal
waves propagation. The former prevails, mainly, during the first few
second of flare energy release, the latter is more significant in the
succedding period of time. The dependence of thermal wave velocity on
initial thermodynamic state of plasma has been determined. It is found
that the mean velocity depends on density rather than on the intitial
temperature. The calculations show a good qualitative agreement with
the observational data, and make it possible to estimate the parameters
of coronal plasma in magnetic arch structures.
Title: Low Frequency Oscillations in Slowly Rotating Stars - Part
Three - Kelvin-Helmholtz Instability
Authors: Dziembowski, W.; Kosovichev, A.
Bibcode: 1987AcA....37..341D
Altcode:
Adiabatic instabilities of stellar rotation to nonaxisymmetric
perturbations are investigated. Exact instability criteria are obtained
assuming various forms of the longitudinal differential rotation. It
is shown that the forms determined for the sun imply an instability,
but limitation of the formalism when applied to the convective zone
are pointed out.
Title: On the possibility of rapid rotation of the solar core
Authors: Kotov, V. A.; Kosovichev, A. G.
Bibcode: 1987BCrAO..77...80K
Altcode: 1989BCrAO..77...80K
No abstract at ADS
Title: Interpretations of the 160-minute solar oscillation
Authors: Kosovichev, A. G.; Severnyj, A. B.
Bibcode: 1986PAZh...12..238K
Altcode:
Identification of solar oscillation with the 160-min period as a
non-radial mode of the sun has been considered on the basis of the
observational data recently available from several observatories. The
amplitudes and phases observed allow the most probable identification
of such an oscillation as a mode with degree l = 3 of a spherical
harmonic. On the other hand however, the observed rotational splitting
of the eigenfrequency due to solar rotation gives l = 1. New methods
for future observations are considered to permit more reliable
identification of the 160-min oscillations.
Title: Interpretations of the 160-MINUTE Solar Oscillation
Authors: Kosovichev, A. G.; Severnyi, A. B.
Bibcode: 1986SvAL...12...97K
Altcode:
No abstract at ADS
Title: Possible effects on the sun and close binary systems from
background gravitational radiation with period 160 min.
Authors: Kosovichev, A. G.
Bibcode: 1986BCrAO..75...30K
Altcode: 1987BCrAO..75...30K
No abstract at ADS
Title: Solution of an inverse helioseismological problem from
observations on solar gravitational oscillations.
Authors: Kosovichev, A. G.
Bibcode: 1986BCrAO..75...36K
Altcode: 1987BCrAO..75...36K
No abstract at ADS
Title: Processing photodiode-array solar-oscillation observations.
Authors: Kosovichev, A. G.
Bibcode: 1986BCrAO..75...19K
Altcode: 1987BCrAO..75...19K
No abstract at ADS
Title: Solution of the inverse problem of helioseismology on the
basis of observations of gravity-mode oscillations of the sun.
Authors: Kosovichev, A. G.
Bibcode: 1986IzKry..75...40K
Altcode:
A technique for inverting the observed frequencies of solar
g-mode oscillations is considered. The inversion procedure gives
a change in density distribution with respect to an original solar
model. Calculations based on the Crimean solar oscillations data show
possible fine-scale density structures in the solar core. However, at
present one cannot draw a final conclusion on the solar core structure
by reason of low accuracy of the data. The results of calculations
with an artificial data set argue that significant corrections to the
original solar model will be possible, when the more extensive and
precise data are available on the g-modes.
Title: Simulating thermal and gasdynamic processes in solar-flare
pulse phases
Authors: Kosovichev, A. G.
Bibcode: 1986BCrAO..75....6K
Altcode: 1987BCrAO..75....6K; 1986BuCri..75....6K
No abstract at ADS
Title: Mathematical method of analysis of solar oscillation data
obtained from a photodiode array.
Authors: Kosovichev, A. G.
