Author name code: khomenko ADS astronomy entries on 2022-09-14 author:"Khomenko, Elena V." ------------------------------------------------------------------------ Title: Observational and numerical characterization of a recurrent arc-shaped front propagating along a coronal fan Authors: Sieyra, M. V.; Krishna Prasad, S.; Stenborg, G.; Khomenko, E.; Van Doorsselaere, T.; Costa, A.; Esquivel, A.; Riedl, J. M. Bibcode: 2022arXiv220810857S Altcode: Recurrent, arc-shaped intensity disturbances were detected by EUV channels in an active region. The fronts were observed to propagate along a coronal loop bundle rooted in a small area within a sunspot umbra. Previous works have linked these intensity disturbances to slow magnetoacoustic waves that propagate from the lower atmosphere to the corona along the magnetic field. The slow magnetoacoustic waves propagate at the local cusp speed. However, the measured propagation speeds from the intensity images are usually smaller as they are subject to projection effects due to the inclination of the magnetic field with respect to the line-of-sight. Here, we aim to understand the effect of projection by comparing observed speeds with those from a numerical model. Using multi-wavelength data we determine the periods present in the observations at different heights of the solar atmosphere through Fourier analysis. We calculate the plane-of-sky speeds along one of the loops from the cross-correlation time lags obtained as a function of distance along the loop. We perform a 2D ideal MHD simulation of an active region embedded in a stratified atmosphere. We drive slow waves from the photosphere with a 3 minutes periodicity. Synthetic time-distance maps are generated from the forward-modelled intensities in coronal wavelengths and the projected propagation speeds are calculated. The intensity disturbances show a dominant period between [2-3] minutes at different heights of the atmosphere. The apparent propagation speeds calculated for coronal channels exhibit an accelerated pattern with values increasing from 40 to 120 km/s as the distance along the loop rises. The propagation speeds obtained from the synthetic time-distance maps also exhibit accelerated profiles within a similar range of speeds. We conclude that the accelerated propagation in our observations is due to the projection effect. Title: Coupling between different atmospheric layers: waves and energy transfer Authors: Khomenko, Elena Bibcode: 2022cosp...44.2545K Altcode: Solar atmosphere is nowadays viewed as an entire coupled system, where the dynamical events observed in a certain layer can be consequences or causes of those detected in the layers above or below. Photospheric flows, interacting with magnetic structures, help energy propagation to the chromosphere and above in the form of waves or vortex flows. These processes are frequency dependent, and there are indications that the high-frequency end of the spectrum is important for energizing the solar atmosphere. Nevertheless, going towards high frequencies is challenging both from the instrumental and modeling point of view. In the case of the latter, new physical aspects are being included, such as interaction between neutrals and plasma. In this talk I will review the recent progress in observational and theoretical works on high-frequency waves, shocks and vorticity propagation through the solar atmosphere, with an emphasis onto multi-fluid modeling of these dynamical features. Title: Acoustic-gravity wave propagation characteristics in 3D radiation hydrodynamic simulations of the solar atmosphere Authors: Fleck, Bernhard; Khomenko, Elena; Carlsson, Mats; Rempel, Matthias; Steiner, Oskar; Riva, Fabio; Vigeesh, Gangadharan Bibcode: 2022cosp...44.2503F Altcode: There has been tremendous progress in the degree of realism of three-dimensional radiation magneto-hydrodynamic simulations of the solar atmosphere in the past decades. Four of the most frequently used numerical codes are Bifrost, CO5BOLD, MANCHA3D, and MURaM. Here we test and compare the wave propagation characteristics in model runs from these four codes by measuring the dispersion relation of acoustic-gravity waves at various heights. We find considerable differences between the various models. Title: Modeling the thermal conduction in the solar atmosphere with the code MANCHA3D Authors: Navarro, A.; Khomenko, E.; Modestov, M.; Vitas, N. Bibcode: 2022A&A...663A..96N Altcode: 2022arXiv220508846N Context. Thermal conductivity is one of the important mechanisms of heat transfer in the solar corona. In the limit of strongly magnetized plasma, it is typically modeled by Spitzer's expression where the heat flux is aligned with the magnetic field.
Aims: This paper describes the implementation of the heat conduction into the code MANCHA3D with an aim of extending single-fluid MHD simulations from the upper convection zone into the solar corona.
Methods: Two different schemes to model heat conduction are implemented: (1) a standard scheme where a parabolic term is added to the energy equation, and (2) a scheme where the hyperbolic heat flux equation is solved.
