Author name code: shelyag ADS astronomy entries on 2022-09-14 author:"Shelyag, Sergey I." ------------------------------------------------------------------------ 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.
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.
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.
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 χ2 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
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.
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.
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.
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.
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.
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.
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.
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 asy 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-1, 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 vx and vy 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 104 K with the distribution of vx, 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
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.
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.
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.
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-1. 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-1, 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-1 to ∼1 m s-1) 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-1). 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-1) 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 & Moradi 2013; Moradi & Cally 2014). In this study, we extend our directional time-distance analysis to more realistic sunspot-like atmospheres to better understand the direct effects of the magnetic field on helioseismic travel-time measurements in sunspots. In line with our previous findings, we uncover a distinct frequency-dependent directional behaviour in the travel-time measurements, consistent with the signatures of magnetohydrodynamic mode conversion. We found this to be the case regardless of the sunspot field strength or depth of its Wilson depression. We also isolated and analysed the direct contribution from purely thermal perturbations to the measured travel times, finding that waves propagating in the umbra are much more sensitive to the underlying thermal effects of the sunspot. Title: Ellerman Bombs with Jets: Cause and Effect Authors: Reid, A.; Mathioudakis, M.; Scullion, E.; Doyle, J. G.; Shelyag, S.; Gallagher, P. 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-1, while their chromospheric counterparts range from 50 to 80 km s-1. 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-1. 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-1, 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
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.
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.
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.

Appendix A is available in electronic form at http://www.aanda.org 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-1, whereas their chromospheric counterparts have a slightly higher average velocity of 1.4 km s-1. 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-1 was found in the simulated G-band images and an average of 1.8 km s-1 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 Mm2 areas on the solar surface, to within ~ ±20 cm s-1 on a ~100 m s-1 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-1, with maximum values of 7 km s-1. 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
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.
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.
Results: The Hα images are highly dynamic in nature, with estimated jet velocities as high as 45 km s-1. These jets are one-directional, with their origin seemingly linked to underlying Ca ii K brightenings and G-band magnetic bright points.
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
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.
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.
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 km2, 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
Aims: We analyse mesogranular flow patterns in a three-dimensional hydrodynamical simulation of solar surface convection in order to determine its characteristics.
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.
Results: The characteristics of the resulting flow patterns depend on the averaging time and length used in the analysis.
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
Aims: A magneto-hydrostatic model is constructed with spectropolarimetric properties close to those of solar photospheric magnetic bright points.
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.
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.
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.
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.
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.
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.
Aims: This paper is devoted to an analysis of the influence of steady flows on the propagation of sound waves through the solar interior.
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.
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.
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, kh) 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 < B > = 10 to 140 G. The effects of spatial resolution and of the amount of magnetic flux in the simulation box on the profiles shapes, amplitudes and shifts are discussed. The synthetic spectra show many properties in common with those observed in quiet solar regions. In particular, the simulations reproduce the width and depth of spatially averaged Stokes I profiles, the basic classes of the Stokes V profiles and their amplitude and area asymmetries, as well as the abundance of the irregular-shaped Stokes V profiles. It is demonstrated that the amplitudes of the 1.56 μm lines observed in the inter-network are consistent with a "true" average unsigned magnetic field strength of 20 G. We show that observations using these and visible lines, carried out under different seeing conditions (e.g., simultaneous observations at different telescopes), may result in different asymmetries and even opposite polarities of the profiles in the two spectral regions observed at the same spatial point. Title: Simulations of magneto-convection in the solar photosphere. Equations, methods, and results of the MURaM code Authors: Vögler, A.; Shelyag, S.; Schüssler, M.; Cattaneo, F.; Emonet, T.; Linde, T. 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.

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.