Bibcode: 1986IzKry..75...22K
Altcode:
A method is presented for the determination of eigenmodes of
oscillation of the sun in the brightness fluctuations which can be
measured by a photodiode array. Two cases of the observations are
considered. First, the solar disc image covers all pixels of the square
diode array. Second, the array covers the entire solar disc, in which
case a part of the array is used to measure the brightness. It is shown
that the second scheme is more suitable for detection of the modes
of oscillation of the sun. The selection of the modes specified by a
spherical harmonic is carried out. The optimal sets of the coefficients
for a diode array of 16×16 pixels and the spherical harmonics with
degree l ≤ 20 are calculated. Resolving powers of the method are
determined for some harmonics.
Title: On possible effects of background gravitational waves with
a period of 160 min on the sun and close binaries.
Authors: Kosovichev, A. G.
Bibcode: 1986IzKry..75...33K
Altcode:
The resonant effect of background (cosmological) gravitational
radiation on the sun and binary systems is calculated. The upper
limits to the amplitude of the 160-min solar pulsation and to the
changes in the periods of the orbital rotation of the binary systems
are established. When the energy density of the gravitational radiation
does not exceed that of a closed Universe, the effects associated with
the gravitational waves are much less than the ones observed.
Title: Numerical simulations of gas-dynamical and thermal processes
in the impulsive phase of solar flares.
Authors: Kosovichev, A. G.
Bibcode: 1986IzKry..75....8K
Altcode:
Nonlinear time-dependent numerical simulation of the gas-dynamical
and thermal processes in the solar atmosphere heated by non-thermal
electrons is carried out. The structure of the shock wave in the
chromosphere and the thermal instability of hot flare plasma are
investigated. A thin zone of thermal relaxation (with thickness Δz
≈ 2×105cm) is found to be formed behind the shock
front. In this zone the temperature and density of plasma vary
from T ≈ 3×105K, n ≈ 1013cm-3
to T ≈ 9×103K, n ≈ 1015cm-3
because of strong radiative energy losses. During the relaxation
process thin cold condensations (Δz ≈ 103cm) arise
behind the shock front as a result of thermal instability. The life
time of these structures approximately equals to 10-2s. They
disappear when the thermal balance is set in the surrounding plasma. The
formation of a cold and dense structure (Δz ≈ 5×105cm,
T ≈ 9×103K, n ≈ 2×1013cm-3) in
high-temperature plasma due to the thermal instability is investigated
in detail.
Title: Excitation of oscillations in a stellar encounter
Authors: Kosovichev, A. G.; Severnyi, A. B.
Bibcode: 1985BCrAO..70...13K
Altcode: 1987BCrAO..70...13K
No abstract at ADS
Title: Chemical composition effects on the stability of the Sun's
natural gravitational oscillations.
Authors: Kosovichev, A. G.; Severnyj, A. B.
Bibcode: 1985BCrAO..72..162K
Altcode: 1985BuCri..72..162K; 1987BCrAO..72..162K
No abstract at ADS
Title: Excitation of stellar oscillations during close encounters
Authors: Kosovichev, A. G.; Severnyi, A. B.
Bibcode: 1985IzKry..70...14K
Altcode:
The oscillations arising at close encounters of stars are considered
with the aid of numerical methods in linear adiabatic approximation. It
is shown that two types of oscillatory motions appear due to the
action of tidal force. The first one is a normal mode described by
a spherical harmonic of degree l = 2 and order m = 0. The second one,
the more important type of oscillation of several times higher amplitude
(l = 2, m = +2) is a running wave propagating in the direction of the
parabolic motion of the disturbing star. The spectral distribution
of amplitudes is investigated for polytropic stellar models with
the index of polytropes N = 2, 2.5, 3, 3.25, 3.5 and for different
periastron distances. It is suggested that the 160-min oscillations
of the sun can be a relict phenomenon of encounter of the sun with a
star, or a cloud, of small (1/5 of solar) mass at the earlier stages
of solar evolution when it was among the stars of an early-type star
cluster. It is also pointed out that the oscillations arising at close
encounters are very similar to those of β Cep stars.
Title: Influence of the chemical composition on the stability of
solar gravity mode oscillations.