Results: The first scheme limits the time step due to the explicit integration of a parabolic term, which makes the simulations computationally expensive. The second scheme solves the limitations on the time step by artificially limiting the heat conduction speed to computationally manageable values. The validation of both schemes is carried out with standard tests in one, two, and three spatial dimensions. Furthermore, we implement the model for heat flux derived by Braginskii (1965, Reviews of Plasma Physics, 205) in its most general form, when the expression for the heat flux depends on the ratio of the collisional to cyclotron frequencies of the plasma, and, therefore on the magnetic field strength. Additionally, our implementation takes into account the heat conduction in parallel, perpendicular, and transverse directions, and provides the contributions from ions and electrons separately. The model recovers Spitzer's expression for parallel thermal conductivity in the strongly magnetized limit but also transitions smoothly between field-aligned conductivity and isotropic conductivity for regions with a low or null magnetic field. We describe the details of the implementation of Braginskii's thermal conductivity using a combination of the first scheme for the perpendicular and transverse directions and the second scheme for the parallel component. We estimate thermal conductivities in a quiet-Sun model. In this model, we find that the perpendicular and transverse components for electrons and ions and the parallel component for ions might have some significance below the transition region. Above the transition region only the parallel component for ions might be important. Finally, we present a two-dimensional test for heat conduction using realistic values of the solar atmosphere where we prove the robustness of the two schemes implemented and show that our adaptation of the hyperbolic treatment offers a great advantage over the computational cost of the simulations. Title: The European Solar Telescope Authors: Quintero Noda, C.; Schlichenmaier, R.; Bellot Rubio, L. R.; Löfdahl, M. G.; Khomenko, E.; Jurcak, J.; Leenaarts, J.; Kuckein, C.; González Manrique, S. J.; Gunar, S.; Nelson, C. J.; de la Cruz Rodríguez, J.; Tziotziou, K.; Tsiropoula, G.; Aulanier, G.; Collados, M.; the EST team Bibcode: 2022arXiv220710905Q Altcode: The European Solar Telescope (EST) is a project aimed at studying the magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. Its design combines the knowledge and expertise gathered by the European solar physics community during the construction and operation of state-of-the-art solar telescopes operating in visible and near-infrared wavelengths: the Swedish 1m Solar Telescope (SST), the German Vacuum Tower Telescope (VTT) and GREGOR, the French Télescope Héliographique pour l'Étude du Magnétisme et des Instabilités Solaires (THÉMIS), and the Dutch Open Telescope (DOT). With its 4.2 m primary mirror and an open configuration, EST will become the most powerful European ground-based facility to study the Sun in the coming decades in the visible and near-infrared bands. EST uses the most innovative technological advances: the first adaptive secondary mirror ever used in a solar telescope, a complex multi-conjugate adaptive optics with deformable mirrors that form part of the optical design in a natural way, a polarimetrically compensated telescope design that eliminates the complex temporal variation and wavelength dependence of the telescope Mueller matrix, and an instrument suite containing several (etalon-based) tunable imaging spectropolarimeters and several integral field unit spectropolarimeters. This publication summarises some fundamental science questions that can be addressed with the telescope, together with a complete description of its major subsystems. Title: Doppler-velocity Drifts Detected in a Solar Prominence Authors: Zapiór, Maciej; Heinzel, Petr; Khomenko, Elena Bibcode: 2022ApJ...934...16Z Altcode: We analyzed multiline observations of a quiescent prominence from the slit spectrograph located at the Ondřejov Observatory. Dopplergrams and integrated intensity maps of the whole prominence were obtained from observations in six spectral lines: Ca II H, Hϵ, Hβ, He I D3, Hα, and Ca II IR. By combining integrated intensity maps with non-LTE radiative-transfer modeling, we carefully identified areas in an optically thin regime. The comparison of the Doppler-velocity maps and scatterplots from different lines shows the existence of differences in the velocity of ions and neutrals called velocity drift. The drift is of a local nature, present mostly at prominence edges in the area with a large velocity gradient, as can be tentatively expected based on multifluid MHD models. We could not explore the time evolution of the drift, since our data set consists of a single scan only. Our paper brings another contribution to a rather controversial problem of the detection of multifluid effects in solar prominences. Title: Numerical simulations of large-amplitude oscillations in solar prominences triggered by an eruptive event Authors: Liakh, Valeriia; Khomenko, Elena; Luna, Manuel Bibcode: 2022cosp...44.2541L Altcode: Large-amplitude oscillations (LAOs) in solar prominences are very spectacular phenomena. Their activation is often associated with external energetic disturbance, such as waves from the flare or eruption, nearby magnetic reconnection, jets. This study represents the first attempt to understand how the perturbations produced by the eruptive event trigger LAOs in the solar prominences using the capabilities of numerical simulations. In these numerical experiments, the prominence model consists of the flux rope magnetic structure formed from the sheared arcade using the footpoints driving and the prominence plasma loaded into the bottom helical part of the pre-formed flux rope. The main source of the disturbances is represented by the eruptive flux rope formed similarly from the sheared arcade with a combination of the converging and shearing motions at the base of the magnetic field lines. On the one hand, we have investigated the possibility of the