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Author name code: shelyag
ADS astronomy entries on 2022-09-14
author:"Shelyag, Sergey I."
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Title: Constraining the magnetic vector in the quiet solar photosphere
and the impact of instrumental degradation
Authors: Campbell, R. J.; Shelyag, S.; Quintero Noda, C.; Mathioudakis,
M.; Keys, P. H.; Reid, A.
2021A&A...654A..11C Altcode: 2021arXiv210701519C
Context. With the advent of next generation high resolution telescopes,
our understanding of how the magnetic field is organized in the
internetwork (IN) photosphere is likely to advance significantly. <BR />
Aims: We aim to evaluate the extent to which we can retrieve accurate
information about the magnetic vector in the IN photosphere using
inversion techniques. <BR /> Methods: We use a snapshot produced from
high resolution three-dimensional magnetohydrodynamic (MHD) simulations
and employ the Stokes Inversions based on Response functions (SIR) code
to produce synthetic observables in the same near infrared spectral
window as observed by the GREGOR Infrared Spectrograph (GRIS), which
contains the highly magnetically sensitive photospheric Fe I line pair
at 15 648.52 Å and 15 652.87 Å. We then use a parallelized wrapper to
SIR to perform nearly 14 million inversions of the synthetic spectra to
test how well the `true' MHD atmospheric parameters can be constrained
statistically. Finally, we degrade the synthetic Stokes vector
spectrally and spatially to GREGOR resolutions and examine how this
influences real observations, considering the impact of stray light,
spatial resolution and signal-to-noise (S/N) in particular. <BR />
Results: We find that the depth-averaged parameters can be recovered
by the inversions of the undegraded profiles, and by adding simple
gradients to magnetic field strength, inclination, and line of sight
velocity we show that an improvement in the χ<SUP>2</SUP> value
is achieved. We also evaluate the extent to which we can constrain
these parameters at various optical depths, with the kinematic and
thermodynamic parameters sensitive deeper in the atmosphere than
the magnetic parameters. We find the S/N and spatial resolution both
play a significant role in determining how the degraded atmosphere
appears. At the same time, we find that the magnetic and kinematic
parameters are invariant upon inclusion of an unpolarized stray
light. We compare our results to recent IN observations obtained
by GREGOR. We studied a linear polarization feature which resembles
those recently observed by GRIS in terms of appearing as `loop-like'
structures and exhibiting very similar magnetic flux density. Thus,
we demonstrate that realistic MHD simulations are capable of showing
close agreement with real observations, and the symbiosis between them
and observations continues to prove essential. We finally discuss the
considerations that must be made for DKIST-era observations.
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Title: Using the Stokes V widths of Fe I lines for diagnostics of
the intrinsic solar photospheric magnetic field
Authors: Gordovskyy, M.; Shelyag, S.; Browning, P. K.; Lozitsky, V. G.
2020A&A...633A.136G Altcode: 2019arXiv191203340G
<BR /> Aims: The goal of this study is to explore a novel method
for the solar photospheric magnetic field diagnostics using Stokes
V widths of different magnetosensitive Fe I spectral lines. <BR
/> Methods: We calculate Stokes I and V profiles of several Fe I
lines based on a one-dimensional photospheric model VAL C using the
NICOLE radiative transfer code. These profiles are used to produce
calibration curves linking the intrinsic magnetic field values with the
widths of blue peaks of Stokes V profiles. The obtained calibration
curves are then tested using the Stokes profiles calculated for
more realistic photospheric models based on magnetohydrodynamic of
magneto-convection. <BR /> Results: It is shown that the developed
Stokes V widths method can be used with various optical and
near-infrared lines. Out of six lines considered in this study,
Fe I 6301 line appears to be the most effective: it is sensitive
to fields over ∼200 G and does not show any saturation up to ∼2
kG. Other lines considered can also be used for the photospheric field
diagnostics with this method, however, only in narrower field value
ranges, typically from about 100 G to 700-1000 G. <BR /> Conclusions:
The developed method can be a useful alternative to the classical
magnetic line ratio method, particularly when the choice of lines
is limited.
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Title: The Effect of the 21 August 2017 Total Solar Eclipse on the
Phase of VLF/LF Signals
Authors: Rozhnoi, A.; Solovieva, M.; Shalimov, S.; Ouzounov, D.;
Gallagher, P.; Verth, G.; McCauley, J.; Shelyag, S.; Fedun, V.
2020E&SS....700839R Altcode:
An experimental study of the phase and amplitude observations
of sub-ionospheric very low and low frequency (VLF/LF) signals is
performed to analyze the response of the lower ionosphere during the 21
August 2017 total solar eclipse in the United States of America. Three
different sub-ionospheric wave paths are investigated. The length of
the paths varies from 2,200 to 6,400 km, and the signal frequencies
are 21.4, 25.2, and 40.75 kHz. The two paths cross the region of
the total eclipse, and the third path is in the region of 40-60%
of obscuration. None of the signals reveal any noticeable amplitude
changes during the eclipse, while negative phase anomalies (from
-33° to -95°) are detected for all three paths. It is shown that
the effective reflection height of the ionosphere in low and middle
latitudes is increased by about 3-5 km during the eclipse. Estimation
of the electron density change in the lower ionosphere caused by
the eclipse, using linear recombination law, shows that the average
decrease is by 2.1 to 4.5 times.
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Title: High-resolution spectropolarimetric observations of the
temporal evolution of magnetic fields in photospheric bright points
Authors: Keys, P. H.; Reid, A.; Mathioudakis, M.; Shelyag, S.;
Henriques, V. M. J.; Hewitt, R. L.; Del Moro, D.; Jafarzadeh, S.;
Jess, D. B.; Stangalini, M.
2020A&A...633A..60K Altcode: 2019arXiv191108436K
Context. Magnetic bright points (MBPs) are dynamic, small-scale
magnetic elements often found with field strengths of the order of a
kilogauss within intergranular lanes in the photosphere. <BR /> Aims:
Here we study the evolution of various physical properties inferred from
inverting high-resolution full Stokes spectropolarimetry data obtained
from ground-based observations of the quiet Sun at disc centre. <BR
/> Methods: Using automated feature-tracking algorithms, we studied
300 MBPs and analysed their temporal evolution as they evolved to
kilogauss field strengths. These properties were inferred using
both the NICOLE and SIR Stokes inversion codes. We employ similar
techniques to study radiative magnetohydrodynamical simulations
for comparison with our observations. <BR /> Results: Evidence was
found for fast (∼30-100 s) amplification of magnetic field strength
(by a factor of 2 on average) in MBPs during their evolution in our
observations. Similar evidence for the amplification of fields is seen
in our simulated data. <BR /> Conclusions: Several reasons for the
amplifications were established, namely, strong downflows preceding
the amplification (convective collapse), compression due to granular
expansion and mergers with neighbouring MBPs. Similar amplification of
the fields and interpretations were found in our simulations, as well
as amplification due to vorticity. Such a fast amplification will have
implications for a wide array of topics related to small-scale fields
in the lower atmosphere, particularly with regard to propagating wave
phenomena in MBPs.
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Title: Stellar Surface Magnetoconvection as a Source of Astrophysical
Noise. III. Sun-as-a-Star Simulations and Optimal Noise Diagnostics
Authors: Cegla, H. M.; Watson, C. A.; Shelyag, S.; Mathioudakis, M.;
Moutari, S.
2019ApJ...879...55C Altcode: 2019arXiv190308446C
Stellar surface magnetoconvection (granulation) creates asymmetries in
the observed stellar absorption lines that can subsequently manifest
themselves as spurious radial velocities (RVs) shifts. In turn, this
can then mask the Doppler reflex motion induced by orbiting planets on
their host stars and represents a particular challenge for determining
the masses of low-mass, long-period planets. Herein, we study this
impact by creating Sun-as-a-star observations that encapsulate
the granulation variability expected from 3D magnetohydrodynamic
simulations. These Sun-as-a-star model observations are in good
agreement with empirical observations of the Sun but may underestimate
the total variability relative to the quiet Sun due to the increased
magnetic field strength in our models. We find numerous line profile
characteristics that linearly correlate with the disk-integrated
convection-induced velocities. Removing the various correlations
with the line bisector, equivalent width, and the V <SUB>asy</SUB>
indicator may reduce ∼50%-60% of the granulation noise in the measured
velocities. We also find that simultaneous photometry may be a key
diagnostic, as our proxy for photometric brightness also allowed us to
remove ∼50% of the granulation-induced RV noise. These correlations
and granulation-noise mitigations break down in the presence of low
instrumental resolution and/or increased stellar rotation, as both
act to smooth the observed line profile asymmetries.
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Title: In situ generation of coronal Alfvén waves by jets
Authors: González-Avilés, J. J.; Guzmán, F. S.; Fedun, V.; Verth,
G.; Sharma, R.; Shelyag, S.; Regnier, S.
2019MNRAS.484.1936G Altcode: 2019MNRAS.tmp...67G; 2018arXiv180704224G
Within the framework of 3D resistive magnetohydrodynamic, we simulate
the formation of a plasma jet with the morphology, upward velocity up
to 130 km s<SUP>-1</SUP>, and time-scale formation between 60 and 90
s after beginning of simulation, similar to those expected for type II
spicules. Initial results of this simulation were published in paper by,
e.g. González-Avilés et al. (2018), and present paper is devoted to
the analysis of transverse displacements and rotational-type motion
of the jet. Our results suggest that 3D magnetic reconnection may be
responsible for the formation of the jet in paper by González-Avilés
et al. (2018). In this paper, by calculating times series of the
velocity components v<SUB>x</SUB> and v<SUB>y</SUB> in different points
near to the jet for various heights we find transverse oscillations in
agreement with spicule observations. We also obtain a time-distance
plot of the temperature in a cross-cut at the plane x = 0.1 Mm and
find significant transverse displacements of the jet. By analysing
temperature isosurfaces of 10<SUP>4</SUP> K with the distribution of
v<SUB>x</SUB>, we find that if the line-of-sight (LOS) is approximately
perpendicular to the jet axis then there is both motion towards and
away from the observer across the width of the jet. This red-blue
shift pattern of the jet is caused by rotational motion, initially
clockwise and anti-clockwise afterwards, which could be interpreted
as torsional motion and may generate torsional Alfvén waves in the
corona region. From a nearly vertical perspective of the jet the LOS
velocity component shows a central blue-shift region surrounded by
red-shifted plasma.
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Title: Flows and magnetic field structures in reconnection regions
of simulations of the solar atmosphere: Do flux pile-up models work?
Authors: Shelyag, S.; Litvinenko, Y. E.; Fedun, V.; Verth, G.;
González-Avilés, J. J.; Guzmán, F. S.
