explanation blue bibcodes open ADS page with paths to full text
Author name code: georgobiani
ADS astronomy entries on 2022-09-14
author:"Georgobiani, Dali"
---------------------------------------------------------
Title: Realistic numerical simulations of solar convection: emerging
flux, pores, and Stokes spectra
Authors: Georgobiani, D.; Stein, R.; Nordlund, A.
2012IAUSS...6E.102G Altcode:
We report on magneto-convection simulations of magnetic flux
emerging through the upper layers of the solar convection zone into
the photosphere. Simulations by Georgobiani, Stein and Nordlund start
from minimally structured, uniform, untwisted horizontal field advected
into the computational domain by supergranule scale inflows at 20 Mm
depth. At the opposite extreme, simulations by Cheung (2007, 2008,
2011) start with a coherent flux tube inserted into or forced into
the bottom of the computational domain. Several robust results have
emerged from the comparison of results from these two very different
initial states. First, rising magnetic flux gets deformed into
undulating, serpentine shapes by the influence of the convective up-
and down-flows. The flux develops fine structure and appears at the
surface first as a "pepper and salt" pattern of mixed polarity. Where
magnetic flux approaches the surface, granules become darker and
elongated in the direction of the field. Subsequently, the underlying
large scale magnetic structures make the field collect into unipolar
regions. Magneto-convection produces a complex, small-scale magnetic
field topology, whatever the initial state. A heirarchy of magnetic
loops corresponding to the different scales of convective motions are
produced. Vertical vortex tubes form at intergranule lane vertices which
can lead to tornado-like magnetic fields in the photosphere. Gradients
in field strength and velocity produce asymmetric Stokes spectra. Where
emerging Omega loops leave behind nearly vertical legs, long lived
pores can spontaneously form. The field in the pores first becomes
concentrated and evacuated near the surface and the evacuated flux
concentration then extends downward.
---------------------------------------------------------
Title: Helioseismic Data from Emerging Flux Simulations
Authors: Stein, R. F.; Lagerfjärd, A.; Nordlund, Å.; Georgobiani, D.
2012ASPC..462..345S Altcode:
Data from solar magneto-convection emerging flux simulations is
available for validating helioseismic inversion procedures. In these
simulations a uniform, untwisted, horizontal magnetic field is advected
by inflows at the bottom of the domain 48 Mm wide by 20 Mm deep and
rises to the surface. The evolution for different field strengths at
20 Mm depth has been investigated. The field emerges first in a mixed
polarity pepper and salt pattern, but then collects into separate,
unipolar concentrations and when enough flux has reached the surface,
pores are produced. In one case the field strength was artificially
increased and then the pores grew into spot-like structures with
penumbral-like borders. The online data consists of slices of vertical
and horizontal velocity and magnetic field strength at continuum
optical depths of 0.01, 0.1 and 1 as well as the emergent intensity
at one minute intervals plus four hour averages (with 2 hour cadence)
of the three-dimensional (3D) density, velocity, temperature, energy,
sound speed and magnetic field. The data can be found as links from the
web page: http://steinr.pa.msu.edu/∼bob/data.html. These calculation
were performed on the supercomputers of the NASA Advanced Supercomputing
Division and were supported by grants from NASA and NSF.
---------------------------------------------------------
Title: Emerging Flux Simulations
Authors: Stein, R. F.; Lagerfjärd, A.; Nordlund, Å.; Georgobiani, D.
2012ASPC..454..193S Altcode:
We simulate the rise through the upper convection zone and emergence
through the solar surface of initially uniform, untwisted, horizontal
magnetic flux that is advected into a domain 48 Mm wide by 20 Mm deep,
with the same entropy as the non-magnetic plasma. The magnetic field
is transported upward by the diverging upflows and pulled down in
the downdrafts, which produces a hierarchy of loop-like structures of
increasingly smaller scale as the surface is approached. 20 kG fields at
the bottom significantly modify the convective flows, leading to long
thin cells of ascending fluid aligned with the magnetic field. Their
magnetic buoyancy makes them rise to the surface faster than the
fluid rise time. A large scale magnetic loop is produced that, as
it emerges through the surface, leads to the formation of a bipolar
pore-like structure.
---------------------------------------------------------
Title: Emerging Flux Simulations and Proto-Active Regions
Authors: Stein, R. F.; Lagerfjärd, A.; Nordlund, &.; Georgobiani, D.