excitation of Moreton and EIT waves by the eruptive flux rope. On the other hand, we have studied the triggering of LAOs by these energetic waves. In the second phase of the numerical experiment, the tearing instability develops in the elongated current sheet below the erupting flux rope. This phase is characterized by the chaotic formation of the magnetic islands in the current sheet. When merging with post-reconnection loops, the formed plasmoids bring additional perturbations in the velocity field. Title: Transverse kink oscillations of inhomogeneous prominence threads Authors: Martínez-Gómez, David; Terradas, Jaume; Soler, Roberto; Khomenko, Elena Bibcode: 2022cosp...44.2556M Altcode: Observations of transverse oscillations in solar prominence threads are usually interpreted as the fundamental kink mode, while the detection of the first harmonic remains elusive. It is known that the properties of these oscillations are greatly affected by the density distribution along the magnetic flux tube. In this talk, we present the results of a numerical study on how the density inhomogeneity in the longitudinal and radial directions modify the periods and damping times of kink oscillations, and how this effect would be reflected in the observations. We have solved the ideal magnetohydrodynamics equations through two different methods: a) performing 3D numerical simulations, and b) solving a 2D generalised eigenvalue problem. We have studied the dependence of the periods, damping times and amplitudes of transverse kink oscillations on the ratio between the densities at the centre and at the ends of the tube, and also on the average density. Then, we have applied forward modelling on our 3D simulations and computed synthetic H-alpha profiles to obtain the observational signatures of several linear and non-linear features of the oscillations. Our results confirm that the ratio of the period of the fundamental oscillation mode to the period of the first harmonic increases as the ratio of the central density to the footpoint density is increased or as the average density of the tube is decreased. The damping times due to resonant absorption decrease as the central to footpoint density increases. Contrary to the case of longitudinally homogeneous threads, we found that the damping time to period ratio also increases as the density ratio is increases or the average density is reduced. The analysis of the synthetic profiles shows that the H-alpha emission can be used to detect the fundamental mode of oscillation, but the first harmonic is barely detectable in H-alpha, which may explain the lack of observations of this mode. Therefore, a combination of different spectral lines is required to get information about the period ratio and to use it to infer physical properties of the threads. Title: Generalized Fluid Models of the Braginskii Type Authors: Hunana, P.; Passot, T.; Khomenko, E.; Martínez-Gómez, D.; Collados, M.; Tenerani, A.; Zank, G. P.; Maneva, Y.; Goldstein, M. L.; Webb, G. M. Bibcode: 2022ApJS..260...26H Altcode: 2022arXiv220111561H Several generalizations of the well-known fluid model of Braginskii (1965) are considered. We use the Landau collisional operator and the moment method of Grad. We focus on the 21-moment model that is analogous to the Braginskii model, and we also consider a 22-moment model. Both models are formulated for general multispecies plasmas with arbitrary masses and temperatures, where all of the fluid moments are described by their evolution equations. The 21-moment model contains two "heat flux vectors" (third- and fifth-order moments) and two "viscosity tensors" (second- and fourth-order moments). The Braginskii model is then obtained as a particular case of a one ion-electron plasma with similar temperatures, with decoupled heat fluxes and viscosity tensors expressed in a quasistatic approximation. We provide all of the numerical values of the Braginskii model in a fully analytic form (together with the fourth- and fifth-order moments). For multispecies plasmas, the model makes the calculation of the transport coefficients straightforward. Formulation in fluid moments (instead of Hermite moments) is also suitable for implementation into existing numerical codes. It is emphasized that it is the quasistatic approximation that makes some Braginskii coefficients divergent in a weakly collisional regime. Importantly, we show that the heat fluxes and viscosity tensors are coupled even in the linear approximation, and that the fully contracted (scalar) perturbations of the fourth-order moment, which are accounted for in the 22-moment model, modify the energy exchange rates. We also provide several appendices, which can be useful as a guide for deriving the Braginskii model with the moment method of Grad. Title: Transverse kink oscillations of inhomogeneous prominence threads: Numerical analysis and Hα forward modelling Authors: Martínez-Gómez, David; Soler, Roberto; Terradas, Jaume; Khomenko, Elena Bibcode: 2022A&A...658A.106M Altcode: 2021arXiv211109036M Context. Prominence threads are very long and thin flux tubes that are partially filled with cold plasma. Observations have shown that transverse oscillations are frequent in these solar structures. The observations are usually interpreted as the fundamental kink mode, while the detection of the first harmonic remains elusive.
Aims: The properties of oscillations in threads are greatly affected by the density distribution along the flux tube. Here, we aim to study how the density inhomogeneities in the longitudinal and radial directions modify the periods and damping times of kink oscillations and how this effect would be reflected in the observations.
Methods: We solved the ideal magnetohydrodynamics equations using two different methods: (a) performing 3D numerical simulations and (b) solving a 2D generalised eigenvalue problem. We studied the dependence of the periods, damping times, and amplitudes of transverse kink oscillations on the ratio between the densities at the centre and at the ends of the tube, and also on the average density. We applied forward modelling to our 3D simulations to compute synthetic Hα profiles.