2018A&A...620A.159S Altcode: 2018arXiv180900587S
<BR /> Aims: We study the process of magnetic field annihilation
and reconnection in simulations of magnetised solar photosphere
and chromosphere with magnetic fields of opposite polarities and
constant numerical resistivity. <BR /> Methods: Exact analytical
solutions for reconnective annihilations were used to interpret the
features of magnetic reconnection in simulations of flux cancellation
in the solar atmosphere. We used MURaM high-resolution photospheric
radiative magneto-convection simulations to demonstrate the presence of
magnetic field reconnection consistent with the magnetic flux pile-up
models. Also, a simulated data-driven chromospheric magneto-hydrodynamic
simulation is used to demonstrate magnetic field and flow structures,
which are similar to the theoretically predicted ones. <BR /> Results:
Both simulations demonstrate flow and magnetic field structures
roughly consistent with accelerated reconnection with magnetic flux
pile-up. The presence of standard Sweet-Parker type reconnection is
also demonstrated in stronger photospheric magnetic fields.
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Title: The multi fluid description of the chromospheric motions
Authors: Tkachenko, A.; Shelyag, S.; Krasnosselskikh, V.; Le Phuong, L.
2018sf2a.conf..469T Altcode:
The major diagnostics of solar chromosphere comes from spectroscopic
observations of different spectral lines, which provide the information
about the dynamics of minor ions, using strong lines, associated with
them (observed by SOHO, SDO, IRIS etc.). They indicate the variation
of temperature with altitude and time. On the other hand, general
dynamics of the chromosphere is determined by three main components:
electrons, protons and neutral hydrogen atoms (probably sometimes
by helium component also). We consider the model, similar to plasma
discharge, describing it by the system of three-fluid hydrodynamic
equations for partially ionized plasma in the presence of electric
and magnetic fields. Our description accounts for the processes of
ionization and recombination as well as the dissipation of electric
currents. Minor ions and atoms are included into our description as
impurities, diffusing in the dominant component's gas. Ionization of
minor ions is supposed to happen due to collisions with electrons. To
achieve this goal, we have the numerical code, written by Sergiy
Shelyag and Linh Le Phuong, which is a multi-dimensional parallel
solver of systems of hyperbolic differential equations on an arbitrary
Cartesian grid. It has already been configured to solve the systems of
hydrodynamic, magneto-hydrodynamic and two-fluid magneto-hydrodynamic
(ions + neutrals) equations. The code takes into account ionization,
recombination and collisional processes for non-linear simulations of
solar partially- and fully-ionized plasmas. Actually we are going to
test different properties of the code.
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Title: Analysis of unresolved photospheric magnetic field structure
using Fe I 6301 and 6302 lines
Authors: Gordovskyy, M.; Shelyag, S.; Browning, P. K.; Lozitsky, V. G.
2018A&A...619A.164G Altcode: 2018arXiv180806862G
Context.Early magnetographic observations indicated that the magnetic
field in the solar photosphere has an unresolved small-scale
structure. Near-infrared and optical data with extremely high
spatial resolution show that these structures have scales of a few
tens of kilometres, which are not resolved in the majority of solar
observations. Aims.The goal of this study is to establish the effect of
the unresolved photospheric magnetic field structure on Stokes profiles
observed with relatively low spatial resolution. Ultimately, we aim
to develop methods for fast estimation of the photospheric magnetic
filling factor and line-of-sight gradient of the photospheric magnetic
field, which can be applied to large observational data sets. Methods.We
exploit 3D magnetohydrodynamic models of magneto-convection developed
using the MURAM code. Corresponding profiles of Fe I 6301.5 and
6302.5 Å spectral lines are calculated using the NICOLE radiative
transfer code. The resulting I and V Stokes [x, y, λ] cubes with a
reduced spatial resolution of 150 km are used to calculate magnetic
field values as they would be obtained in observations with the Solar
Optical Telescope (SOT) onboard Hinode or the Helioseismic and Magnetic
Imager (HMI) onboard the Solar Dynamic Observatory (SDO) mission. <BR />
Results: Three different methods of magnetic filling factor estimation
are considered: the magnetic line ratio method, the Stokes V width
method, and a simple statistical method. We find that the statistical
method and the Stokes V width method are sufficiently reliable for
fast filling factor estimations. Furthermore, we find that the Stokes
I ± V bisector splitting gradient can be used for fast estimation of
the line-of-sight gradient of the photospheric magnetic field.
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Title: Stellar Surface Magneto-convection as a Source of Astrophysical
Noise. II. Center-to-limb Parameterization of Absorption Line Profiles
and Comparison to Observations
Authors: Cegla, H. M.; Watson, C. A.; Shelyag, S.; Chaplin, W. J.;
Davies, G. R.; Mathioudakis, M.; Palumbo, M. L., III; Saar, S. H.;
Haywood, R. D.
2018ApJ...866...55C Altcode: 2018arXiv180711423C
Manifestations of stellar activity (such as star-spots, plage/faculae,
and convective flows) are well-known to induce spectroscopic
signals often referred to as astrophysical noise by exoplanet
hunters. For example, setting an ultimate goal of detecting true
Earth analogs demands reaching radial velocity (RV) precisions of
∼9 cm s<SUP>-1</SUP>. While this is becoming technically feasible
with the latest generation of highly stabilized spectrographs, it
is astrophysical noise that sets the true fundamental barrier on
attainable RV precisions. In this paper, we parameterize the impact
of solar surface magneto-convection on absorption line profiles, and
extend the analysis from the solar disk center (Paper I) to the solar
limb. Off disk-center, the plasma flows orthogonal to the granule tops
begin to lie along the line of sight, and those parallel to the granule
tops are no longer completely aligned with the observer. Moreover, the
granulation is corrugated and the granules can block other granules, as
well as the intergranular lane components. Overall, the visible plasma
flows and geometry of the corrugated surface significantly impact the
resultant line profiles and induce center-to-limb variations in shape
and net position. We detail these herein, and compare to various solar
observations. We find our granulation parameterization can recreate
realistic line profiles and induced radial velocity shifts, across
the stellar disk, indicative of both those found in computationally
heavy radiative 3D magnetohydrodynamical simulations and empirical
solar observations.
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Title: Signatures of quiet Sun reconnection events in Ca II, Hα,
and Fe I
Authors: Shetye, J.; Shelyag, S.; Reid, A. L.; Scullion, E.; Doyle,
J. G.; Arber, T. D.
2018MNRAS.479.3274S Altcode: 2018MNRAS.tmp.1509S; 2017arXiv170310968S
We use observations of quiet Sun (QS) regions in the Hα 6563 Å, Ca II
8542 Å, and Fe I 6302 Å lines. We observe brightenings in the wings
of the Hα and Ca II combined with observations of the interacting
magnetic concentrations observed in the Stokes signals of Fe I. These
brightenings are similar to Ellerman bombs (EBs), i.e. impulsive
bursts in the wings of the Balmer lines that leave the line cores
unaffected. Such enhancements suggest that these events have similar
formation mechanisms to the classical EBs found in active regions,
with the reduced intensity enhancements found in the QS regions due to
a weaker feeding magnetic flux. The observations also show that the
quiet Sun Ellerman bombs are formed at a higher height in the upper
photosphere than the photospheric continuum level. Using simulations,
we investigate the formation mechanism associated with the events and
suggest that these events are driven by the interaction of magnetic
field lines in the upper photospheric regions. The results of the
simulation are in agreement with observations when comparing the light
curves, and in most cases, we found that the peak in the Ca II 8542
Å wing occurred before the peak in Hα wing. Moreover, in some cases,
the line profiles observed in Ca II are asymmetrical with a raised core
profile. The source of heating in these events is shown by the MURaM
simulations and is suggested to occur 430 km above the photosphere.
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Title: I. Jet Formation and Evolution Due to 3D Magnetic Reconnection
Authors: González-Avilés, J. J.; Guzmán, F. S.; Fedun, V.; Verth,
G.; Shelyag, S.; Regnier, S.
2018ApJ...856..176G Altcode: 2017arXiv170905066G
Using simulated data-driven, 3D resistive MHD simulations of the solar
atmosphere, we show that 3D magnetic reconnection may be responsible for
the formation of jets with the characteristics of Type II spicules. We
numerically model the photosphere-corona region using the C7 equilibrium
atmosphere model. The initial magnetic configuration is a 3D potential
magnetic field, extrapolated up to the solar corona region from a
dynamic realistic simulation of the solar photospheric magnetoconvection
model that mimics the quiet-Sun. In this case, we consider a uniform
and constant value of the magnetic resistivity of 12.56 Ω m. We
have found that the formation of the jet depends on the Lorentz
force, which helps to accelerate the plasma upward. Analyzing various
properties of the jet dynamics, we found that the jet structure shows
a Doppler shift close to regions with high vorticity. The morphology,
the upward velocity covering a range up to 130 km s<SUP>-1</SUP>,
and the timescale formation of the structure between 60 and 90 s,
are similar to those expected for Type II spicules.
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Title: I. Jet Formation and Evolution due to 3D Magnetic Reconnection
Authors: González, J. J.; Guzmán, F.; Fedun, V.; Verth, G.; Shelyag,
S.; Regnier, S.
2017AGUFMSH43A2807G Altcode:
Using simulated data-driven three-dimensional resistive MHD simulations
of the solar atmosphere, we show that magnetic reconnection can be
responsible of the formation of jets with characteristic of Type
II spicules. For this, we numerically model the photosphere-corona
region using the C7 equilibrium atmosphere model. The initial magnetic
configuration is a 3D potential magnetic field, extrapolated up to
the solar corona region from a dynamic realistic simulation of solar
photospheric magnetoconvection model which is mimicking quiet-Sun. In
this case we consider a uniform and constant value of the magnetic
resistivity of 12.56 Ω m. We have found that formation of the jets
depends on the Lorentz force, which helps to accelerate the plasma
upwards. Analyzing various properties of the jet dynamics, we found
that the jet structure shows Doppler shift near to regions with high
vorticity. The morphology, upward velocity, covering a range up to
100 km s-1, and life-time of the estructure, bigger than 100 s, are
similar to those expected for Type II spicules.
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Title: Project SunbYte: solar astronomy on a budget
Authors: Alvarez Gonzalez, F.; Badilita, A. -M.; Baker, A.; Cho,
Y. -H.; Dhot, N.; Fedun, V.; Hare, C.; He, T.; Hobbs, M.; Javed,
M.; Lovesey, H.; Lord, C.; Panoutsos, G.; Permyakov, A.; Pope, S.;
Portnell, M.; Rhodes, L.; Sharma, R.; Taras, P.; Taylor, J.; Tilbrook,
R.; Verth, G.; Wrigley, S. N.; Yaqoob, M.; Cook, R.; McLaughlin, J.;
Morton, R.; Scullion, E.; Shelyag, S.; Hamilton, A.; Zharkov, S.;
Jess, D.; Wrigley, M.