Authors: Kosovichev, A. G.; Severnyi, A. B.
Bibcode: 1985IzKry..72..188K
Altcode:
Solar models with low heavy elements abundance interior (model C) and
with mixing in the core are calculated. It is suggested that mixing
leads to more homogeneous distribution of hydrogen concentration in the
core compared with the standard model. The non-radial oscillations
stability of these models are investigated by quasi-adiabatic
approximation. For models C g1- and g2-modes
belonging to dipole (l = 1) oscillations are found unstable due to
the ɛ-mechanism. It means that the mechanism of nonlinear resonance
excitation of long-period (over the range 120 - 200m)
solar oscillations for this model is confirmed. The instability of a
gravity mode is also revealed in the mixing models when the ratio of
hydrogen abundance at the centre and the surface equals approximately
to 0.7 - 0.8. It is pointed out that only the models with low Z are
in agreement with the solar neutrino experiment and the observed solar
gravity mode oscillations.
Title: On the influence of gravitational radiation on the 160-min
solar oscillation.
Authors: Kosovichev, A. G.
Bibcode: 1985IzKry..73...15K
Altcode:
The resonance amplitude of solar oscillations forced by gravitational
waves is calculated in connection with the recent idea, that the 160-min
oscillation might be caused by gravitational waves from the nearby
binary system Geminga. It is found that the maximum possible amplitude
of the oscillation is at least 103 times smaller than the
observed value. The author concludes that the 160-min oscillation in
the sun cannot be excited by gravitational waves emitted from the binary
system Geminga but by any other source. He calculates the gravitational
wave luminosity of the sun due to the 160-min oscillation and finds
that the influence of the gravitational radiation on the damping of
the oscillation is negligible.
Title: The Effects of Gravitational Radiation on the Solar 160-Min
Pulsations
Authors: Kosovichev, A. G.
Bibcode: 1985BCrAO..72...13K
Altcode:
No abstract at ADS
Title: The stability of solar gravity-mode oscillations and the
structure of the sun
Authors: Kosovichev, A. G.; Severnyj, A. B.
Bibcode: 1984PAZh...10..679K
Altcode:
The g-mode oscillations stability for the interior of solar models
with low heavy-element abundances and models with turbulent diffusion
mixing inside is investigated by a quasi-adiabatic approximation. The
models with low Z are found to be most unstable. It is pointed out that
only the models with low Z are in agreement with the solar neutrino
experiment and the observed solar gravity mode oscillations.
Title: The stability of solar gravity-mode oscillations and the
structure of the sun
Authors: Kosovichev, A. G.; Severnyj, A. B.
Bibcode: 1984SvAL...10..284K
Altcode:
No abstract at ADS
Title: Instability of non-radial g-mode oscillations of the sun with
low-Z interior
Authors: Kosovichev, A. G.; Severnyi, A. B.
Bibcode: 1984MmSAI..55..129K
Altcode:
A quasi-adiabatic approximation is used to calculate the stability of
nonradial g-mode oscillations in evolutionary solar models with initial
heavy-element abundances (Z0) = 0.02 and 0.001. The model parameters
and the calculated periods and damping times are presented in tables,
and a graph of the evolution of growth rates is provided. In the
model with Z0 = 0.02, low g-modes with l = 1 are unstable during the
main-sequence phase from 250 Myr to 3.8 Gyr, but all modes are stable
(except for the l = 1 g2-mode) in the present. In the model with Z0 =
0.001, however, the g1 and g2 modes with l = 1 are unstable due to a
destabilization epsilon mechanism, and the amplitude growth time is
of the order 10 Myr. These results suggest that high-degree g-modes
are resonantly excited.
Title: The 160-minute solar pulsations are not excited by
gravitational waves
Authors: Kosovichev, A. G.