2012ASPC..455..133S Altcode: 2011arXiv1102.1049S
The emergence of minimally structured (uniform and horizontal) magnetic
field from a depth of 20 Mm has been simulated. The field emerges first
in a mixed polarity pepper and salt pattern, but then collects into
separate, unipolar concentrations and produces pores. The field strength
was then artificially increased to produce spot-like structures. The
field strength at continuum optical depth unity peaks at 1 kG, with
a maximum of 4 kG. Where the vertical field is strong, the spots
persist (at present an hour of solar time has been simulated). Where
the field is weak, the spot gets filled in and disappears. Stokes
profiles have been calculated and processed with the Hinode annular
Modulation Transfer Function, the slit diffraction, and frequency
smoothing. These data are available at steinr.pa.msu.edu/∼bob/stokes.
---------------------------------------------------------
Title: Photospheric Magnetic Fields from Magneto-Convection
Simulations
Authors: Stein, Robert F.; Nordlund, Aake; Georgobiani, Dali
2012decs.confE..95S Altcode:
We present the properties of photospheric magnetic fields from
magneto-convection simulations and as they would be observed by Hinode,
for both quiet Sun and plage regions. This will include statistical
properties, morphology, Stokes spectra, energy fluxes and correlations
with convection dynamics. The rate of flux emergence will be discussed
as a constraint on model parameters.
---------------------------------------------------------
Title: Magnetic Fields: Modeling And ATST Observations
Authors: Stein, Robert F.; Georgobiani, D.; Nordlund, A.; Lagerfjard,
A.
2011SPD....42.0804S Altcode: 2011BAAS..43S.0804S
We have performed magneto-convection simulations starting from
snapshots of hydrodynamic convection with initial conditions both
of uniform vertical magnetic field and with minimally structured
(uniform, untwisted), horizontal magnetic field advected into
the computational domain from a depth of 20 Mm. One clear result
is that while the magnetic field can collect into large-scale
concentrations - pores and sunspots - most of the magnetic flux is
in small concentrations with steep horizontal gradients in the field
and plasma properties. Furthermore, the field strength distribution
is a power law with slope between -1 and -2, so most of the field
at the surface is weak. A large aperture telescope, such as ATST, is
needed both to collect sufficient photons to measure the ubiquitous
weak fields and to resolve the small-scale magnetic features. <P
/>We present results on flux emergence, pore formation, and Stokes
spectra as they would appear in Hinode and ATST compared with the
raw simulation.For those interested in analyzing the simulation data,
it is available online at steinr.pa.msu.edu/ bob/data.html. There are
slices of the velocity and magnetic field vectors at continuum optical
depths of 1, 0.1, and 0.01 and the emergent intensity have been saved
at 1 minute intervals. Four hour averages, with 2 hour cadence for the
3D cube for variables: velocity, magnetic field, density, temperature,
sound speed, and internal energy have been computed. Stokes spectra
have been computed for the Hinode FeI 630 nm lines, processed with the
Hinode annular mtf, the slit diffraction and frequency smoothing. <P
/>This work has been supported by NASA grants NNX07AO71G, NNX07AH79G and
NNX08AH44G and NSF grant AST0605738. The simulations where performed
on the Pleiades cluster of the NASA Advanced Supercomputing Division
at the Ames Research Center.
---------------------------------------------------------
Title: Solar Flux Emergence Simulations
Authors: Stein, R. F.; Lagerfjärd, A.; Nordlund, Å.; Georgobiani, D.
2011SoPh..268..271S Altcode: 2009arXiv0912.4938S; 2010SoPh..tmp...34S
We simulate the rise through the upper convection zone and emergence
through the solar surface of initially uniform, untwisted, horizontal
magnetic flux, with the same entropy as the nonmagnetic plasma,
that is advected into a domain 48 Mm wide by 20 Mm deep. The magnetic
field is advected upward by the diverging upflows and pulled down in
the downdrafts, which produces a hierarchy of loop-like structures
of increasingly smaller scale as the surface is approached. There are
significant differences between the behavior of fields of 10 kG and 20
or 40 kG strength at 20 Mm depth. The 10 kG fields have little effect
on the convective flows and show small magnetic-buoyancy effects,
reaching the surface in the typical fluid rise time from 20 Mm depth
of 32 hours. 20 and 40 kG fields significantly modify the convective
flows, leading to long, thin cells of ascending fluid aligned with
the magnetic field and their magnetic buoyancy makes them rise to the
surface faster than the fluid rise time. The 20 kG field produces a
large-scale magnetic loop that as it emerges through the surface leads
to the formation of a bipolar, pore-like structure.
---------------------------------------------------------
Title: Supergranule Scale Flux Emergence Simulations
Authors: Stein, Robert F.; Lagerfjard, A.; Nordlund, A.; Georgobiani,
D.