2017A&G....58d2.24A Altcode:
The Sheffield University Nova Balloon Lifted Solar Telescope (SunbYte)
is a high-altitude balloon experiment devised and run largely by
students at the University of Sheffield, and is scheduled for launch
in October 2017. It was the only UK project in 2016 to be selected for
the balloon side of the Swedish-German student programme REXUS/BEXUS
(Rocket and Balloon Experiments for University Students; see box on
p2.25). The success of the SunbYte team in the REXUS/BEXUS selection
process is an unprecedented opportunity for the students to gain
valuable experience working in the space engineering industry, using
their theoretical knowledge and networking with students and technology
companies from all over Europe.
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Title: The role of partial ionization in solar chromospheric heating
Authors: Shelyag, S.; Khomenko, E.; Przybylski, D.; Vitas, N.; de
Vicente, A.
2016AGUFMSH21E2565S Altcode:
The most energetic part of the Sun, its interior, due to its plasma
parameters is hidden below the solar surface and invisible to the
observer. Nevertheless, the solar interior generates the energy and
provokes atmospheric magnetic activity. Despite great progress in
both observational and simulational methods, the mechanism of energy
transport from the solar convection zone into the upper atmosphere,
and the upper-atmospheric heating mechanism remain the main unresolved
problems in solar and stellar structure. In this presentation, we
analyse the role of non-ideal plasma effects and partial ionization in
the solar atmospheric energy transport and chromospheric heating. Using
numerical magneto-hydrodynamic modelling we create detailed models
of magnetic flux tubes and realistic simulations of the coupled
solar interior and atmosphere with different levels of magnetic
activity, which take into account the effects of partial ionisation
and ion-neutral interaction in the solar atmospheric plasma. We show
that compressible and incompressible oscillations in solar magnetic
fields, indeed, are able to provide sufficient energy to compensate
chromospheric radiative losses. Detailed radiative diagnostics of the
simulated models is carried out to create a link between the simulations
and observational data. This gives an opportunity to directly compare
the simulation results with modern solar observations.
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Title: Modeling the Rossiter-McLaughlin Effect: Impact of the
Convective Center-to-limb Variations in the Stellar Photosphere
Authors: Cegla, H. M.; Oshagh, M.; Watson, C. A.; Figueira, P.;
Santos, N. C.; Shelyag, S.
2016ApJ...819...67C Altcode: 2016arXiv160102054C
Observations of the Rossiter-McLaughlin (RM) effect provide information
on star-planet alignments, which can inform planetary migration and
evolution theories. Here, we go beyond the classical RM modeling
and explore the impact of a convective blueshift that varies across
the stellar disk and non-Gaussian stellar photospheric profiles. We
simulated an aligned hot Jupiter with a four-day orbit about a Sun-like
star and injected center-to-limb velocity (and profile shape) variations
based on radiative 3D magnetohydrodynamic simulations of solar surface
convection. The residuals between our modeling and classical RM modeling
were dependent on the intrinsic profile width and v sin I; the amplitude
of the residuals increased with increasing v sin I and with decreasing
intrinsic profile width. For slowly rotating stars the center-to-limb
convective variation dominated the residuals (with amplitudes of 10
s of cm s<SUP>-1</SUP> to ∼1 m s<SUP>-1</SUP>) however, for faster
rotating stars the dominant residual signature was due a non-Gaussian
intrinsic profile (with amplitudes from 0.5 to 9 m s<SUP>-1</SUP>). When
the impact factor was 0, neglecting to account for the convective
center-to-limb variation led to an uncertainty in the obliquity of
∼10°-20°, even though the true v sin I was known. Additionally,
neglecting to properly model an asymmetric intrinsic profile had a
greater impact for more rapidly rotating stars (e.g., v sin I = 6 km
s<SUP>-1</SUP>) and caused systematic errors on the order of ∼20°
in the measured obliquities. Hence, neglecting the impact of stellar
surface convection may bias star-planet alignment measurements and
consequently theories on planetary migration and evolution.
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Title: Heating of the Partially Ionized Solar Chromosphere by Waves
in Magnetic Structures
Authors: Shelyag, S.; Khomenko, E.; de Vicente, A.; Przybylski, D.
2016ApJ...819L..11S Altcode: 2016arXiv160203373S
In this paper, we show a “proof of concept” of the heating
mechanism of the solar chromosphere due to wave dissipation caused by
the effects of partial ionization. Numerical modeling of non-linear wave
propagation in a magnetic flux tube, embedded in the solar atmosphere,
is performed by solving a system of single-fluid quasi-MHD equations,
which take into account the ambipolar term from the generalized Ohm’s
law. It is shown that perturbations caused by magnetic waves can be
effectively dissipated due to ambipolar diffusion. The energy input
by this mechanism is continuous and shown to be more efficient than
dissipation of static currents, ultimately leading to chromospheric
temperature increase in magnetic structures.
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Title: Alfvén Wave Heating of the Solar Chromosphere: 1.5D Models
Authors: Arber, T. D.; Brady, C. S.; Shelyag, S.
2016ApJ...817...94A Altcode: 2015arXiv151205816A
Physical processes that may lead to solar chromospheric heating
are analyzed using high-resolution 1.5D non-ideal MHD modeling. We
demonstrate that it is possible to heat the chromospheric plasma by
direct resistive dissipation of high-frequency Alfvén waves through
Pedersen resistivity. However, this is unlikely to be sufficient
to balance radiative and conductive losses unless unrealistic field
strengths or photospheric velocities are used. The precise heating
profile is determined by the input driving spectrum, since in 1.5D
there is no possibility of Alfvén wave turbulence. The inclusion
of the Hall term does not affect the heating rates. If plasma
compressibility is taken into account, shocks are produced through
the ponderomotive coupling of Alfvén waves to slow modes and shock
heating dominates the resistive dissipation. In 1.5D shock coalescence
amplifies the effects of shocks, and for compressible simulations
with realistic driver spectra, the heating rate exceeds that required
to match radiative and conductive losses. Thus, while the heating
rates for these 1.5D simulations are an overestimate, they do show
that ponderomotive coupling of Alfvén waves to sound waves is more
important in chromospheric heating than Pedersen dissipation through
ion-neutral collisions.
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Title: Spectropolarimetrically Accurate Magnetohydrostatic Sunspot
Model for Forward Modeling in Helioseismology
Authors: Przybylski, D.; Shelyag, S.; Cally, P. S.
2015ApJ...807...20P Altcode: 2015arXiv150402189P
We present a technique to construct a spectropolarimetrically
accurate magnetohydrostatic model of a large-scale solar magnetic
field concentration, mimicking a sunspot. Using the constructed model
we perform a simulation of acoustic wave propagation, conversion,
and absorption in the solar interior and photosphere with the sunspot
embedded into it. With the 6173 Å magnetically sensitive photospheric
absorption line of neutral iron, we calculate observable quantities
such as continuum intensities, Doppler velocities, as well as the full
Stokes vector for the simulation at various positions at the solar disk,
and analyze the influence of non-locality of radiative transport in
the solar photosphere on helioseismic measurements. Bisector shapes
were used to perform multi-height observations. The differences
in acoustic power at different heights within the line formation
region at different positions at the solar disk were simulated
and characterized. An increase in acoustic power in the simulated
observations of the sunspot umbra away from the solar disk center was
confirmed as the slow magnetoacoustic wave.
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Title: Directional time-distance probing of model sunspot atmospheres
Authors: Moradi, H.; Cally, P. S.; Przybylski, D.; Shelyag, S.
2015MNRAS.449.3074M Altcode: 2015arXiv150304270M
A crucial feature not widely accounted for in local helioseismology is
that surface magnetic regions actually open a window from the interior
into the solar atmosphere, and that the seismic waves leak through this
window, reflect high in the atmosphere, and then re-enter the interior
to rejoin the seismic wave field normally confined there. In a series
of recent numerical studies using translation invariant atmospheres,
we utilized a `directional time-distance helioseismology' measurement
scheme to study the implications of the returning fast and Alfvén waves
higher up in the solar atmosphere on the seismology at the photosphere
(Cally & Moradi 2013; Moradi & Cally 2014). In this study,
we extend our directional time-distance analysis to more realistic
sunspot-like atmospheres to better understand the direct effects
of the magnetic field on helioseismic travel-time measurements
in sunspots. In line with our previous findings, we uncover a
distinct frequency-dependent directional behaviour in the travel-time
measurements, consistent with the signatures of magnetohydrodynamic
mode conversion. We found this to be the case regardless of the sunspot
field strength or depth of its Wilson depression. We also isolated and
analysed the direct contribution from purely thermal perturbations
to the measured travel times, finding that waves propagating in the
umbra are much more sensitive to the underlying thermal effects of
the sunspot.
---------------------------------------------------------
Title: Ellerman Bombs with Jets: Cause and Effect
Authors: Reid, A.; Mathioudakis, M.; Scullion, E.; Doyle, J. G.;
Shelyag, S.; Gallagher, P.
2015ApJ...805...64R Altcode: 2015arXiv150305359R
Ellerman Bombs (EBs) are thought to arise as a result of photospheric
magnetic reconnection. We use data from the Swedish 1 m Solar
Telescope to study EB events on the solar disk and at the limb. Both
data sets show that EBs are connected to the foot points of forming
chromospheric jets. The limb observations show that a bright structure
in the Hα blue wing connects to the EB initially fueling it, leading
to the ejection of material upwards. The material moves along a loop
structure where a newly formed jet is subsequently observed in the
red wing of Hα. In the disk data set, an EB initiates a jet which
propagates away from the apparent reconnection site within the EB
flame. The EB then splits into two, with associated brightenings in
the inter-granular lanes. Micro-jets are then observed, extending
to 500 km with a lifetime of a few minutes. Observed velocities of
the micro-jets are approximately 5-10 km s<SUP>-1</SUP>, while their
chromospheric counterparts range from 50 to 80 km s<SUP>-1</SUP>. MURaM
simulations of quiet Sun reconnection show that micro-jets with
properties similar to those of the observations follow the line of
reconnection in the photosphere, with associated Hα brightening at
the location of increased temperature.
---------------------------------------------------------
Title: The Dynamics of Rapid Redshifted and Blueshifted Excursions
in the Solar Hα Line
Authors: Kuridze, D.; Henriques, V.; Mathioudakis, M.; Erdélyi, R.;
Zaqarashvili, T. V.; Shelyag, S.; Keys, P. H.; Keenan, F. P.