Results: We confirm that the ratio of the period of the fundamental oscillation mode to the period of the first harmonic increases as the ratio of the central density to the footpoint density is increased, or as the averaged density of the tube is decreased. We find that the damping times due to resonant absorption decrease as the central-to-footpoint density ratio increases. Contrary to the case of longitudinally homogeneous tubes, we find that the damping-time-to-period ratio also increases as the density ratio is increased or the average density is reduced. We present snapshots and time-distance diagrams of the emission in the Hα line.
Conclusions: The results presented here have implications for the field of prominence seismology. While the Hα emission can be used to detect the fundamental mode, the first harmonic is barely detectable in Hα. This may explain the lack of detections of the first harmonic. A combination of different spectral lines is required to obtain information about the period ratio and to use it to infer physical properties of the threads.

Movies associated to Figs. 11-14 are available at https://www.aanda.org Title: Ambipolar Diffusion in the Lower Solar Atmosphere: MHD Simulations of a Sunspot Authors: MacBride, Conor; Jess, David; Khomenko, Elena Bibcode: 2021AGUFMSH25A2065M Altcode: Behind MHD is the assumption that there is a low degree of plasma ionization. However, in the upper photosphere and chromosphere a reduced temperature often results in the partial ionization of the plasma. The interaction between the decoupled neutral and ionized components of such a partially ionized plasma produce ambipolar diffusion. To investigate the role of ambipolar diffusion in energy transfer and dissipation in the chromosphere, we analyze a 2D numerical model of magnetoacoustic waves propagating through the chromosphere above the umbra of a sunspot. We solve the ideal MHD equations for perturbations to the magnetostatic equilibrium using the Mancha code. The effect of ambipolar diffusion is isolated by varying the inclusion of additional terms in the MHD equations which account for this process. Data-driven perturbations are introduced in the photosphere using observational SDO/HMI umbral velocity time series. Using energy spectra, we analyze the chromospheric dynamics of our simulation and gather insights into the role of ambipolar diffusion, and partial ionization, in energy transfer and dissipation in the lower solar atmosphere. Title: Magnetic field amplification and structure formation by the Rayleigh-Taylor instability Authors: Popescu Braileanu, B.; Lukin, V. S.; Khomenko, E. Bibcode: 2021arXiv211213043P Altcode: We report on results of high resolution two fluid non-linear simulations of the Rayleigh Taylor Instability (RTI) at the interface between a solar prominence and the corona. These follow results reported earlier by Popescu Braileanu et al. (2021a,b) on linear and early non-linear RTI dynamics in this environment. The simulations use a two fluid model that includes collisions between neutrals and charges, including ionization/recombination, energy and momentum transfer, and frictional heating. High resolution 2.5D magnetized RTI simulations with the magnetic field dominantly normal to and slightly sheared with respect to the prominence plane demonstrate that in a fully developed state of RTI a large fraction of the gravitational energy of a prominence thread can be converted into quasi-turbulent energy of the magnetic field. RTI magnetic energy generation is further accompanied by magnetic and plasma density structure formation, including dynamic formation, break-up, and merging of current sheets and plasmoid sub-structures. The simulations show the role of flow decoupling and ionization/recombination reactions between the neutrals and charges on the structure formation in magnetized RTI. We provide a careful examination of sources and form of numerical dissipation of the evolving magnetic field structures. Title: Large-amplitude longitudinal oscillations in solar prominences simulated with different resolutions Authors: Liakh, V.; Luna, M.; Khomenko, E. Bibcode: 2021A&A...654A.145L Altcode: 2021arXiv210801143L Context. Large-amplitude longitudinal oscillations (LALOs) in solar prominences have been widely studied in recent decades. However, their damping and amplification mechanisms are not well understood.
Aims: In this study, we investigate the attenuation and amplification of LALOs using high-resolution numerical simulations with progressively increasing spatial resolutions.
Methods: We performed time-dependent numerical simulations of LALOs using the 2D magnetic configuration that contains a dipped region. After the prominence mass loading in the magnetic dips, we triggered LALOs by perturbing the prominence mass along the magnetic field. We performed the experiments with four values of spatial resolution.
Results: In the simulations with the highest resolution, the period shows good agreement with the pendulum model. The convergence experiment revealed that the damping time saturates at the bottom prominence region with increasing resolution, indicating the existence of a physical reason for the damping of oscillations. At the prominence top, the oscillations are amplified during the first minutes and are then slowly attenuated. The characteristic time suggests more significant amplification in the experiments with the highest spatial resolution. The analysis revealed that the energy exchange between the bottom and top prominence regions is responsible for the attenuation and amplification of LALOs.
Conclusions: High-resolution experiments are crucial when studying the periods and the damping mechanism of LALOs. The period agrees with the pendulum model only when using a sufficiently high spatial resolution. The results suggest that numerical diffusion in simulations with insufficient spatial resolution can hide important physical mechanisms, such as amplification of oscillations. Title: Simulations of the Biermann battery mechanism in two-fluid partially ionised plasmas Authors: Martínez-Gómez, D.; Popescu Braileanu, B.; Khomenko, E.; Hunana, P. Bibcode: 2021A&A...650A.123M Altcode: 2021arXiv210406956M Context. In the absence of an initial seed, the Biermann battery term of a non-ideal induction equation acts as a source that generates weak magnetic fields. These fields are then amplified via a dynamo mechanism. The Kelvin-Helmholtz instability is a fluid phenomenon that takes place in many astrophysical scenarios and can trigger the action of the Biermann battery and dynamo processes.