2010AAS...21621103S Altcode:
We simulate the rise of initially horizontal, untwisted magnetic flux
from 20 Mm depth through the near surface convection to the solar
surface in a domain 48 Mm wide. The magnetic field is transported
upward by diverging upflows aided by magnetic buoyancy, and pushed
down by downdrafts, which produces a hierarchy of loop like structures,
of increasingly smaller scale as the surface is approached. We compare
two cases with field strengths of 5 and 20 kG at 20 Mm depth. In the
stronger field strength case, the magnetic field significantly disturbs
the convection below 3 Mm, inhibiting the vertical motion, shutting
off convective energy transport and producing elongated cellular
structures in the field direction. Shallower than 3 Mm the convection
appears normal, but with concentrated vertical magnetic concentrations
("flux tubes") extending through the surface and producing pores where
the field is greatest. Even in the weaker field case, the magnetic
field inhibits vertical motions and the convective transport of
energy although the convective cellular pattern is not significantly
distorted. This work was supported by NSF grant AST065738 and NASA
grants NNX08AH44G, NNX07AH79G and NNX07AO71G. The simulations were
performed at the NASA Advanced Supercomputing Division of the Ames
Research Center.
---------------------------------------------------------
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.
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: Supergranulation Scale Convection Simulations
Authors: Stein, R. F.; Lagerfjård, A.; Nordlund, Å.; Georgobiani,
D.; Benson, D.; Schaffenberger, W.
2009ASPC..415...63S Altcode:
Results of realistic simulations of solar surface convection on
the scale of supergranules (48 and 96 Mm wide by 20 Mm deep) are
presented. The simulations include the hydrogen, first and most
of the second helium ionization zones. Horizontal magnetic field is
advected into the domain by upflows at the bottom. Upflows stretch the
field lines upward, while downflows push them down, thus producing
loop like magnetic structures. The mass mixing length is 1.8 scale
heights. Two thirds of the area is upflowing fluid except very close
to the surface. The internal (ionization) energy flux is the largest
contributor to the convective flux for temperatures less than 40,000
K and the thermal energy flux is the largest contributor at higher
temperatures. The data is available for evaluating local helioseismic
procedures.
---------------------------------------------------------
Title: Solar Magneto-Convection Simulations
Authors: Stein, Robert F.; Lagerfjard, A.; Nordlund, A.; Benson, D.;
Georgobiani, D.; Schaffenberger, W.
2009SPD....40.0401S Altcode:
We present preliminary results of magneto-convection simulations
of the rise of initially horizontal magnetic flux from 20 Mm deep
through the solar surface in a domain 48 Mm wide. The magnetic field
is stretched upward in the diverging upflows and pulled down in the
downdrafts which produces a hierarchy of loop like structures. The
strength varies with depth as the square root of the density. The field
is swept to the boundaries of small supergranular like structures to
form a magnetic network.
---------------------------------------------------------
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.
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: Supergranulation Scale Convection Simulations
Authors: Stein, Robert F.; Georgobiani, Dali; Schafenberger, Werner;
Nordlund, Åke; Benson, David
2009AIPC.1094..764S Altcode: 2009csss...15..764S
Results of realistic simulations of solar surface convection on the
scale of supergranules (96 Mm wide by 20 Mm deep) are presented. The
simulations cover only 10% of the geometric depth of the solar
convection zone, but half its pressure scale heights. They include the
hydrogen, first and most of the second helium ionization zones. The
horizontal velocity spectrum is a power law and the horizontal
size of the dominant convective cells increases with increasing
depth. Convection is driven by buoyancy work which is largest close
to the surface, but significant over the entire domain. Close to the
surface buoyancy driving is balanced by the divergence of the kinetic
energy flux, but deeper down it is balanced by dissipation. The
damping length of the turbulent kinetic energy is 4 pressure scale
heights. The mass mixing length is 1.8 scale heights. Two thirds of the
area is upflowing fluid except very close to the surface. The internal
(ionization) energy flux is the largest contributor to the convective
flux for temperatures less than 40,000 K and the thermal energy flux
is the largest contributor at higher temperatures.
---------------------------------------------------------
Title: Supergranulation Scale Connection Simulations
Authors: Stein, R. F.; Nordlund, A.; Georgobiani, D.; Benson, D.;
Schaffenberger, W.
2008arXiv0811.0472S Altcode:
Results of realistic simulations of solar surface convection on the
scale of supergranules (96 Mm wide by 20 Mm deep) are presented. The
simulations cover only 10% of the geometric depth of the solar
convection zone, but half its pressure scale heights. They include the
hydrogen, first and most of the second helium ionization zones. The
horizontal velocity spectrum is a power law and the horizontal
size of the dominant convective cells increases with increasing
depth. Convection is driven by buoyancy work which is largest close
to the surface, but significant over the entire domain. Close to the
surface buoyancy driving is balanced by the divergence of the kinetic
energy flux, but deeper down it is balanced by dissipation. The
damping length of the turbulent kinetic energy is 4 pressure scale
heights. The mass mixing length is 1.8 scale heights. Two thirds of the
area is upflowing fluid except very close to the surface. The internal
(ionization) energy flux is the largest contributor to the convective
flux for temperatures less than 40,000 K and the thermal energy flux is
the largest contributor at higher temperatures. This data set is useful
for validating local helioseismic inversion methods. Sixteen hours
of data are available as four hour averages, with two hour cadence,
at steinr.msu.edu/~bob/96averages, as idl save files. The variables
stored are the density, temperature, sound speed, and three velocity
components. In addition, the three velocity components at 200 km above
mean continuum optical depth unity are available at 30 sec. cadence.