2015ApJ...802...26K Altcode: 2015arXiv150106205K
We analyze high temporal and spatial resolution time-series of spectral
scans of the Hα line obtained with the CRisp Imaging SpectroPolarimeter
instrument mounted on the Swedish Solar Telescope. The data reveal
highly dynamic, dark, short-lived structures known as Rapid Redshifted
and Blueshifted Excursions (RREs, RBEs) that are on-disk absorption
features observed in the red and blue wings of spectral lines formed
in the chromosphere. We study the dynamics of RREs and RBEs by tracking
their evolution in space and time, measuring the speed of the apparent
motion, line of sight (LOS) Doppler velocity, and transverse velocity
of individual structures. A statistical study of their measured
properties shows that RREs and RBEs have similar occurrence rates,
lifetimes, lengths, and widths. They also display non-periodic,
nonlinear transverse motions perpendicular to their axes at speeds of
4-31 km s<SUP>-1</SUP>. Furthermore, both types of structures either
appear as high speed jets and blobs that are directed outwardly from
a magnetic bright point with speeds of 50-150 km s<SUP>-1</SUP>,
or emerge within a few seconds. A study of the different velocity
components suggests that the transverse motions along the LOS of
the chromospheric flux tubes are responsible for the formation and
appearance of these redshifted/blueshifted structures. The short
lifetime and fast disappearance of the RREs/RBEs suggests that, similar
to type II spicules, they are rapidly heated to transition region
or even coronal temperatures. We speculate that the Kelvin-Helmholtz
instability triggered by observed transverse motions of these structures
may be a viable mechanism for their heating.
---------------------------------------------------------
Title: Spectro-polarimetric Simulations of the Solar Limb:
Absorption-emission Fe I 6301.5 Å and 6302.5 Å Line Profiles and
Torsional Flows in the Intergranular Magnetic Flux Concentrations
Authors: Shelyag, S.
2015ApJ...801...46S Altcode: 2015arXiv150100870S
Using radiative magnetohydrodynamic simulations of the magnetized solar
photosphere and detailed spectro-polarimetric diagnostics with the Fe I
6301.5 Å and 6302.5 Å photospheric lines in the local thermodynamic
equilibrium approximation, we model active solar granulation as if it
was observed at the solar limb. We analyze general properties of the
radiation across the solar limb, such as the continuum and the line
core limb darkening and the granulation contrast. We demonstrate the
presence of profiles with both emission and absorption features at the
simulated solar limb, and pure emission profiles above the limb. These
profiles are associated with the regions of strong linear polarization
of the emergent radiation, indicating the influence of the intergranular
magnetic fields on the line formation. We analyze physical origins of
the emission wings in the Stokes profiles at the limb, and demonstrate
that these features are produced by localized heating and torsional
motions in the intergranular magnetic flux concentrations.
---------------------------------------------------------
Title: Understanding Astrophysical Noise from Stellar Surface
Magneto-Convection
Authors: Cegla, H. M.; Watson, C. A.; Shelyag, S.; Mathioudakis, M.
2015csss...18..567C Altcode: 2014arXiv1408.2301C
To obtain cm s^{-1} precision, stellar surface magneto-convection
must be disentangled from observed radial velocities (RVs). In
order to understand and remove the convective signature, we create
Sun-as-a-star model observations based off a 3D magnetohydrodynamic
solar simulation. From these Sun-as-a-star model observations, we
find several line characteristics are correlated with the induced RV
shifts. The aim of this campaign is to feed directly into future high
precision RV studies, such as the search for habitable, rocky worlds,
with forthcoming spectrographs such as ESPRESSO.
---------------------------------------------------------
Title: Plasma properties and Stokes profiles during the lifetime of
a photospheric magnetic bright point
Authors: Hewitt, R. L.; Shelyag, S.; Mathioudakis, M.; Keenan, F. P.
2014A&A...565A..84H Altcode: 2014arXiv1404.0132H
<BR /> Aims: In this paper we aim to investigate the evolution of plasma
properties and Stokes parameters in photospheric magnetic bright points
using 3D magneto-hydrodynamical simulations and radiative diagnostics of
solar granulation. <BR /> Methods: Simulated time-dependent radiation
parameters and plasma properties were investigated throughout the
evolution of a bright point. Synthetic Stokes profiles for the FeI
630.25 nm line were calculated, which also allowed the evolution of the
Stokes-I line strength and Stokes-V area and amplitude asymmetries
to be investigated. <BR /> Results: Our results are consistent
with theoretical predictions and published observations describing
convective collapse, and confirm this as the bright point formation
process. Through degradation of the simulated data to match the spatial
resolution of SOT, we show that high spatial resolution is crucial for
the detection of changing spectro-polarimetric signatures throughout
a magnetic bright point's lifetime. We also show that the signature
downflow associated with the convective collapse process tends towards
zero as the radiation intensity in the bright point peaks, because
of the magnetic forces present restricting the flow of material in
the flux tube. <P />Appendix A is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201322882/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Ellerman Bombs—Evidence for Magnetic Reconnection in the
Lower Solar Atmosphere
Authors: Nelson, C. J.; Shelyag, S.; Mathioudakis, M.; Doyle, J. G.;
Madjarska, M. S.; Uitenbroek, H.; Erdélyi, R.
2013ApJ...779..125N Altcode: 2013arXiv1310.7756N
The presence of photospheric magnetic reconnection has long been thought
to give rise to short and impulsive events, such as Ellerman bombs (EBs)
and Type II spicules. In this article, we combine high-resolution,
high-cadence observations from the Interferometric BIdimensional
Spectrometer and Rapid Oscillations in the Solar Atmosphere instruments
at the Dunn Solar Telescope, National Solar Observatory, New Mexico,
with co-aligned Solar Dynamics Observatory Atmospheric Imaging Assembly
and Hinode Solar Optical Telescope (SOT) data to observe small-scale
events situated within an active region. These data are then compared
with state-of-the-art numerical simulations of the lower atmosphere
made using the MURaM code. It is found that brightenings, in both the
observations and the simulations, of the wings of the Hα line profile,
interpreted as EBs, are often spatially correlated with increases in
the intensity of the Fe I λ6302.5 line core. Bipolar regions inferred
from Hinode/SOT magnetic field data show evidence of flux cancellation
associated, co-spatially, with these EBs, suggesting that magnetic
reconnection could be a driver of these high-energy events. Through
the analysis of similar events in the simulated lower atmosphere,
we are able to infer that line profiles analogous to the observations
occur co-spatially with regions of strong opposite-polarity magnetic
flux. These observed events and their simulated counterparts are
interpreted as evidence of photospheric magnetic reconnection at scales
observable using current observational instrumentation.
---------------------------------------------------------
Title: Alfvén Waves in Simulations of Solar Photospheric Vortices
Authors: Shelyag, S.; Cally, P. S.; Reid, A.; Mathioudakis, M.
2013ApJ...776L...4S Altcode: 2013arXiv1309.2019S
Using advanced numerical magneto-hydrodynamic simulations of the
magnetized solar photosphere, including non-gray radiative transport and
a non-ideal equation of state, we analyze plasma motions in photospheric
magnetic vortices. We demonstrate that apparent vortex-like motions in
photospheric magnetic field concentrations do not exhibit "tornado"-like
behavior or a "bath-tub" effect. While at each time instance the
velocity field lines in the upper layers of the solar photosphere show
swirls, the test particles moving with the time-dependent velocity
field do not demonstrate such structures. Instead, they move in a
wave-like fashion with rapidly changing and oscillating velocity field,
determined mainly by magnetic tension in the magnetized intergranular
downflows. Using time-distance diagrams, we identify horizontal
motions in the magnetic flux tubes as torsional Alfvén perturbations
propagating along the nearly vertical magnetic field lines with local
Alfvén speed.
---------------------------------------------------------
Title: A Pathway to Earth-like Worlds: Overcoming Astrophysical
Noise due to Convection
Authors: Cegla, Heather M.; Watson, C.; Shelyag, S.; Mathioudakis, M.
2013AAS...22230402C Altcode:
One of the consequences of the plasma motions within the convective
envelopes of low-mass stars (i.e. potential planet hosting stars)
are the radial velocity (RV) shifts due to variable stellar
line profile asymmetries, known as astrophysical noise (or
stellar jitter). This can pose a major problem for planet hunters
because RV follow-up is mandatory for most planet confirmation and
characterization. Furthermore, as the net RV shifts produced from
these photospheric convective motions are around the m/s level this is
especially troublesome for confirmation of Earth-analogs that induce
Doppler-wobbles on the cm/s level. The currently implemented noise
removal technique for granulation rests on adapting observational
strategies to average out such noise. However, this technique is
extremely observationally intensive and does not provide information on
the nature of jitter. We aim to go beyond these previous techniques
by understanding the physical processes involved in granulation
and removing the actual RV signature from granulation. We outline
our techniques to characterize photospheric granulation as an
astrophysical noise source. The backbone of this characterization is
a state-of-the-art 3D magnetohydrodynamic solar simulation, coupled
with detailed wavelength-dependent radiative transfer. Due to the
time-intensive nature of these simulations, we use a short time-series
to parameterize the granulation signal. This parameterization is then
used to create full Sun-as-a-star observations from which we examine
the convective noise. We present the results of this study, as well as
the identification of variable gravitational redshift as a potential
source of stellar jitter, both of which could impact the RV follow-up
and confirmation of low-mass terrestrial planets and Earth-like worlds.
---------------------------------------------------------
Title: Evidence for the Photospheric Excitation of Incompressible
Chromospheric Waves
Authors: Morton, R. J.; Verth, G.; Fedun, V.; Shelyag, S.; Erdélyi, R.
2013ApJ...768...17M Altcode: 2013arXiv1303.2356M
Observing the excitation mechanisms of incompressible transverse
waves is vital for determining how energy propagates through the lower
solar atmosphere. We aim to show the connection between convectively
driven photospheric flows and incompressible chromospheric waves. The
observations presented here show the propagation of incompressible
motion through the quiet lower solar atmosphere, from the photosphere
to the chromosphere. We determine photospheric flow vectors to search
for signatures of vortex motion and compare results to photospheric
flows present in convective simulations. Further, we search for the
chromospheric response to vortex motions. Evidence is presented that
suggests incompressible waves can be excited by the vortex motions of a
strong magnetic flux concentration in the photosphere. A chromospheric
counterpart to the photospheric vortex motion is also observed,
presenting itself as a quasi-periodic torsional motion. Fine-scale,
fibril structures that emanate from the chromospheric counterpart
support transverse waves that are driven by the observed torsional
motion. A new technique for obtaining details of transverse waves from
time-distance diagrams is presented and the properties of transverse
waves (e.g., amplitudes and periods) excited by the chromospheric
torsional motion are measured.
---------------------------------------------------------
Title: Tracking magnetic bright point motions through the solar
atmosphere
Authors: Keys, P. H.; Mathioudakis, M.; Jess, D. B.; Shelyag, S.;
Christian, D. J.; Keenan, F. P.