Aims: We aim to investigate the effect of the ionisation degree of the plasma and the interaction between the charged and neutral species on the generation and amplification of magnetic fields during the different stages of the instability.
Methods: We use the two-fluid model implemented in the numerical code MANCHA-2F. We perform 2D simulations starting from a configuration with no initial magnetic field and which is unstable due to a velocity shear. We vary the ionisation degree of the plasma and we analyse the role that the different collisional terms included in the equations of the model play on the evolution of the instability and the generation of magnetic field.
Results: We find that when no collisional coupling is considered between the two fluids, the effect of the Biermann battery mechanism does not depend on the ionisation degree. However, when elastic collisions are taken into account, the generation of magnetic field is increased as the ionisation degree is reduced. This behaviour is slightly enhanced if the process of charge-exchange is also considered. We also find a dependence on the total density of the plasma related to the dependence on the coupling degree between the two fluids. As the total density is increased, the results from the two-fluid model converge to the predictions of single-fluid models.
Conclusions: The charged-neutral interaction in a partially ionised plasmas has a non-negligible effect on the Biermann battery mechanism and it effectively enhances the generation of a magnetic field. In addition, single-fluid models, which assume a very strong coupling between the two species, may overestimate the contribution of this interaction in comparison with two-fluid models.

Movies associated to Figs. 2 and A.2 are available at https://www.aanda.org Title: Two-fluid simulations of Rayleigh-Taylor instability in a magnetized solar prominence thread. II. Effects of collisionality Authors: Popescu Braileanu, B.; Lukin, V. S.; Khomenko, E.; de Vicente, Á. Bibcode: 2021A&A...650A.181P Altcode: 2021arXiv210112731P Solar prominences are formed by partially ionized plasma with inter-particle collision frequencies generally warranting magnetohydrodynamic treatment. In this work we explore the dynamical impacts and observable signatures of two-fluid effects in the parameter regimes when ion-neutral collisions do not fully couple the neutral and charged fluids. We perform 2.5D two-fluid (charges-neutrals) simulations of the Rayleigh-Taylor instability (RTI) at a smoothly changing interface between a solar prominence thread and the corona. The purpose of this study is to deepen our understanding of the RTI and the effects of the partial ionization on the development of RTI using nonlinear two-fluid numerical simulations. Our two-fluid model takes into account neutral viscosity, thermal conductivity, and collisional interaction between neutrals and charges: ionization-recombination, energy and momentum transfer, and frictional heating. In this paper, the sensitivity of the RTI dynamics to collisional effects for different magnetic field configurations supporting the prominence thread is explored. This is done by artificially varying, or eliminating, effects of both elastic and inelastic collisions by modifying the model equations. We find that ionization and recombination reactions between ionized and neutral fluids do not substantially impact the development of the primary RTI. However, such reactions can impact the development of secondary structures during the mixing of the cold prominence and hotter surrounding coronal material. We find that collisionality within and between ionized and neutral particle populations plays an important role in both linear and nonlinear development of RTI; ion-neutral collision frequency is the primary determining factor in development or damping of small-scale structures. We also observe that the degree and signatures of flow decoupling between ion and neutral fluids can depend on the inter-particle collisionality and on the magnetic field configuration of the prominence thread. Title: Chromospheric Heating by Magnetohydrodynamic Waves and Instabilities Authors: Srivastava, A. K.; Ballester, J. L.; Cally, P. S.; Carlsson, M.; Goossens, M.; Jess, D. B.; Khomenko, E.; Mathioudakis, M.; Murawski, K.; Zaqarashvili, T. V. Bibcode: 2021JGRA..12629097S Altcode: 2021arXiv210402010S The importance of the chromosphere in the mass and energy transport within the solar atmosphere is now widely recognized. This review discusses the physics of magnetohydrodynamic waves and instabilities in large-scale chromospheric structures as well as in magnetic flux tubes. We highlight a number of key observational aspects that have helped our understanding of the role of the solar chromosphere in various dynamic processes and wave phenomena, and the heating scenario of the solar chromosphere is also discussed. The review focuses on the physics of waves and invokes the basics of plasma instabilities in the context of this important layer of the solar atmosphere. Potential implications, future trends and outstanding questions are also delineated. Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST) Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio, Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart; Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa, Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler, Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun, Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres, Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.; Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini, Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena; Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor; Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael; Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli, Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys, Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.; Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis, Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson, Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.; Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.; Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava, Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas, Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST Instrument Scientists; DKIST Science Working Group; DKIST Critical Science Plan Community Bibcode: 2021SoPh..296...70R Altcode: 2020arXiv200808203R The National