Bibcode: 1984PAZh...10..457K
Altcode:
Theoretical calculations are carried out of solar oscillations caused
by gravitational waves from a binary system such as the gamma ray
emitter Geminga. The waves would raise tides in the sun as they
passed through it. The analytical model describes fluctuations in
the curvature tensor, due to the gravitational waves. The waves are
treated in terms of a time delayed reduced quadrupole mass-moment
tensor emitted from the center of mass of the binary system. Account is
taken of the complex spherical harmonics in the waves. An equation is
obtained for the forced adiabatic solar oscillation and its amplitude,
and to dissipation of the pulsational energy in the sun. It is shown
that the peak amplitude of the projected oscillation will be at least
a thousand times smaller than that of the solar 160 min oscillations,
so Geminga cannot be causing the oscillations.
Title: The 160-MINUTE Solar Pulsations are not Excited by
Gravitational Waves
Authors: Kosovichev, A. G.
Bibcode: 1984SvAL...10..190K
Altcode:
No abstract at ADS
Title: On the Stability of Solar Gravity Mode Oscillations and the
Structure of the Sun
Authors: Kosovichev, A. G.; Severny, A. B.
Bibcode: 1984LIACo..25..278K
Altcode: 1984tpss.conf..278K; 1984trss.conf..278K
No abstract at ADS
Title: Numerical Analysis of Nonlinear Radial Pulsations of Stars
Authors: Kosovichev, A. G.
Bibcode: 1984BCrAO..69..101K
Altcode: 1986BCrAO..69...93B
No abstract at ADS
Title: A method for the numerical calculation of the nonlinear radial
pulsations of stars
Authors: Kosovichev, A. G.
Bibcode: 1984IzKry..69..108K
Altcode:
The use of the finite difference method to calculate the nonlinear
radial pulsations of stars is examined. The mathematical model of these
pulsations is described by equations of time-dependent gravitational
gas dynamics in the one-dimensional (spherically symmetric) case. A
two-parameter family of fully conservative difference schemes is
obtained; these schemes provide for a more precise calculation of
nonlinear flows with shocks than other difference schemes of the same
order of approximation. Methods for the numerical solution of implicit
(absolutely stable) difference schemes from the family considered
are discussed.
Title: Stellar oscillations triggered by close encounters
Authors: Kosovichev, A. G.; Severnyj, A. B.
Bibcode: 1983PAZh....9..424K
Altcode:
The oscillations arising at close encounters of stars are considered
with the aid of numerical methods in linear adiabatic approximations. It
is shown that two types of oscillatory motions appear due to the action
of tidal forces. The first is a normal mode described by the spherical
harmonic of degree l = 2 and of order m = 0. The second more important
type of oscillation, with a several times higher amplitude (l = 2, |m| =
2) is a running wave, propagating in the direction of parabolic motion
of the disturbing star. It is suggested that the 160-min oscillations
of the sun can be a relict phenomenon of encounter of the sun with a
low-mass star or a cloud at the earlier stage of solar evolution. It
is also pointed out that the oscillations arising at close encounters
are very similar to those of β Cephei stars.
Title: Stellar Oscillations Triggered by Close Encounters
Authors: Kosovichev, A. G.; Severnyi, A. B.
Bibcode: 1983SvAL....9..223K
Altcode:
No abstract at ADS
Title: On the Excitation of Oscillations of the Sun - Numerical Models
Authors: Kosovichev, A. G.; Severny, A. B.
Bibcode: 1983SoPh...82..323K
Altcode: 1983IAUCo..66..323K
Numerical solutions of the general time-dependent gas-dynamical
equations in linear adiabatic approximation are given for initial
conditions imitating: (a) a central perturbation, (b) a boundary
perturbation (in the convective envelope), and (c) a `shrinking' of
the Sun as a whole. For a variety of models of the Sun it is found
that at the surface the radial component vr of velocity is
much greater than the tangential component vt, and that the
period T of stationary oscillations does not exceed 131m. The
appearance at the surface of a g mode with period 160m
is found to be improbable.
