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.
Bibcode: 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.

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.
Bibcode: 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.

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.
Bibcode: 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.

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.
Bibcode: 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.

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.
Bibcode: 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.

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.
Bibcode: 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.

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.
Bibcode: 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.

Title: The multi fluid description of the chromospheric motions
Authors: Tkachenko, A.; Shelyag, S.; Krasnosselskikh, V.; Le Phuong, L.
Bibcode: 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.

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.
Bibcode: 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.

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.
Bibcode: 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.

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.
Bibcode: 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.

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.
Bibcode: 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.

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.
Bibcode: 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.

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.
Bibcode: 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.

Title: The role of partial ionization in solar chromospheric heating
Authors: Shelyag, S.; Khomenko, E.; Przybylski, D.; Vitas, N.; de
   Vicente, A.
Bibcode: 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.

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.
Bibcode: 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.

Title: Heating of the Partially Ionized Solar Chromosphere by Waves
    in Magnetic Structures
Authors: Shelyag, S.; Khomenko, E.; de Vicente, A.; Przybylski, D.
Bibcode: 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.

Title: Alfvén Wave Heating of the Solar Chromosphere: 1.5D Models
Authors: Arber, T. D.; Brady, C. S.; Shelyag, S.
Bibcode: 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.

Title: Spectropolarimetrically Accurate Magnetohydrostatic Sunspot
    Model for Forward Modeling in Helioseismology
Authors: Przybylski, D.; Shelyag, S.; Cally, P. S.
Bibcode: 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.

Title: Directional time-distance probing of model sunspot atmospheres
Authors: Moradi, H.; Cally, P. S.; Przybylski, D.; Shelyag, S.
Bibcode: 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 &amp; Moradi 2013; Moradi &amp; 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 &lt; B &gt; = 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.