2013MNRAS.428.3220K Altcode: 2012MNRAS.tmp..182K; 2012arXiv1210.5904K
High-cadence, multiwavelength observations and simulations are
employed for the analysis of solar photospheric magnetic bright
points (MBPs) in the quiet Sun. The observations were obtained with
the Rapid Oscillations in the Solar Atmosphere (ROSA) imager and
the Interferometric Bidimensional Spectrometer at the Dunn Solar
Telescope. Our analysis reveals that photospheric MBPs have an average
transverse velocity of approximately 1 km s<SUP>-1</SUP>, whereas their
chromospheric counterparts have a slightly higher average velocity of
1.4 km s<SUP>-1</SUP>. Additionally, chromospheric MBPs were found to be
around 63 per cent larger than the equivalent photospheric MBPs. These
velocity values were compared with the output of numerical simulations
generated using the muram code. The simulated results were similar,
but slightly elevated, when compared to the observed data. An average
velocity of 1.3 km s<SUP>-1</SUP> was found in the simulated G-band
images and an average of 1.8 km s<SUP>-1</SUP> seen in the velocity
domain at a height of 500 km above the continuum formation layer. Delays
in the change of velocities were also analysed. Average delays of ∼4
s between layers of the simulated data set were established and values
of ∼29 s observed between G-band and Ca ii K ROSA observations. The
delays in the simulations are likely to be the result of oblique
granular shock waves, whereas those found in the observations are
possibly the result of a semi-rigid flux tube.
---------------------------------------------------------
Title: Stellar Surface Magneto-convection as a Source of Astrophysical
Noise. I. Multi-component Parameterization of Absorption Line Profiles
Authors: Cegla, H. M.; Shelyag, S.; Watson, C. A.; Mathioudakis, M.
2013ApJ...763...95C Altcode: 2012arXiv1212.0236C
We outline our techniques to characterize photospheric granulation
as an astrophysical noise source. A four-component parameterization
of granulation is developed that can be used to reconstruct stellar
line asymmetries and radial velocity shifts due to photospheric
convective motions. The four components are made up of absorption
line profiles calculated for granules, magnetic intergranular lanes,
non-magnetic intergranular lanes, and magnetic bright points at disk
center. These components are constructed by averaging Fe I 6302 Å
magnetically sensitive absorption line profiles output from detailed
radiative transport calculations of the solar photosphere. Each
of the four categories adopted is based on magnetic field and
continuum intensity limits determined from examining three-dimensional
magnetohydrodynamic simulations with an average magnetic flux of 200
G. Using these four-component line profiles we accurately reconstruct
granulation profiles, produced from modeling 12 × 12 Mm<SUP>2</SUP>
areas on the solar surface, to within ~ ±20 cm s<SUP>-1</SUP> on a
~100 m s<SUP>-1</SUP> granulation signal. We have also successfully
reconstructed granulation profiles from a 50 G simulation using the
parameterized line profiles from the 200 G average magnetic field
simulation. This test demonstrates applicability of the characterization
to a range of magnetic stellar activity levels.
---------------------------------------------------------
Title: Vortices in the Solar Photosphere
Authors: Shelyag, S.; Fedun, V.; Erdélyi, R.; Keenan, F. P.;
Mathioudakis, M.
2012ASPC..463..107S Altcode: 2012arXiv1202.1966S
Using numerical simulations of the magnetized solar photosphere
and radiative diagnostics of the simulated photospheric models,
we further analyse the physical nature of magnetic photospheric
intergranular vortices. We confirm the magnetic nature of the vortices
and find that most MHD Umov-Poynting flux is produced by horizontal
vortex motions in the magnetised intergranular lanes. In addition,
we consider possible ways to directly observe photospheric magnetic
vortices using spectropolarimetry. Although horizontal plasma motions
cannot be detected in the spectropolarimetric observations of solar disk
center, we find an observational signature of photospheric vortices in
simulated observations of Stokes-V amplitude asymmetry close to the
solar limb. Potential ways to find the vortices in the observations
are discussed.
---------------------------------------------------------
Title: Mechanisms for MHD Poynting Flux Generation in Simulations
of Solar Photospheric Magnetoconvection
Authors: Shelyag, S.; Mathioudakis, M.; Keenan, F. P.
2012ApJ...753L..22S Altcode: 2012arXiv1206.0030S
We investigate the generation mechanisms of MHD Poynting flux in the
magnetized solar photosphere. Using radiative MHD modeling of the
solar photosphere with initial magnetic configurations that differ
in their field strength and geometry, we show the presence of two
different mechanisms for MHD Poynting flux generation in simulations
of solar photospheric magnetoconvection. The weaker mechanism is
connected to vertical transport of weak horizontal magnetic fields in
the convectively stable layers of the upper photosphere, while the
stronger is the production of Poynting flux in strongly magnetized
intergranular lanes experiencing horizontal vortex motions. These
mechanisms may be responsible for the energy transport from the solar
convection zone to the higher layers of the solar atmosphere.
---------------------------------------------------------
Title: Stellar jitter from variable gravitational redshift:
implications for radial velocity confirmation of habitable exoplanets
Authors: Cegla, H. M.; Watson, C. A.; Marsh, T. R.; Shelyag, S.;
Moulds, V.; Littlefair, S.; Mathioudakis, M.; Pollacco, D.; Bonfils, X.
2012MNRAS.421L..54C Altcode: 2011arXiv1112.1553C
A variation of gravitational redshift, arising from stellar radius
fluctuations, will introduce astrophysical noise into radial velocity
measurements by shifting the centroid of the observed spectral
lines. Shifting the centroid does not necessarily introduce line
asymmetries. This is fundamentally different from other types
of stellar jitter so far identified, which do result from line
asymmetries. Furthermore, only a very small change in stellar radius,
∼0.01 per cent, is necessary to generate a gravitational redshift
variation large enough to mask or mimic an Earth-twin. We explore
possible mechanisms for stellar radius fluctuations in low-mass
stars. Convective inhibition due to varying magnetic field strengths and
the Wilson depression of starspots are both found to induce substantial
gravitational redshift variations. Finally, we investigate a possible
method for monitoring/correcting this newly identified potential source
of jitter and comment on its impact for future exoplanet searches.
---------------------------------------------------------
Title: Propagating Wave Phenomena Detected in Observations and
Simulations of the Lower Solar Atmosphere
Authors: Jess, D. B.; Shelyag, S.; Mathioudakis, M.; Keys, P. H.;
Christian, D. J.; Keenan, F. P.
2012ApJ...746..183J Altcode: 2012arXiv1201.1981J
We present high-cadence observations and simulations of the solar
photosphere, obtained using the Rapid Oscillations in the Solar
Atmosphere imaging system and the MuRAM magnetohydrodynamic (MHD) code,
respectively. Each data set demonstrates a wealth of magnetoacoustic
oscillatory behavior, visible as periodic intensity fluctuations
with periods in the range 110-600 s. Almost no propagating waves with
periods less than 140 s and 110 s are detected in the observational
and simulated data sets, respectively. High concentrations of power
are found in highly magnetized regions, such as magnetic bright points
and intergranular lanes. Radiative diagnostics of the photospheric
simulations replicate our observational results, confirming that the
current breed of MHD simulations are able to accurately represent the
lower solar atmosphere. All observed oscillations are generated as
a result of naturally occurring magnetoconvective processes, with no
specific input driver present. Using contribution functions extracted
from our numerical simulations, we estimate minimum G-band and 4170 Å
continuum formation heights of 100 km and 25 km, respectively. Detected
magnetoacoustic oscillations exhibit a dominant phase delay of -8°
between the G-band and 4170 Å continuum observations, suggesting
the presence of upwardly propagating waves. More than 73% of MBPs
(73% from observations and 96% from simulations) display upwardly
propagating wave phenomena, suggesting the abundant nature of
oscillatory behavior detected higher in the solar atmosphere may be
traced back to magnetoconvective processes occurring in the upper
layers of the Sun's convection zone.
---------------------------------------------------------
Title: Towards Earth-like Worlds: Identifying and Removing Stellar
Jitter
Authors: Cegla, Heather M.; Watson, C.; Marsh, T.; Shelyag, S.;
Moulds, V.; Littlefair, S.; Mathioudakis, M.; Pollacco, D.; Bonfils, X.
2012AAS...21943203C Altcode:
Space-based, photometric surveys have moved us into a new era
of exoplanet discovery. In order to confirm the masses and hence
the planetary nature of exoplanet candidates from such surveys,
radial velocity (RV) follow up is mandatory. To do this for low-mass
planets typically requires cm/s RV precision. However, astrophysical
noise sources (or stellar jitter) due to spots, plages, granulation
and stellar oscillations, for example, become an issue at the m/s
level. These phenomena alter the shape of the stellar absorption lines,
injecting spurious or systematic RV signals that may mask or mimic
planetary signals. As such, `quiet’ stars (those with little activity)
are the most likely candidates for the detection of low-mass planets,
but even these stars will still exhibit some stellar jitter. We present
our techniques to explore the stellar jitter due to granulation through
the use of sophisticated 3D magnetohydrodynamical simulations of the
Sun. In addition, we also present the identification of an entirely new
source of stellar jitter that has been hitherto unrecognized and that
could impact the RV follow-up and confirmation of low-mass terrestrial
planets and Earth-like worlds.
---------------------------------------------------------
Title: The Velocity Distribution of Solar Photospheric Magnetic
Bright Points
Authors: Keys, P. H.; Mathioudakis, M.; Jess, D. B.; Shelyag, S.;
Crockett, P. J.; Christian, D. J.; Keenan, F. P.
2011ApJ...740L..40K Altcode: 2011arXiv1109.3565K
We use high spatial resolution observations and numerical simulations to
study the velocity distribution of solar photospheric magnetic bright
points. The observations were obtained with the Rapid Oscillations in
the Solar Atmosphere instrument at the Dunn Solar Telescope, while the
numerical simulations were undertaken with the MURaM code for average
magnetic fields of 200 G and 400 G. We implemented an automated bright
point detection and tracking algorithm on the data set and studied
the subsequent velocity characteristics of over 6000 structures,
finding an average velocity of approximately 1 km s<SUP>-1</SUP>, with
maximum values of 7 km s<SUP>-1</SUP>. Furthermore, merging magnetic
bright points were found to have considerably higher velocities,
and significantly longer lifetimes, than isolated structures. By
implementing a new and novel technique, we were able to estimate the
background magnetic flux of our observational data, which is consistent
with a field strength of 400 G.
---------------------------------------------------------
Title: Small-scale Hα jets in the solar chromosphere
Authors: Kuridze, D.; Mathioudakis, M.; Jess, D. B.; Shelyag, S.;
Christian, D. J.; Keenan, F. P.; Balasubramaniam, K. S.