---------------------------------------------------------
Title: Surface Convection
Authors: Stein, Robert F.; Benson, David; Georgobiani, Dali; Nordlund,
Åke; Schaffenberger, Werner
2007AIPC..948..111S Altcode:
What are supergranules? Why do they stand out? Preliminary results from
realistic simulations of solar convection on supergranule scales (96 Mm
wide by 20 Mm deep) are presented. The solar surface velocity amplitude
is a decreasing power law from the scale of granules up to giant cells
with a slight enhancement at supergranule scales. The simulations show
that the size of the horizontal convective cells increases gradually
and continuously with increasing depth. Without magnetic fields
present there is, as yet, no enhancement at supergranule scales at the
surface. A hypothesis is presented that it is the balance between the
rate of magnetic flux emergence and the horizontal sweeping of magnetic
flux by convective motions that determines the size of the magnetic
network and produces the extra power at supergranulation scales.
---------------------------------------------------------
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.
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: Application of convection simulations to oscillation excitation
and local helioseismology
Authors: Stein, Robert F.; Benson, David; Georgobiani, Dali; Nordlund,
Åke
2007IAUS..239..331S Altcode:
No abstract at ADS
---------------------------------------------------------
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
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: 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
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: Excitation of solar-like oscillations across the HR diagram
Authors: Samadi, R.; Georgobiani, D.; Trampedach, R.; Goupil, M. J.;
Stein, R. F.; Nordlund, Å.
2007A&A...463..297S Altcode: 2006astro.ph.11762S
Aims:We extend semi-analytical computations of excitation rates for
solar oscillation modes to those of other solar-like oscillating stars
to compare them with recent observations <BR />Methods: Numerical
3D simulations of surface convective zones of several solar-type
oscillating stars are used to characterize the turbulent spectra
as well as to constrain the convective velocities and turbulent
entropy fluctuations in the uppermost part of the convective zone of
such stars. These constraints, coupled with a theoretical model for
stochastic excitation, provide the rate P at which energy is injected
into the p-modes by turbulent convection. These energy rates are
compared with those derived directly from the 3D simulations. <BR
/>Results: The excitation rates obtained from the 3D simulations
are systematically lower than those computed from the semi-analytical
excitation model. We find that P<SUB>max</SUB>, the P maximum, scales as
(L/M)<SUP>s</SUP> where s is the slope of the power law and L and M are
the mass and luminosity of the 1D stellar model built consistently
with the associated 3D simulation. The slope is found to depend
significantly on the adopted form of χ_k, the eddy time-correlation;
using a Lorentzian, χ_k^L, results in s=2.6, whereas a Gaussian,
χ_k^G, gives s=3.1. Finally, values of V_max, the maximum in the mode
velocity, are estimated from the computed power laws for P_max and we
find that V<SUB>max</SUB> increases as (L/M)<SUP>sv</SUP>. Comparisons
with the currently available ground-based observations show that the
computations assuming a Lorentzian χ<SUB>k</SUB> yield a slope, sv,
closer to the observed one than the slope obtained when assuming a
Gaussian. We show that the spatial resolution of the 3D simulations
must be high enough to obtain accurate computed energy rates.
---------------------------------------------------------
Title: Spatial and Temporal Spectra of Solar Convection
Authors: Georgobiani, D.; Stein, R. F.; Nordlund, Å.
2006ASPC..354..109G Altcode:
Recent observations support the theory that solar-type oscillations
are stochastically excited by turbulent convection in the outer
layers of the solar-like stars. The acoustic power input rates
depend on the details of the turbulent energy spectrum. <P />We
use numerical simulations to study the spectral properties of solar
convection. We find that spatial turbulent energy spectra vary at
different temporal frequencies, while temporal turbulent spectra show
various features at different spatial wavenumbers, and their best fit
at all frequencies is a generalized power law Power = Amplitude ×
(frequency^2 + width^2)^{-n(k)}, where n(k) depends on the spatial
wavenumber. Therefore, it is impossible to separate the spatial and
temporal components of the turbulent spectra.
---------------------------------------------------------
Title: Supergranule scale convection simulations
Authors: Stein, R. F.; Benson, D.; Georgobiani, D.; Nordlund, Å.