Title: The Optical Continuum of Solar and Stellar Flares
Authors: Livshits, M. A.; Badalian, O. G.; Kosovichev, A. G.; Katsova,
M. M.
Bibcode: 1981SoPh...73..269L
Altcode:
A further development of the Kostyuk-Pikelner's model is presented. The
response of the chromosphere heated by non-thermal electrons of
the power-law energy spectrum has been studied on the basis of the
numerical solution of the one-dimensional time-dependent equations
of gravitational gas dynamics. The ionization and energy loss for
the emissions in the Lyman and Balmer lines have been determined
separately for the optically thin and thick Lα-line layers. Due to
the initial heating, a higher-pressure region is formed. From this
region, disturbances propagate upwards (a shock wave with a velocity
of more than 1000 km s-1) and downwards. A temperature
jump propagates downwards, and a shock is formed in front of the
thermal wave. During a period of several seconds after the beginning
of this process, the temperature jump intensifies the downward shock
wave and the large radiative loss gives rise to the high density jump
(ϱ2/ϱ1 ∼ 100). The numerical solution has
been analyzed in detail for the case heating of the ionized and neutral
plasma, and a value of this heating is close to the upper limit of the
admissible values. In this case, the condensation located between the
temperature jump and the shock wave front, may emit in the observed
optical continuum.
Title: On the excitation of oscillations in the sun
Authors: Kosovichev, A. G.; Severnyj, A. B.
Bibcode: 1981PAZh....7..304K
Altcode:
The nonstationary equations of gas dynamics are solved numerically to
analyze radial adiabatic oscillations of the sun that result from local
perturbations of the thermodynamic parameters in the energy-generating
core region of the sun. Calculations are performed for a polytropic
sphere and a standard solar model. Results show that a spherical
wave produced by the perturbation will travel from the center to the
surface and back, while undergoing successive reflections from the lower
boundary of the atmosphere and from the center of the sun. Calculated
oscillations in the radiation flux and the oscillation power spectrum
are compared with observed solar pulsations.
Title: On the Excitation of Oscillations in the Sun
Authors: Kosovichev, A. G.; Severnyi, A. B.
Bibcode: 1981SvAL....7..168K
Altcode:
No abstract at ADS
Title: Origin of the optical continuum of flares on red dwarfs
Authors: Katsova, M. M.; Kosovichev, A. G.; Livshits, M. A.
Bibcode: 1981Afz....17..285K
Altcode:
Gas dynamic processes in the chromosphere of a red dwarf, on which
a beam of accelerated electrons with a power spectrum is incident
for 10 sec, are considered. The ionization and radiation losses of
hydrogen are determined separately for layers that are transparent
and opaque in the line L-alpha. A discontinuity of the temperature
propagates downward in the chromosphere, and a shock wave with high
compression is formed in front of it. A dense region is formed at a
height of about 1500 km with a density greater than 10 to the 15th/cu
cm and a T of 9000 K; this region expands from a thickness of 1 km
to 10 km. White radiation in the flares results when the dense region
becomes strongly opaque in the Balmer lines. The Balmer losses cannot
compensate for the heating, and a rise in the temperature in the dense
region leads to the appearance of the optical continuum.
Title: Origin of the Optical Continuum of Flares on Red Dwarfs
Authors: Katsova, M. M.; Kosovichev, A. G.; Livshits, M. A.
Bibcode: 1981Ap.....17..156K
Altcode:
No abstract at ADS
Title: Quasiperiodic oscillations in the solar atmosphere
Authors: Kosovichev, A. G.; Popov, Yu. P.
Bibcode: 1981BCrAO..63...15K
Altcode: 1981BuCri..63...15K
No abstract at ADS
Title: On quasi-periodic oscillations in the solar atmosphere.
Authors: Popov, Iu. P.; Kosovichev, A. G.
Bibcode: 1981IzKry..63...15P
Altcode:
A numerical solution of the nonlinear equations of one-dimensional
gas dynamics is used to examine the propagation of perturbations in
the convective zone and atmosphere of the sun. The inhomogeneity of
the solar plasma in the gravitational field leads to the generation
of oscillations in the atmosphere with a period of approximately 300
sec. The effect of nonlinearity on the character of the oscillations is
clarified, and the present results are compared with experimental data.