2011A&A...533A..76K Altcode: 2011arXiv1108.1043K
<BR /> Aims: High temporal and spatial resolution observations from
the Rapid Oscillations in the Solar Atmosphere (ROSA) multiwavelength
imager on the Dunn Solar Telescope are used to study the velocities
of small-scale Hα jets in an emerging solar active region. <BR
/> Methods: The dataset comprises simultaneous imaging in the Hα
core, Ca ii K, and G band, together with photospheric line-of-sight
magnetograms. Time-distance techniques are employed to determine
projected plane-of-sky velocities. <BR /> Results: The Hα images
are highly dynamic in nature, with estimated jet velocities as high
as 45 km s<SUP>-1</SUP>. These jets are one-directional, with their
origin seemingly linked to underlying Ca ii K brightenings and G-band
magnetic bright points. <BR /> Conclusions: It is suggested that the
siphon flow model of cool coronal loops is suitable for interpreting our
observations. The jets are associated with small-scale explosive events,
and may provide a mass outflow from the photosphere to the corona.
---------------------------------------------------------
Title: Vorticity in the solar photosphere
Authors: Shelyag, S.; Keys, P.; Mathioudakis, M.; Keenan, F. P.
2011A&A...526A...5S Altcode: 2010arXiv1010.5604S
<BR /> Aims: We use magnetic and non-magnetic 3D numerical simulations
of solar granulation and G-band radiative diagnostics from the resulting
models to analyse the generation of small-scale vortex motions in
the solar photosphere. <BR /> Methods: Radiative MHD simulations of
magnetoconvection are used to produce photospheric models. Our starting
point is a non-magnetic model of solar convection, where we introduce
a uniform magnetic field and follow the evolution of the field in the
simulated photosphere. We find two different types of photospheric
vortices, and provide a link between the vorticity generation and the
presence of the intergranular magnetic field. A detailed analysis of
the vorticity equation, combined with the G-band radiative diagnostics,
allows us to identify the sources and observational signatures of
photospheric vorticity in the simulated photosphere. <BR /> Results: Two
different types of photospheric vorticity, magnetic and non-magnetic,
are generated in the domain. Non-magnetic vortices are generated by the
baroclinic motions of the plasma in the photosphere, while magnetic
vortices are produced by the magnetic tension in the intergranular
magnetic flux concentrations. The two types of vortices have different
shapes. We find that the vorticity is generated more efficiently in
the magnetised model. Simulated G-band images show a direct connection
between magnetic vortices and rotary motions of photospheric bright
points, and suggest that there may be a connection between the magnetic
bright point rotation and small-scale swirl motions observed higher
in the atmosphere.
---------------------------------------------------------
Title: Numerical Modeling of Footpoint-driven Magneto-acoustic Wave
Propagation in a Localized Solar Flux Tube
Authors: Fedun, V.; Shelyag, S.; Erdélyi, R.
2011ApJ...727...17F Altcode:
In this paper, we present and discuss results of two-dimensional
simulations of linear and nonlinear magneto-acoustic wave propagation
through an open magnetic flux tube embedded in the solar atmosphere
expanding from the photosphere through to the transition region
and into the low corona. Our aim is to model and analyze the
response of such a magnetic structure to vertical and horizontal
periodic motions originating in the photosphere. To carry out
the simulations, we employed our MHD code SAC (Sheffield Advanced
Code). A combination of the VALIIIC and McWhirter solar atmospheres
and coronal density profiles were used as the background equilibrium
model in the simulations. Vertical and horizontal harmonic sources,
located at the footpoint region of the open magnetic flux tube,
are incorporated in the calculations, to excite oscillations in the
domain of interest. To perform the analysis we have constructed a
series of time-distance diagrams of the vertical and perpendicular
components of the velocity with respect to the magnetic field lines
at each height of the computational domain. These time-distance
diagrams are subject to spatio-temporal Fourier transforms allowing
us to build ω-k dispersion diagrams for all of the simulated regions
in the solar atmosphere. This approach makes it possible to compute
the phase speeds of waves propagating throughout the various regions
of the solar atmosphere model. We demonstrate the transformation of
linear slow and fast magneto-acoustic wave modes into nonlinear ones,
i.e., shock waves, and also show that magneto-acoustic waves with a
range of frequencies efficiently leak through the transition region
into the solar corona. It is found that the waves interact with the
transition region and excite horizontally propagating surface waves
along the transition region for both types of drivers. Finally, we
estimate the phase speed of the oscillations in the solar corona and
compare it with the phase speed derived from observations.
---------------------------------------------------------
Title: The Area Distribution of Solar Magnetic Bright Points
Authors: Crockett, P. J.; Mathioudakis, M.; Jess, D. B.; Shelyag,
S.; Keenan, F. P.; Christian, D. J.
2010ApJ...722L.188C Altcode: 2010arXiv1009.2410C
Magnetic bright points (MBPs) are among the smallest observable objects
on the solar photosphere. A combination of G-band observations and
numerical simulations is used to determine their area distribution. An
automatic detection algorithm, employing one-dimensional intensity
profiling, is utilized to identify these structures in the observed and
simulated data sets. Both distributions peak at an area of ≈45,000
km<SUP>2</SUP>, with a sharp decrease toward smaller areas. The
distributions conform with log-normal statistics, which suggests
that flux fragmentation dominates over flux convergence. Radiative
magneto-convection simulations indicate an independence in the MBP
area distribution for differing magnetic flux densities. The most
commonly occurring bright point size corresponds to the typical width
of inter-granular lanes.
---------------------------------------------------------
Title: Mesogranular structure in a hydrodynamical simulation
Authors: Matloch, Ł.; Cameron, R.; Shelyag, S.; Schmitt, D.;
Schüssler, M.
2010A&A...519A..52M Altcode: 2010arXiv1007.0387M
<BR /> Aims: We analyse mesogranular flow patterns in
a three-dimensional hydrodynamical simulation of solar surface
convection in order to determine its characteristics. <BR /> Methods:
We calculate divergence maps from horizontal velocities obtained with
the local correlation tracking (LCT) method. Mesogranules are identified
as patches of positive velocity divergence. We track the mesogranules
to obtain their size and lifetime distributions. We vary the analysis
parameters to verify if the pattern has characteristic scales. <BR />
Results: The characteristics of the resulting flow patterns depend on
the averaging time and length used in the analysis. <BR /> Conclusions:
We conclude that the mesogranular patterns do not exhibit intrinsic
length and time scales.
---------------------------------------------------------
Title: A photospheric bright point model
Authors: Shelyag, S.; Mathioudakis, M.; Keenan, F. P.; Jess, D. B.
2010A&A...515A.107S Altcode: 2010arXiv1003.1653S
<BR /> Aims: A magneto-hydrostatic model is constructed with
spectropolarimetric properties close to those of solar photospheric
magnetic bright points. <BR /> Methods: Results of solar radiative
magneto-convection simulations are used to produce the spatial
structure of the vertical component of the magnetic field. The
horizontal component of magnetic field is reconstructed using the
self-similarity condition, while the magneto-hydrostatic equilibrium
condition is applied to the standard photospheric model with the
magnetic field embedded. Partial ionisation processes are found to be
necessary for reconstructing the correct temperature structure of the
model. <BR /> Results: The structures obtained are in good agreement
with observational data. By combining the realistic structure of
the magnetic field with the temperature structure of the quiet solar
photosphere, the continuum formation level above the equipartition
layer can be found. Preliminary results are shown of wave propagation
through this magnetic structure. The observational consequences of the
oscillations are examined in continuum intensity and in the Fe I 6302
Å magnetically sensitive line.
---------------------------------------------------------
Title: Analysis of Acoustic Wave Propagation in the Subphotosphere
with Localized Magnetic Field Concentration
Authors: Zharkov, S.; Shelyag, S.; Thompson, M. J.
2009ASPC..416...75Z Altcode:
We present the time-distance analysis of numerical simulations of
acoustic wave propagation and dispersion in the solar subphotosphere
with a localized magnetic field concentration.
---------------------------------------------------------
Title: Numerical Simulation of Acoustic Wave Propagation in the
Solar Sub-Photosphere with Localized Magnetic Field Concentration
Authors: Shelyag, S.; Zharkov, S.; Fedun, V.; Erdélyi, R.; Thompson,
M. J.
2009ASPC..416..167S Altcode:
The results of numerical simulations of acoustic wave propagation and
dispersion in the solar sub-photosphere with a localised magnetic
field concentration are presented. The initial equilibrium density
and pressure stratifications are derived from a standard solar
model and adjusted to maintain magnetohydrostatic and convective
stability. Acoustic waves are generated by a perturbation located
at the height corresponding to the visible surface of the Sun. The
time-distance diagram of the vertical velocity perturbation at the level
corresponding to the visible solar surface shows that the magnetic
field perturbs and scatters acoustic waves and absorbs the acoustic
power of the wave packet.
---------------------------------------------------------
Title: Photospheric high-frequency acoustic power excess in sunspot
umbra: signature of magneto-acoustic modes
Authors: Zharkov, S.; Shelyag, S.; Fedun, V.; Erdélyi, R.; Thompson,
M. J.
2009arXiv0909.5332Z Altcode:
We present observational evidence for the presence of MHD waves in
the solar photosphere deduced from SOHO/MDI Dopplergram velocity
observations. The magneto-acoustic perturbations are observed as
acoustic power enhancement in the sunspot umbra at high-frequency bands
in the velocity component perpendicular to the magnetic field. We use
numerical modelling of wave propagation through localised non-uniform
magnetic field concentration along with the same filtering procedure
as applied to the observations to identify the observed waves. Guided
by the results of the numerical simulations we classify the observed
oscillations as magneto-acoustic waves excited by the trapped
sub-photospheric acoustic waves. We consider the potential application
of the presented method as a diagnostic tool for magnetohelioseismology.
---------------------------------------------------------
Title: Acoustic wave propagation in the solar sub-photosphere with
localised magnetic field concentration: effect of magnetic tension
Authors: Shelyag, S.; Zharkov, S.; Fedun, V.; Erdélyi, R.; Thompson,
M. J.
2009A&A...501..735S Altcode: 2009arXiv0901.3680S
Aims: We analyse numerically the propagation and dispersion of acoustic
waves in the solar-like sub-photosphere with localised non-uniform
magnetic field concentrations, mimicking sunspots with various
representative magnetic field configurations. <BR />Methods: Numerical
simulations of wave propagation through the solar sub-photosphere
with a localised magnetic field concentration are carried out using
SAC, which solves the MHD equations for gravitationally stratified
plasma. The initial equilibrium density and pressure stratifications
are derived from a standard solar model. Acoustic waves are generated
by a source located at the height corresponding approximately to
the visible surface of the Sun. By means of local helioseismology we
analyse the response of vertical velocity at the level corresponding
to the visible solar surface to changes induced by magnetic field in
the interior. <BR />Results: The results of numerical simulations of
acoustic wave propagation and dispersion in the solar sub-photosphere
with localised magnetic field concentrations of various types are
presented. Time-distance diagrams of the vertical velocity perturbation
at the level corresponding to the visible solar surface show that the
magnetic field perturbs and scatters acoustic waves and absorbs the
acoustic power of the wave packet. For the weakly magnetised case, the
effect of magnetic field is mainly thermodynamic, since the magnetic
field changes the temperature stratification. However, we observe
the signature of slow magnetoacoustic mode, propagating downwards,
for the strong magnetic field cases.