2006ESASP.624E..79S Altcode: 2006soho...18E..79S
No abstract at ADS
---------------------------------------------------------
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.
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: Time-distance analysis of realistic simulations of solar
convection
Authors: Georgobiani, D.; Zhao, J.; Benson, D.; Stein, R. F.;
Kosovichev, A. G.; Nordlund, A.
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: Excitation of Solar-like Oscillations: From PMS to MS Stellar
Models
Authors: Samadi, R.; Goupil, M. -J.; Alecian, E.; Baudin, F.;
Georgobiani, D.; Trampedach, R.; Stein, R.; Nordlund, Å.
2005JApA...26..171S Altcode:
The amplitude of solar-like oscillations results from a balance between
excitation and damping. As in the sun, the excitation is attributed
to turbulent motions that stochastically excite the p modes in the
upper-most part of the convective zone. We present here a model for the
excitation mechanism. Comparisons between modeled amplitudes and helio
and stellar seismic constraints are presented and the discrepancies
discussed. Finally the possibility and the interest of detecting
such stochastically excited modes in pre-main sequence stars are
also discussed.
---------------------------------------------------------
Title: Excitation of P-Modes in the Sun and Stars
Authors: Stein, Robert; Georgobiani, Dali; Trampedach, Regner; Ludwig,
Hans-Günter; Nordlund, Åke
2005HiA....13..411S Altcode:
We describe the stochastic excitation of p-mode oscillations by solar
convection. We discuss the role of Reynolds stresses and entropy
fluctuations what controls the excitation spectrum the depth of the
driving and the location of the driving. We then present results for
a range of other stars and discuss the similarities and differences
with the Sun.
---------------------------------------------------------
Title: Excitation rates of p modes: mass luminosity relation across
the HR diagram
Authors: Samadi, R.; Georgobiani, D.; Trampedach, R.; Goupil, M. J.;
Stein, R. F.; Nordlund, Å.
2004sf2a.conf..323S Altcode: 2004astro.ph.10043S
We compute the rates P at which energy is injected into the p modes for
a set of 3D simulations of outer layers of stars. We found that Pmax
- the maximum in P - scales as (L/M)^s where s is the slope of the
power law, L and M are the luminosity and the mass of the 1D stellar
models associated with the simulations. The slope is found to depend
significantly on the adopted representation for the turbulent eddy-time
correlation function, chi_k. According to the expected performances
of COROT, it will likely be possible to measure Pmax as a function
of L/M and to constrain the properties of stellar turbulence as the
turbulent eddy time-correlation.
---------------------------------------------------------
Title: High Degree Solar Oscillations in 3d Numerical Simulations
Authors: Georgobiani, D.; Stein, R. F.; Nordlund, Å.; Kosovichev,
A. G.; Mansour, N. N.
2004ESASP.559..267G Altcode: 2004soho...14..267G
No abstract at ADS
---------------------------------------------------------
Title: Oscillation Power Spectra of the Sun and of CEN a: Observations
Versus Models
Authors: Samadi, R.; Goupil, M. J.; Baudin, F.; Georgobiani, D.;
Trampedach, R.; Stein, R.; Nordlund, A.
2004ESASP.559..615S Altcode: 2004astro.ph..9325S; 2004soho...14..615S
Hydrodynamical, 3D simulations of the outer layers of the Sun and Alpha
Cen A are used to obtain constraints on the properties of turbulent
convection in such stars. These constraints enable us to compute -
on the base of a theoretical model of stochastic excitation - the
rate P at which p modes are excited by turbulent convection in those
two stars. Results are then compared with solar seismic observations
and recent observations of Alpha Cen A. For the Sun, a good agreement
between observations and computed P is obtained. For Alpha Cen A a
large discrepancy is obtained which origin cannot be yet identified:
it can either be caused by the present data quality which is not
sufficient for our purpose or by the way the intrinsic amplitudes and
the life-times of the modes are determined or finally attributed to
our present modelling. Nevertheless, data with higher quality or/and
more adapted data reductions will likely provide constraints on the
p-mode excitation mechanism in Alpha Cen A.
---------------------------------------------------------
Title: Turbulence Convection and Oscillations in the Sun
Authors: Mansour, N. N.; Kosovichev, A. G.; Georgobiani, D.; Wray,
A.; Miesch, M.