Science Foundation's Daniel K. Inouye Solar Telescope (DKIST) will revolutionize our ability to measure, understand, and model the basic physical processes that control the structure and dynamics of the Sun and its atmosphere. The first-light DKIST images, released publicly on 29 January 2020, only hint at the extraordinary capabilities that will accompany full commissioning of the five facility instruments. With this Critical Science Plan (CSP) we attempt to anticipate some of what those capabilities will enable, providing a snapshot of some of the scientific pursuits that the DKIST hopes to engage as start-of-operations nears. The work builds on the combined contributions of the DKIST Science Working Group (SWG) and CSP Community members, who generously shared their experiences, plans, knowledge, and dreams. Discussion is primarily focused on those issues to which DKIST will uniquely contribute. Title: Modeling of 3d Atmospheres of Cool Stars with the Mancha Code Authors: Perdomo, Andrea; Vitas, Nikola; Khomenko, Elena; Collados, Manuel Bibcode: 2021csss.confE.129P Altcode: The first results of the application of the MANCHA code to the case of stars beyond the solar case are presented: hydrodynamical simulations of stars of spectral type K0V and M0V compared with the solar case. Title: Accurately constraining velocity information from spectral imaging observations using machine learning techniques Authors: MacBride, Conor D.; Jess, David B.; Grant, Samuel D. T.; Khomenko, Elena; Keys, Peter H.; Stangalini, Marco Bibcode: 2021RSPTA.37900171M Altcode: 2020arXiv200707904M Determining accurate plasma Doppler (line-of-sight) velocities from spectroscopic measurements is a challenging endeavour, especially when weak chromospheric absorption lines are often rapidly evolving and, hence, contain multiple spectral components in their constituent line profiles. Here, we present a novel method that employs machine learning techniques to identify the underlying components present within observed spectral lines, before subsequently constraining the constituent profiles through single or multiple Voigt fits. Our method allows active and quiescent components present in spectra to be identified and isolated for subsequent study. Lastly, we employ a Ca ɪɪ 8542 Å spectral imaging dataset as a proof-of-concept study to benchmark the suitability of our code for extracting two-component atmospheric profiles that are commonly present in sunspot chromospheres. Minimization tests are employed to validate the reliability of the results, achieving median reduced χ2-values equal to 1.03 between the observed and synthesized umbral line profiles.

This article is part of the Theo Murphy meeting issue `High-resolution wave dynamics in the lower solar atmosphere'. Title: Two-fluid simulations of Rayleigh-Taylor instability in a magnetized solar prominence thread. I. Effects of prominence magnetization and mass loading Authors: Popescu Braileanu, B.; Lukin, V. S.; Khomenko, E.; de Vicente, Á. Bibcode: 2021A&A...646A..93P Altcode: 2020arXiv200715984P Solar prominences are formed by partially ionized plasma with inter-particle collision frequencies, which generally warrant magnetohydrodynamic treatment. In this work, we explore the dynamical impacts and observable signatures of two-fluid effects in the parameter regimes when ion-neutral collisions do not fully couple the neutral and charged fluids. We performed 2.5D two-fluid (charge - neutrals) simulations of the Rayleigh-Taylor instability (RTI) at a smoothly changing interface between a solar prominence thread and the corona. The purpose of this study is to deepen our understanding of the RTI and the effects of partial ionization on the development of the RTI using nonlinear two-fluid numerical simulations. Our two-fluid model takes into account viscosity, thermal conductivity, and collisional interaction between neutrals and charge: ionization or recombination, energy and momentum transfer, and frictional heating. In this paper, we explore the sensitivity of the RTI dynamics to the prominence equilibrium configuration, including the impact of the magnetic field strength and shear supporting the prominence thread, and the amount of prominence mass-loading. We show that at small scales, a realistically smooth prominence-corona interface leads to qualitatively different linear RTI evolution than that which is expected for a discontinuous interface, while magnetic field shear has the stabilizing effect of reducing the growth rate or eliminating the instability. In the nonlinear phase, we observe that in the presence of field shear the development of the instability leads to formation of coherent and interacting 2.5D magnetic structures, which, in turn, can lead to substantial plasma flow across magnetic field lines and associated decoupling of the fluid velocities of charged particles and neutrals. Title: Influence of ambipolar and Hall effects on vorticity in three-dimensional simulations of magneto-convection Authors: Khomenko, E.; Collados, M.; Vitas, N.; González-Morales, P. A. Bibcode: 2021RSPTA.37900176K Altcode: 2020arXiv200909753K This paper presents the results of the analysis of three-dimensional simulations of solar magneto-convection that include the joint action of the ambipolar diffusion and the Hall effect. Three simulation runs are compared: one including both ambipolar diffusion and the Hall effect; one including only ambipolar diffusion and one without any of these two effects. The magnetic field is amplified from initial field to saturation level by the action of turbulent local dynamo. In each of these cases, we study 2 h of simulated solar time after the local dynamo reaches the saturation regime. We analyse the power spectra of vorticity, of magnetic field fluctuations and of the different components of the magnetic Poynting flux responsible for the transport of vertical or horizontal perturbations. Our preliminary results show that the ambipolar diffusion produces a strong reduction of vorticity in the upper chromospheric layers and that it dissipates the vortical perturbations converting them into thermal energy. The Hall effect acts in the opposite way, strongly enhancing the vorticity. When the Hall effect is included, the magnetic field in the simulations becomes, on average, more vertical and long-lived flux tube-like structures are produced. We trace a single magnetic structure to study its evolution pattern and the magnetic field intensification, and their possible relation to the Hall effect.