---------------------------------------------------------
Title: Magnetohydrodynamic code for gravitationally-stratified media
Authors: Shelyag, S.; Fedun, V.; Erdélyi, R.
2008A&A...486..655S Altcode:
Aims: We describe a newly-developed magnetohydrodynamic (MHD)
code with the capacity to simulate the interaction of any arbitrary
perturbation (i.e., not necessarily limited to the linearised limit)
with a magnetohydrostatic equilibrium background. <BR />Methods: By
rearranging the terms in the system of MHD equations and explicitly
taking into account the magnetohydrostatic equilibrium condition,
we define the equations governing the perturbations that describe
the deviations from the background state of plasma for the density,
internal energy and magnetic field. We found it was advantageous to use
this modified form of the MHD equations for numerical simulations of
physical processes taking place in a stable gravitationally-stratified
plasma. The governing equations are implemented in a novel way in
the code. Sub-grid diffusion and resistivity are applied to ensure
numerical stability of the computed solution of the MHD equations. We
apply a fourth-order central difference scheme to calculate the
spatial derivatives, and implement an arbitrary Runge-Kutta scheme
to advance the solution in time. <BR />Results: We have built the
proposed method, suitable for strongly-stratified magnetised plasma,
on the base of the well-documented Versatile Advection Code (VAC)
and performed a number of one- and multi-dimensional hydrodynamic and
MHD tests to demonstrate the feasibility and robustness of the code
for applications to astrophysical plasmas.
---------------------------------------------------------
Title: Forward modelling of sub-photospheric flows for time-distance
helioseismology
Authors: Shelyag, S.; Erdélyi, R.; Thompson, M. J.
2007A&A...469.1101S Altcode: 2007astro.ph..3067S
Context: The results of forward modelling of acoustic wave propagation
in a realistic solar sub-photosphere with two cases of steady
horizontal flows are presented and analysed by the means of local
helioseismology. <BR />Aims: This paper is devoted to an analysis
of the influence of steady flows on the propagation of sound waves
through the solar interior. <BR />Methods: The simulations are based
on fully compressible ideal hydrodynamical modelling in a Cartesian
grid. The initial model is characterised by solar density and pressure
stratifications taken from the standard Model S and is adjusted in
order to suppress convective instability. Acoustic waves are excited
by a non-harmonic source located below the depth corresponding to
the visible surface of the Sun. Numerical experiments with coherent
horizontal flows of linear and Gaussian dependences of flow speed on
depth are carried out. These flow fields may mimic horizontal motions
of plasma surrounding a sunspot, differential rotation or meridional
circulation. An inversion of the velocity profiles from the simulated
travel time differences is carried out. The inversion is based on
the ray approximation. The results of inversion are then compared
with the original velocity profiles. <BR />Results: The results
of forward modelling of acoustic wave propagation in a realistic
solar sub-photosphere with two cases of steady horizontal flows are
presented. The influence of steady flow on the propagation of sound
waves through the solar interior is analysed. A time-distance analysis
technique is applied to compute the direct observable signatures of
the background bulk motions on travel times and phase shifts. This
approach allows direct comparison with observational data. Further,
we propose a method of obtaining the travel-time differences for the
waves propagating in sub-photospheric solar regions with horizontal
flows. The method employs directly the difference between travel-time
diagrams of waves propagating with and against the background flow. <BR
/>Conclusions: The analysis shows that the flow speed profiles obtained
from inversion based on the ray approximation differ from the original
ones. The difference between the original and observed profiles is
caused by the fact that the wave packets propagate along the ray
bundle, which has a finite extent, and thus reach deeper regions of
the sub-photosphere in comparison with ray theory.
---------------------------------------------------------
Title: Stokes diagnostics of simulated solar magneto-convection
Authors: Shelyag, S.; Schüssler, M.; Solanki, S. K.; Vögler, A.
2007A&A...469..731S Altcode: 2007astro.ph..3490S
We present results of synthetic spectro-polarimetric diagnostics of
radiative MHD simulations of solar surface convection with magnetic
fields. Stokes profiles of Zeeman-sensitive lines of neutral iron in
the visible and infrared spectral ranges emerging from the simulated
atmosphere have been calculated in order to study their relation to the
relevant physical quantities and compare with observational results. We
have analyzed the dependence of the Stokes-I line strength and width as
well as of the Stokes-V signal and asymmetries on the magnetic field
strength. Furthermore, we have evaluated the correspondence between
the actual velocities in the simulation with values determined from
the Stokes-I (Doppler shift of the centre of gravity) and Stokes-V
profiles (zero-crossing shift). We confirm that the line weakening in
strong magnetic fields results from a higher temperature (at equal
optical depth) in the magnetic flux concentrations. We also confirm
that considerable Stokes-V asymmetries originate in the peripheral
parts of strong magnetic flux concentrations, where the line of sight
cuts through the magnetopause of the expanding flux concentration into
the surrounding convective donwflow.
---------------------------------------------------------
Title: Forward Modeling of Acoustic Wave Propagation in the Quiet
Solar Subphotosphere
Authors: Shelyag, S.; Erdélyi, R.; Thompson, M. J.
2006ApJ...651..576S Altcode:
The results of numerical simulations of acoustic wave propagation
and dispersion in the nonmagnetic solar subphotosphere are
presented. Initial equilibrium density and pressure stratifications are
taken from a standard solar model but modified to suppress convective
instabilities in fully compressible two-dimensional ideal hydrodynamical
modeling. Acoustic waves are generated by sources located below the
height corresponding to the visible solar surface. The dynamic response
of the solar interior to two acoustic source types, namely a harmonic
source and one representing downward-propagating photospheric plumes,
is studied. A large number of randomly distributed localized cooling
sources with random amplitudes is also introduced. The methods used to
analyze the simulation data are similar to ones used in observational
studies in local helioseismology. Time-distance diagrams of the pressure
and vertical velocity perturbations at the level corresponding to the
solar surface show the appearance of wave packets propagating with
different speeds, which are reflected at different depths beneath the
subphotosphere. The (ω, k<SUB>h</SUB>) power spectra, derived from the
vertical velocity data, show the existence of g-, f-, and p-modes p-mode
ridges are identifiable up to high radial orders of n~11 g-modes appear
in the simulations, unlike in the real Sun, where they cannot propagate
in the convectively unstable solar subphotosphere. Cross-correlation
analysis of vertical velocity perturbations shows a good correspondence
with the observed time-distance helioseismic data for quiet Sun. Thus,
the ability of the implemented approach of forward modeling to
investigate propagation of acoustic, internal, and surface gravity
waves in a realistic solar interior model is shown.
---------------------------------------------------------
Title: Helioseismology of sub-photospheric flows
Authors: Shelyag, S.; Erdélyi, R.; Thompson, M. J.
2006ESASP.624E.123S Altcode: 2006soho...18E.123S
No abstract at ADS
---------------------------------------------------------
Title: Acoustic Wave Propagation in the Solar Subphotosphere
Authors: Shelyag, S.; Erdelyi, R.; Thompson, M. J.
2005AGUFMSH53A1237S Altcode:
Local time-distance helioseismology aims to get an information about the
internal structure of subphotospheric region of the Sun, like flows and
sound speed perturbations, by investigation of observed travel times
of the oscillatory responses of subphotosphere to a perturbation. We
present the results of simulations of acoustic wave propagation and
dispersion in the non-magnetic solar subphotosphere. We use methods
similar to the observational ones to produce results aiming to reproduce
observational data from local helioseismology. The VAC code (G. Toth)
is used to carry out the simulations. The code can solve the full system
of ideal hydrodynamic equations in two or three dimensions. All the
results presented are carried out in 2D. The simulations use realistic
solar density and pressure stratifications and are based on fully
compressible ideal hydrodynamical model. Acoustic waves are produced
by perturbations located near the layer corresponding to the visible
surface of the Sun. The simulation domain is 150 Mm wide and 52.6 Mm
deep, and has a resolution of 600x4000 grid points; the upper boundary
of the domain is near the temperature minimum. The density profile for
the initial plane-parallel model is taken from Christensen-Dalsgaard's
standard Model S. We suppress convective motions of the fluid in the
domain by slight change of adiabatic index γ_1. This approach has
an advantage among other approaches because the waves we study, while
they propagate through the quiescent medium, can be clearly observed
far from the source undisturbed by convective fluid motions. First
we study the propagation of acoustic waves produced by a harmonic
pressure perturbation, then, in order to follow the criteria of realism
we introduce more involved perturbation source, described as a source
in the energy equation of the HD system of equations. The source used
corresponds to a localized cooling event causing mass inflow and
sound waves extinction (M. Rast, 1999). In order to reproduce the
helioseismogram of the quiet Sun we introduce a number of randomly
distributed sources with random amplitudes and present a comparison
of the simulated data with the observations.
---------------------------------------------------------
Title: Stokes diagnostics of simulations of magnetoconvection of
mixed-polarity quiet-Sun regions
Authors: Khomenko, E. V.; Shelyag, S.; Solanki, S. K.; Vögler, A.
2005A&A...442.1059K Altcode:
Realistic solar magneto-convection simulations including the
photospheric layers are used to study the polarization of the Fe i
Zeeman-sensitive spectral lines at 6301.5, 6302.5, 15 648 and 15 652
Å. The Stokes spectra are synthesized in a series of snapshots with
a mixed-polarity magnetic field whose average unsigned strength varies
from < B > = 10 to 140 G. The effects of spatial resolution and
of the amount of magnetic flux in the simulation box on the profiles
shapes, amplitudes and shifts are discussed. The synthetic spectra show
many properties in common with those observed in quiet solar regions. In
particular, the simulations reproduce the width and depth of spatially
averaged Stokes I profiles, the basic classes of the Stokes V profiles
and their amplitude and area asymmetries, as well as the abundance of
the irregular-shaped Stokes V profiles. It is demonstrated that the
amplitudes of the 1.56 μm lines observed in the inter-network are
consistent with a "true" average unsigned magnetic field strength of
20 G. We show that observations using these and visible lines, carried
out under different seeing conditions (e.g., simultaneous observations
at different telescopes), may result in different asymmetries and
even opposite polarities of the profiles in the two spectral regions
observed at the same spatial point.