2004ESASP.559..164M Altcode: 2004soho...14..164M
No abstract at ADS
---------------------------------------------------------
Title: Excitation of Radial P-Modes in the Sun and Stars
Authors: Stein, Robert; Georgobiani, Dali; Trampedach, Regner; Ludwig,
Hans-Günter; Nordlund, Åke
2004SoPh..220..229S Altcode:
P-mode oscillations in the Sun and stars are excited stochastically
by Reynolds stress and entropy fluctuations produced by convection in
their outer envelopes. The excitation rate of radial oscillations of
stars near the main sequence from K to F and a subgiant K IV star have
been calculated from numerical simulations of their surface convection
zones. P-mode excitation increases with increasing effective temperature
(until envelope convection ceases in the F stars) and also increases
with decreasing gravity. The frequency of the maximum excitation
decreases with decreasing surface gravity.
---------------------------------------------------------
Title: What Causes p-Mode Asymmetry Reversal?
Authors: Georgobiani, Dali; Stein, Robert F.; Nordlund, Åke
2003ApJ...596..698G Altcode: 2002astro.ph..5141G
The solar acoustic p-mode line profiles are asymmetric. Velocity spectra
have more power on the low-frequency sides, whereas intensity profiles
show the opposite sense of asymmetry. Numerical simulations of the
upper convection zone have resonant p-modes with the same asymmetries
and asymmetry reversal as the observed modes. The temperature
and velocity power spectra at optical depth τ<SUB>cont</SUB>=1
have the opposite asymmetry, as is observed for the intensity and
velocity spectra. At a fixed geometrical depth, corresponding to
<τ<SUB>cont</SUB>>=1, however, the temperature and velocity
spectra have the same asymmetry. This indicates that the asymmetry
reversal in the simulation is produced by radiative transfer effects and
not by correlated noise. The cause of this reversal is the nonlinear
amplitude of the displacements in the simulation and the nonlinear
dependence of the H<SUP>-</SUP> opacity on temperature. Where the
temperature is hotter the opacity is larger and photons escape from
higher, cooler layers. This reduces the fluctuations in the radiation
temperature compared to the gas temperature. The mode asymmetry reversal
in the simulation is a small frequency-dependent differential effect
within this overall reduction. Because individual solar modes have
smaller amplitudes than the simulation modes, this effect will be
smaller on the Sun.
---------------------------------------------------------
Title: Understanding the convective Sun
Authors: Trampedach, Regner; Georgobiani, Dali; Stein, Robert F.;
Nordlund, Åke
2003ESASP.517..195T Altcode: 2003soho...12..195T
Hydrodynamical simulations of the surface layers of the Sun, has greatly
improved our understanding and interpretation of solar observations. I
review some past successes in matching spectral lines, improving the
agreement with high-degree p-mode frequencies and matching the depth of
the solar convection zone without adjustable convection-parameters. Our
solar simulations contain p-modes, and are used for studying the
asymmetry of p-mode peaks and to calibrate the conversion between the
observed velocity proxies and the actual velocities.
---------------------------------------------------------
Title: Asymmetry reversal in solar acoustic modes
Authors: Georgobiani, Dali; Stein, Robert F.; Nordlund, Åke
2003ESASP.517..279G Altcode: 2003soho...12..279G
The power spectra of solar acoustic modes are asymmetric, with velocity
having more power on the low frequency side of the peak and intensity
having more power on the high frequency side. This effect exists in both
observations and simulations, and it is believed to be caused by the
correlated background noise. We study the temperature near the solar
surface by means of a 3D hydrodynamic simulation of convection with a
detailed treatment of radiation. The temperature spectrum at optical
depth τ<SUB>cont</SUB> = 1 has opposite asymmetry to the velocity
spectrum, whereas the temperature measured at a fixed geometrical depth,
corresponding to <τ<SUB>cont</SUB>> = 1, has the same asymmetry
as velocity. We believe that the asymmetry reversal in temperature
at τ<SUB>cont</SUB> = 1 (and therefore in intensity) occurs partly
because of the radiative transfer effects. High temperature sensitivity
of the opacity suppresses temperature fluctuations on opposite sides
of the mode peaks differently, thus causing the asymmetry reversal.
---------------------------------------------------------
Title: Solar and Stellar Oscillations
Authors: Stein, Robert; Nordlund, Aake; Georgobiani, Dali; Trampedach,
Regner; Ludwig, Hans-Guenther
2003IAUJD..12E..41S Altcode:
We describe the stochastic excitation of p-mode oscillations by solar
convection. We discuss the role of Reynolds stresses and entropy
fluctuations what controls the excitation spectrum the depth of the
driving and the location of the driving. We then present results for
a range of other stars and discuss the similarities and differences
with the Sun.