This article is part of the Theo Murphy meeting issue `High-resolution wave dynamics in the lower solar atmosphere'. Title: Acoustic-gravity wave propagation characteristics in three-dimensional radiation hydrodynamic simulations of the solar atmosphere Authors: Fleck, B.; Carlsson, M.; Khomenko, E.; Rempel, M.; Steiner, O.; Vigeesh, G. Bibcode: 2021RSPTA.37900170F Altcode: 2020arXiv200705847F There has been tremendous progress in the degree of realism of three-dimensional radiation magneto-hydrodynamic simulations of the solar atmosphere in the past decades. Four of the most frequently used numerical codes are Bifrost, CO5BOLD, MANCHA3D and MURaM. Here we test and compare the wave propagation characteristics in model runs from these four codes by measuring the dispersion relation of acoustic-gravity waves at various heights. We find considerable differences between the various models. The height dependence of wave power, in particular of high-frequency waves, varies by up to two orders of magnitude between the models, and the phase difference spectra of several models show unexpected features, including ±180° phase jumps.

This article is part of the Theo Murphy meeting issue `High-resolution wave dynamics in the lower solar atmosphere'. Title: Effects of neutrals on magnetic Rayleigh Taylor instability in solar prominences Authors: Khomenko, Elena; Lukin, Vyacheslav; Popescu Braileanu, Beatrice Bibcode: 2021cosp...43E.976K Altcode: The Rayleigh Taylor instability has been frequently observed at the interface between solar prominences and corona. Prominence plasma contains a large fraction of neutrals, and their role on the stability of these structures is not fully understood. Here we study the behavior of plasma and neutral components during the Rayleigh Taylor instability in a thread of prominence material, using two-fluid numerical simulations. Our model takes into account elastic collisions, ionization/recombination, thermal exchange, neutral viscosity and conductivity. We study the effects of the magnetic field strength, orientation, and the density contrast of the thread on the growth rate of the instability, both in the linear and non-linear phases. We observe that, while large-scale harmonics grow exponentially in the linear phase of the instability, for intermediate and small scales, affected by viscosity, thermal conduction, and the interaction between neutrals and charges, the linear and the nonlinear phases are superposed. This behavior affects the thermal and dynamic decoupling of the components. We observe differences in the neutral and plasma velocities of the order of several hundreds of m/s, and differences in their temperature of the order of several tens of Kelvin. The ionization-recombination imbalance results in creation of a layer of increased density of charges. This layer follows the evolution of the eddies, and it potentially observable. Elastic collisions influence the growth rate similarly to the viscosity: an increase of collisions results in a larger growth rate at small scales. Larger density contrast increases the growth rate, contrary to the effect of increasing the strength of the magnetic field. Title: Joint action of Hall and ambipolar effects in 3D magneto-convection simulations of the quiet Sun. I. Dissipation and generation of waves Authors: González-Morales, P. A.; Khomenko, E.; Vitas, N.; Collados, M. Bibcode: 2020A&A...642A.220G Altcode: 2020arXiv200810429G The partial ionization of the solar plasma causes several nonideal effects such as the ambipolar diffusion, the Hall effect, and the Biermann battery effect. Here we report on the first three-dimensional realistic simulations of solar local dynamo where all three effects were taken into account. The simulations started with a snapshot of already saturated battery-seeded dynamo, where two new series were developed: one with solely ambipolar diffusion and another one also taking into account the Hall term in the generalized Ohm's law. The simulations were then run for about 4 h of solar time to reach the stationary regime and improve the statistics. In parallel, a purely MHD dynamo simulation was also run for the same amount of time. The simulations are compared in a statistical way. We consider the average properties of simulation dynamics, the generation and dissipation of compressible and incompressible waves, and the magnetic Poynting flux. The results show that, with the inclusion of the ambipolar diffusion, the amplitudes of the incompressible perturbations related to Alfvén waves are reduced, and the Poynting flux is absorbed, with a frequency dependence. The Hall effect causes the opposite action: significant excess of incompressible perturbations is generated and an excess of the Poynting flux is observed in the chromospheric layers. The model with ambipolar diffusion shows, on average, sharper current sheets and slightly more abundant fast magneto-acoustic shocks in the chromosphere. The model with the Hall effect has higher temperatures at the lower chromosphere and stronger and more vertical magnetic field concentrations all over the chromosphere. The study of high-frequency waves reveals that significant power of incompressible perturbations is associated with areas with intense and more vertical magnetic fields and larger temperatures. This behavior explains the large Poynting fluxes in the simulations with the Hall effect and provides confirmation as to the role of Alfvén waves in chromospheric heating in internetwork regions, under the action of both Hall and ambipolar effects. We find a positive correlation