---------------------------------------------------------
Title: Simulations of magneto-convection in the solar photosphere.
Equations, methods, and results of the MURaM code
Authors: Vögler, A.; Shelyag, S.; Schüssler, M.; Cattaneo, F.;
Emonet, T.; Linde, T.
2005A&A...429..335V Altcode:
We have developed a 3D magnetohydrodynamics simulation code for
applications in the solar convection zone and photosphere. The code
includes a non-local and non-grey radiative transfer module and takes
into account the effects of partial ionization. Its parallel design
is based on domain decomposition, which makes it suited for use on
parallel computers with distributed memory architecture. We give a
description of the equations and numerical methods and present the
results of the simulation of a solar plage region. Starting with
a uniform vertical field of 200 G, the processes of flux expulsion
and convective field amplification lead to a dichotomy of strong,
mainly vertical fields embedded in the granular downflow network and
weak, randomly oriented fields filling the hot granular upflows. The
strong fields form a magnetic network with thin, sheet-like structures
extending along downflow lanes and micropores with diameters of up to
1000 km which form occasionally at vertices where several downflow
lanes merge. At the visible surface around optical depth unity,
the strong field concentrations are in pressure balance with their
weakly magnetized surroundings and reach field strengths of up to 2
kG, strongly exceeding the values corresponding to equipartition with
the kinetic energy density of the convective motions. As a result of
the channelling of radiation, small flux concentrations stand out as
bright features, while the larger micropores appear dark in brightness
maps owing to the suppression of the convective energy transport. The
overall shape of the magnetic network changes slowly on a timescale
much larger than the convective turnover time, while the magnetic flux
is constantly redistributed within the network leading to continuous
formation and dissolution of flux concentrations. <P />Appendices A-D
are only available in electronic form at http://www.edpsciences.org
---------------------------------------------------------
Title: The Decay of a Simulated Pore
Authors: Cameron, R.; Vögler, A.; Shelyag, S.; Schüssler, M.
2004ASPC..325...57C Altcode:
Using MURaM -- Max-Planck Institut für Aeronomie University of
Chicago Radiative Magnetohydrodynamics -- an MHD code which includes
radiative transfer and partial ionization, we have studied the decay
phase of a solar pore. The simulations are sufficiently realistic
in their treatment of the photosphere to allow a direct comparison
with observations, both current and those of upcoming missions such
as SolarB. As well as discussing the structure and decay of pores,
we show the formation of shallow, field aligned, convective rolls
which are an important feature of our solutions.
---------------------------------------------------------
Title: G-band spectral synthesis and diagnostics of simulated solar
magneto-convection
Authors: Shelyag, S.; Schüssler, M.; Solanki, S. K.; Berdyugina,
S. V.; Vögler, A.
2004A&A...427..335S Altcode:
Realistic simulations of radiative magneto-convection in the solar
(sub)photosphere are used for a spectral synthesis of Fraunhofer's G
band, which is dominated by spectral lines from the CH molecule. It
is found that the spatial pattern of integrated G-band brightness
closely matches the spatial structure of magnetic flux concentrations
in the convective downflow regions. The brightness contrast is mainly
caused by the weakening of CH lines due to the reduced CH abundance
and the resulting shift of the optical depth scale in the hot and
tenuous magnetic flux concentrations. Various properties of the
synthetic brightness images agree well with G-band observations. These
results lends credit to the observational usage of G-band bright
features as proxies for magnetic flux concentrations in the solar
photosphere. However, the converse is only correct in a limited sense:
only a fraction of the magnetic flux concentrations turn out to be
bright in the G band.
---------------------------------------------------------
Title: Stokes diagnostics of magneto-convection. Profile shapes
and asymmetries
Authors: Khomenko, E. V.; Shelyag, S.; Solanki, S. K.; Vögler, A.;
Schüssler, M.
2004IAUS..223..635K Altcode: 2005IAUS..223..635K
We discuss the polarization signals produced in recent realistic 3D
simulations of solar magnetoconvection. The Stokes profiles of the
Fe I 6301.5, 6302.5, 15648 and 15652 mathrm{Å} Zeeman-sensitive
spectral lines are synthesised and smeared to simulate the image
degradation caused by the Earth's atmosphere and finite telescope
resolution. A Principal Component Analysis approach is applied to
classify the profiles. We find that the classes of Stokes V profiles
as well as their amplitude and area asymmetries are very close to the
observations in the network and inter-network regions.
---------------------------------------------------------
Title: Stokes Diagnostics of Magnetoconvection. Profile shapes
and asymmetries.
Authors: Khomenko, E. V.; Shelyag, S.; Solanki, S. K.; Vogler, A.;
Schussler, M.
2004cosp...35.2131K Altcode: 2004cosp.meet.2131K
Stokes profiles observed in the quiet Sun have a broad range of
asymmetries and show a variety of shapes. These asymmetries are the
result of the velocity and magnetic field gradients both in horizontal
and vertical directions. We use the most recent realistic 3D simulations
of magnetoconvection at the solar surface to synthesize Stokes profiles
of some photospheric lines and to compare them with observations. Such
comparison provides an important constrains on the MHD models allowing
to conclude about their realism and, thus, to understand the nature of
solar magnetoconvection. The following Zeeman-sensitive spectral lines
are considered: Fe I 6301.5, 6302.5, 15648 and 15652 Å. These lines are
extensively used in observations. The computed Stokes profiles of these
lines were spatially smeared to simulate the effects of a telescope and
atmospheric seeing. A Principal Component Analysis approach is applied
to classify the profiles. The effects of spatial resolution and the
amount of the magnetic flux in the MHD model on the profile shapes are
discussed. The profiles of different classes are clustered together and
form patches on the surface. The size of these patches decreases with
increasing spatial resolution. The distributions of the amplitude and
area asymmetries of Stokes V profiles are very close to the observations
in network and inter-network regions. Some 15% of the profiles smeared
with a 0.”5 seeing have irregular shape with 3 or more lobes. Finally,
we show that simultaneous observations of the same area of the solar
disc using infrared Fe I 15648, 15652 Å and the visible Fe I 6301.5,
6302.5 Å lines done under different seeing conditions (for example
in the case of simultaneous observations at different telescopes)
may result in different asymmetries and even different polarities
of the profiles in two spectral regions observed at the same spatial
point. This work was partially supported by INTAS grant 00-00084.
---------------------------------------------------------
Title: Spectro-polarimetric diagnostics of magneto-convection
simulations of the solar photosphere
Authors: Shelyag, Sergey
2004PhDT.......388S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Why Solar Magnetic Flux Concentrations Are Bright in Molecular
Bands
Authors: Schüssler, M.; Shelyag, S.; Berdyugina, S.; Vögler, A.;
Solanki, S. K.
2003ApJ...597L.173S Altcode:
Using realistic ab initio simulations of radiative magnetoconvection,
we show that the bright structures in images taken in the “G band,”
a spectral band dominated by lines of the CH molecule, precisely
outline small-scale concentrations of strong magnetic fields on the
visible solar surface. The brightening is caused by a depletion of CH
molecules in the hot and tenuous magnetic structures, thus confirming
the model of radiatively heated magnetic flux concentrations. These
results provide a firm basis for observational studies of the evolution
and dynamics of the small-scale solar magnetic field derived through
“proxy magnetometry” with G-band images.
---------------------------------------------------------
Title: The influence of mergings on galaxy evolution
Authors: Kontorovich, Victor M.; Shelyag, Sergey I.
2003Ap&SS.284..475K Altcode:
The evolution of the galaxy mass function MF (and luminosity function
LF) depending on merging of galaxies is discussed. The richness and
masses of most massive (and most luminous) galaxies of a cluster
are chosen as a characteristic of the bright edge of LF. Mergers are
simulated by a Monte-Carlo method. The probability of merging depends
on the masses of galaxies. The ratio of the current number of galaxies
to the initial one plays the role of the time scale. Transformation
to real time and redshift is realized with help of the Smoluchowsky
kinetic equation (SE) solution, describing the merging process
and making possible to find the dependence of the galaxy number on
time. The dependencies of richness, masses and magnitude differences
of most massive and brightest galaxies of a cluster on redshift
have been obtained. Creation of cD-galaxy has been retraced on
small redshifts. The masses of the second and less massive cluster
galaxies grow, but after the creation of the cD-galaxy they begin to
decrease. Comparison of obtained results with data of the catalogues
is provided. Correspondence in mentioned dependencies is observed.
---------------------------------------------------------
Title: Simulation of Solar Magnetoconvection
Authors: Vögler, A.; Shelyag, S.; Schüssler, M.; Cattaneo, F.;
Emonet, T.; Linde, T.
2003IAUS..210..157V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Galaxy Cluster Mass Function Evolution Caused by Galaxy
Mergings
Authors: Kontorovich, V. M.; Shelyag, S. I.
2002ASPC..268..397K Altcode: 2002tceg.conf..397K
No abstract at ADS
---------------------------------------------------------
Title: The overflow of density singularity by shock generated by
strong explosion
Authors: Shelyag, S. I.
2001KosNT...7S.101S Altcode:
Cosmological nature of the gamma-ray bursts means that energy discharged
from the GRB is greater than energy emitted from supernova explosion,
and is enough to make shock reach neighbouring stars or gas clouds
and remain strong. It implies the possibility of using the Kompaneets
strong explosion approximation for analysis of "hypernovae" remnants
shapes, which may correspond to the gamma-ray bursts. Forms of the
shock generated by a strong explosion in a medium with quadratic
law of density decrease and coming to constant on big distances are
analysed. The overflow of density singularity is observed. Obtained
results are compared with observational data about hypernova explosions.
---------------------------------------------------------
Title: The Enveloping of a Density Singularity by Shock Generated
by Strong Explosion
Authors: Kontorovich, V.; Shelyag, S.
2001AGM....18.P182K Altcode:
Discovering of cosmological nature of Gamma-Ray Bursts (GRB) means that
energy emitted in the GRB is much greater than energy from supernova
explosion. Strong shock wave from such “hypernova” explosion could
reach neighbouring stars or gas clouds and still stay strong. We use the
Kompaneets shock front equation for analysis of “hypernovae” remnants
shapes, which may correspond to the GRBs. Exact solution of Kompaneets
equation for non-central explosion in a nonuniform medium with bi-power
density coordinate dependence is obtained. The shape of the shock (in
particular, for quadratic law of density decrease and constant on big
distances) is analysed analitically. The effects of enveloping of the
density singularity and self-intersection of the shock front beyond
the singularity are obtained. The results compared with observational
data for the features which regard as the hypernova remnant pretenders.