---------------------------------------------------------
Title: Models of the solar oscillations
Authors: Georgobiani, Dali; Stein, Robert F.; Nordlund, Aake
2001ESASP.464..583G Altcode: 2001soho...10..583G
The shallow upper layer of the solar convection zone is simulated
using the three-dimensional hydrodynamic code of Stein &
Nordlund. The simulation oscillation modes behave similarly to
the SOHO/MDI observations; namely, they have the same asymmetries
and phase relations. Therefore, one can study the properties of the
modes from the simulations to investigate behavior below the surface,
which cannot be observed directly. The asymmetry of the line profiles
varies with depth. At the surface, the velocity asymmetry is the same
as in the SOHO/MDI observations, but deeper down it becomes flipped in
comparison to the surface asymmetry. This behavior is well represented
by the simple model of a potential well with the source inside (or
outside). The simulations can be used to determine the depth of the
driving at different frequencies, while the simulation modes show a
strong correlation of excitation with emergent intensity.
---------------------------------------------------------
Title: A Least-squares Solution for the Effective Conductivity of
the Solar Convection Zone
Authors: Kuhn, J. R.; Georgobiani, D.
2000SSRv...94..161K Altcode:
Here we show how realistic numerical simulations of solar convection can
be parameterized with an effective thermal conductivity tensor. We show
that this diffusive approximation yields an accurate statistical (in the
sense of the χ^2 test) description of the thermal transport properties
of a perturbed solar convection zone. This parameterization will allow
more accurate large scale solar irradiance and luminosity calculations.
---------------------------------------------------------
Title: Numerical Simulations of Oscillation Modes of the Solar
Convection Zone
Authors: Georgobiani, D.; Kosovichev, A. G.; Nigam, R.; Nordlund,
Å.; Stein, R. F.
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: A Least-Squares Solution for the Effective Conductivity of
the Solar Convection Zone
Authors: Kuhn, J. R.; Georgobiani, D.
2000svc..book..161K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Realistic Simulations of Solar Surface Convection
Authors: Stein, R. F.; Bercik, D.; Georgobiani, D.; Nordlund, A.
1999AAS...194.2104S Altcode: 1999BAAS...31R.858S
Results from realistic simulations of near surface solar convection
will be summarized and compared with observations. Solar convection
is driven by radiative cooling from an extremely thin surface thermal
boundary layer, which produces low entropy fluid. Its topology is
controlled by mass conservation and consists of turbulent downdrafts
penetrating nearly laminar upflows. The horizontal scales increase with
depth. Good agreement is found with the of the depth of the convection
zone, p-mode frequencies, excitation, line asymmetries and intensity -
velocity phase differences from helioseismology; with observations of
granulation and profiles of weak Fe lines. This work was supported by
grants from NSF, NASA, and the Danish Research Council. The calculations
were performed at NCSA, MSU and UNIC.
---------------------------------------------------------
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.
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: Magneto-Convection
Authors: Stein, R. F.; Georgobiani, D.; Bercik, D. J.; Brandenburg,
A.; Nordlund, Å.
1999ASPC..173..193S Altcode: 1999sstt.conf..193S
No abstract at ADS
---------------------------------------------------------
Title: Solar P-Mode Spectrum Asymmetries: Testing Theories With
Numerical Simulations
Authors: Georgobiani, Dali; Nigam, Rakesh; Kosovichev, Alexander G.;
Stein, Robert F.
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: Solar Magneto-Convection
Authors: Stein, R. F.; Bercik, D. J.; Brandenburg, A.; Georgobiani,
D.; Nordlund, A.
1998AAS...191.7417S Altcode: 1998BAAS...30..758S
We present results of realistic simulations of magneto-convection near
the solar surface. The simulations were performed with two magnetic
field topologies - (1) a unipolar, initially vertical field, and (2)
a bipolar field, where fluid entering at the base of the computational
domain advects in horizontal field. As the unipolar flux is increased,
the magnetic field concentrates in the intergranule lanes and develops
large, dark, cool regions. These regions surround smaller areas where
convection has not been suppressed. In contrast, for the bipolar case,
the strongest fields appear as bright points in the intergranule lanes.
---------------------------------------------------------
Title: Solar Magneto-Convection
Authors: Bercik, David J.; Basu, Shantanu; Georgobiani, Dali; Nordlund,
Ake; Stein, Robert F.
1998ASPC..154..568B Altcode: 1998csss...10..568B
We have simulated magneto-convection near the solar surface
with two topologies: (1) an initial vertical field; and (2) a
horizontal field carried in with the fluid entering at the base
of the computational domain. We report results on the interaction
of convection and magnetic fields. An MPEG video is viewable at:
http://www.pa.msu.edu/~steinr/images/bhoriz.mpg The MPEG video is also
included on the CS10 CD ROM.
---------------------------------------------------------
Title: Heat Transport in the Convective Zone and Deviations from
the Mixing Length Models
Authors: Georgobiani, D.; Kuhn, J. R.; Nordlund, AA.; Stein, R. F.