Title: The nature of the optical continuum of red-dwarf flares
Authors: Katsova, M. M.; Kosovichev, A. G.; Livshits, M. A.
Bibcode: 1980PAZh....6..498K
Altcode:
The gas-dynamical processes that would result from heating of
a partially ionized, hydrogen stellar chromosphere by accelerated
electrons are investigated numerically. The radiative energy loss in the
optically thick layer is estimated. A condensation should, according
to the calculation, be propagated downward in the chromosphere: it
would have a density n = 10 to the 15th - 10 to the 16th per cu cm,
temperature T = 900 K, thickness 1-10 km, and optical depth tau (4500)
= 1. The luminosity, color indices, and Balmer jump for the optical
continuum of this condensation are consistent with observations of
flares in red dwarf stars.
Title: The Nature of the Optical Continuum of Red Dwarf Flares
Authors: Katsova, M. M.; Kosovichev, A. G.; Livshits, M. A.
Bibcode: 1980SvAL....6..275K
Altcode:
No abstract at ADS
Title: Calculation of one-dimensional unsteady-state problems of
gravitational gasdynamics
Authors: Kosovichev, A. G.; Popov, Iu. P.
Bibcode: 1979ZVMMF..19.1253K
Altcode:
Some features of the numerical solution of one-dimensional
unsteady-state problems of gasdynamics in the presence of gravity
are examined. The problem of shock wave propagation in the solar
atmosphere is analyzed, showing the effectiveness of applying completely
conservative finite-difference schemes to problems of this type. The
process of periodic generation of shock waves in the atmosphere in
the presence of monotonic dynamic effects at its lower boundary is
identified.
Title: Gas-dynamical processes under the action of heating of the
chromosphere by accelerated electrons.
Authors: Kosovichev, A. G.; Livshits, M. A.; Badalyan, O. G.
Bibcode: 1979ATsir1069....1K
Altcode:
No abstract at ADS
Title: Nonlinear stage of instability due to local Joule-overheating
in the solar active regions.
Authors: Sokolov, V. S.; Kosovichev, A. G.
Bibcode: 1978SoPh...57...73S
Altcode:
The numerical solution by a computer of the system of
magnetohydrodynamics equations in the one-dimensional approximation
serves as the basis for studying the non-linear stage of the instability
due to local Joule-overheating of zones with large values of magnetic
field gradients in the active regions of the Sun. We have demonstrated
the formation of a system of current layers responsible for efficient
transformation of magnetic energy into Joule heat and kinetic energy
of the macroscopic motion. The specific features of quasi-stationary
skinning of magnetic field with gravitation have been noted.
Title: Skinning process stability of the magnetic field in the solar
active regions.
Authors: Sokolov, V. S.; Katsnel'Son, S. S.; Katsnelson, S. S.;
Kosovichev, A. G.; Slavin, V. S.
Bibcode: 1977SoPh...51..293S
Altcode:
Skinning process stability of the magnetic field in homogeneous
plasma is studied. A set of magnetohydrodynamic equations is
used. Dependence of electrical conductivity on the plasma parameters
and radiation intensity in grey-body approximation are taken into
account. The investigation is carried out on the model problems
in linear approximation and by means of numerical solution of MHD
equations. Threshold of stability and critical gradient of magnetic
field in skin-layer are obtained. The model of the phenomenon proposed
in the paper indicates on overheating instability of plasma with
electric current in large gradient magnetic field zones as a possible
trigger mechanism of solar flare origin.
Title: On a model of chromospheric flares on the sun.
Authors: Sokolov, V. S.; Kosovichev, A. G.; Slavin, V. S.
Bibcode: 1977mmns.conf..216S
Altcode:
No abstract at ADS
Title: On the stability of the magnetic field skin zone in the solar
lower chromosphere plasma.
Authors: Kosovichev, A. G.; Sokolov, V. S.
Bibcode: 1975AehIs...5...17K
Altcode:
No abstract at ADS