1998ESASP.418..771G Altcode: 1998soho....6..771G
For several decades, the heat transport in the solar convective
zone has been thought to be isotropic. Attempts to describe it in
terms of the mixing length theory seemed to be quite successful. In
contradiction with such an idealized picture, recent numerical
and observational data have demonstrated a highly non-isotropic,
inhomogeneous structure of the convective zone. This work presents the
results of calculations of the thermal conductivity in the convective
zone, using the numerical model of Stein-Nordlund. Thermal conductivity
is assumed to be a 3D tensor. Its vertical and horizontal diagonal
components differ in magnitudes for each given depth. Moreover, the
horizontal component stays negative, while increasing with depth. Both
features are naturally explained by the physical properties of the
solar convective zone. Implications for global questions of solar
convection are considered.
---------------------------------------------------------
Title: Sound speed variations near the photosphere due to entropy
perturbations in 3d numerical experiments
Authors: Georgobiani, D.; Kuhn, J. R.; Stein, R. F.
1997ASSL..225..127G Altcode: 1997scor.proc..127G
Results on how the temperature distribution near the solar photosphere
is altered by perturbing the entropy of rising fluid in the convection
zone several megameters below the surface, are presented. Effects on
the emergent intensity and implications for helioseismic observations
are described.
---------------------------------------------------------
Title: Solar Convection: Comparison of Numerical Simulations and
Mixing-Length Theory
Authors: Abbett, William P.; Beaver, Michelle; Davids, Barry;
Georgobiani, Dali; Rathbun, Pamela; Stein, Robert F.
1997ApJ...480..395A Altcode:
We compare the results of realistic numerical simulations of convection
in the superadiabatic layer near the solar surface with the predictions
of mixing-length theory. We find that the peak values of such quantities
as the temperature gradient, the temperature fluctuations, and the
velocity fluctuations, as well as the entropy jump in the simulation,
can be reproduced by mixing-length theory for a ratio of mixing length
to pressure scale height α ~ 1.5. However, local mixing-length theory
neither reproduces the profiles of these variables with depth nor allows
penetration of convective motions into the overlying stable photosphere.
---------------------------------------------------------
Title: Sound Speed Variations Near the Photosphere due to Entropy
Perturbations in 3D Numerical Experiments
Authors: Georgobiani, D.; Kuhn, J. R.; Stein, R. F.
1996AAS...188.6910G Altcode: 1996BAAS...28..937G
Results on how the temperature distribution near the solar photosphere
is altered by perturbing the entropy of fluid in the convection zone
several megameters below the surface are presented. Effects on the
emergent intensity and implications for helioseismic observations
are described.
---------------------------------------------------------
Title: Using Eclipse Observations to Test Scintillation Models
Authors: Georgobiani, D.; Kuhn, J. R.; Beckers, J. M.
1995SoPh..156....1G Altcode:
Near second and third contact during a solar eclipse the spatial
spectrum of the solar illumination changes as the relative power
at high spatial frequencies increases strongly. Since groundlevel
atmospheric scintillation depends on a weighted integral of the image
power spectrum, we can expect to see a measureable time dependence
to solar scintillation during an eclipse. This effect was observed
during an annular solar eclipse and quantitatively compared with a
scintillation model.
---------------------------------------------------------
Title: Solar corona photometry by electropolarimetric and CCD
observations during the eclipse of July 11, 1991
Authors: Kulidzanishvili, V. I.; Georgobiani, D. G.
1995AN....316...23K Altcode:
The observational data of the solar corona obtained during the solar
eclipse of July 11, 1993, using both a electropolarimeter (EP) and
a CCD matrix were processed. Using these data the photometry of the
solar corona was carried out. The results of EP data were compared
with those of the CCD data. It must be noticed here that the CCD data
give us only the characteristics of the inner corona, while the EP data
show the features of both the inner and the middle corona up to 4 solar
radii. The standard flattening index epsilon was evaluated from both
data. The dependence of epsilon on the distance from the solar limb
was investigated. Three-dimensional images of the coronal intensity
distribution for different spectral lines are shown. Isophotes in Na
and Ca lines with unusual features are plotted. Based on these data
some ideas and conclusions on the type of the solar corona and the
physical conditions in it are presented.
---------------------------------------------------------
Title: July 11, 1991 Eclipse Corona Photometry by the Results of
Electropolarimetric and CCD-Matrix Observations
Authors: Kulidzaniskvili, V.; Georgobiani, D.
1994scs..conf..567K Altcode: 1994IAUCo.144..567K
The observational data of July 11, 1991 eclipse solar corona obtained
by both electropolarimeter (EP) and CCD-matrix were processed and
compared. The dependence of the flattening index on the distance from
the solar limb is investigated. The isophotes in Na and Ca lines are
plotted. Some ideas and conclusions on the type of the solar corona
are presented.