between the magnitude of the ambipolar heating and the temperature increase at the same location after a characteristic time of 102 s. Title: Numerical simulations of large-amplitude oscillations in flux rope solar prominences Authors: Liakh, V.; Luna, M.; Khomenko, E. Bibcode: 2020sea..confE.204L Altcode: This study is based on the 2.5D numerical simulations of the large-amplitude oscillations (LAOs) in the flux rope solar prominences. The prominence models with two different values of the initial shear angle and the density contrast were considered. In order to investigate the possible normal modes of the structure, the prominence was perturbed with the horizontal and vertical disturbances. To study the mechanism of the external LAOs triggering, we used the disturbance placed out of the flux rope. The transverse and longitudinal oscillation periods do not show a strong dependence on the shear angle and the density contrast. The external perturbation excites the oscillations of both polarizations, and their properties are a mixture of those excited by purely horizontal and vertical driving. Title: Local dynamo in stars beyond the Sun: Study for a K0V star Authors: Perdomo García, A.; Vitas, N.; Khomenko, E.; Collados Vera, M. A. Bibcode: 2020sea..confE.206P Altcode: We present the first results of application of the MANCHA3D code (Felipe 2010; Khomenko et al. 2017, 2018) to a K0V cool star. Initially we run the code solving purely hydrodynamic equations until the stationary convection is reached. Then we produce the magnetic field generation and amplification by Biermann's battery seed and local dynamo. We find values around 100 Gauss for the amplified saturated magnetic field, similar to those found in Khomenko et al. (2017) for the solar case. Title: 2D simulations of the Biermann battery mechanism in partially ionized plasmas Authors: Martínez-Gómez, D.; Popescu Braileanu, B.; Khomenko, E.; Hunana, P. Bibcode: 2020sea..confE.205M Altcode: In the absence of an initial seed, the Biermann battery term of a non-ideal induction equation acts a source that generates weak magnetic fields. Here, we study this mechanism in the context of partially ionized plasmas, using a model in which the charged and neutral components of the plasma are treated as two different fluids that interact by means of collisions. We investigate the effect that the ionization degree and the charged-neutral interaction have on the generation of magnetic field. We use the numerical code MANCHA-2F to perform 2D simulations of the Kelvin-Helmholtz instability. We study how the magnetic field generated by the Biermann battery process depends on the ionization degree of the plasma and on the different collisional terms included in the equations of the model. We find that when the collisional coupling is taken into account, the generation of magnetic field is increased as the ionization degree is decreased. We also find that this effect depends on the total density of the plasma and that as this parameter is increased, the numerical two-fluid results converge to the analytical results from a single-fluid model. Title: Numerical simulations of large-amplitude oscillations in flux rope solar prominences Authors: Liakh, V.; Luna, M.; Khomenko, E. Bibcode: 2020A&A...637A..75L Altcode: 2020arXiv200304343L Context. Large-amplitude oscillations (LAOs) of solar prominences are a very spectacular, but poorly understood, phenomena. These motions have amplitudes larger than 10 km s-1 and can be triggered by the external perturbations such as Moreton or EIT waves.
Aims: Our aim is to analyze the properties of LAOs using realistic prominence models and the triggering mechanism by external disturbances.
Methods: We performed time-dependent numerical simulations of LAOs using a magnetic flux rope model with the two values of shear angle and density contrast. We studied the internal modes of the prominence using horizontal and vertical triggering. In addition, we used perturbation that arrives from outside to understand how such external disturbance can produce LAOs.
Results: The period of longitudinal oscillations and its behavior with height show good agreement with the pendulum model. The period of the transverse oscillations remains constant with height, suggesting a global normal mode. The transverse oscillations typically have shorter periods than longitudinal oscillations.
Conclusions: The periods of the longitudinal and transverse oscillations show only weak dependence on the shear angle of the magnetic structure and prominence density contrast. The external disturbance perturbs the flux rope exciting oscillations of both polarizations. Their properties are a mixture of those excited by purely horizontal and vertical driving. Title: Two-dimensional simulations of coronal rain dynamics. I. Model consisting of a vertical magnetic field and an unbounded atmosphere Authors: Martínez-Gómez, D.; Oliver, R.; Khomenko, E.; Collados, M. Bibcode: 2020A&A...634A..36M Altcode: 2019arXiv191106638M Context. Coronal rain often comes about as the final product of evaporation and condensation cycles that occur in active regions. Observations show that the condensed plasma falls with an acceleration that is less than that of free fall.
Aims: We aim to improve the understanding of the physical mechanisms behind the slower than free-fall motion and the two-stage evolution (an initial phase of acceleration followed by an almost constant velocity phase) detected in coronal rain events.
Methods: Using the MANCHA3D code, we solve the 2D ideal magnetohydrodynamic equations. We represent the solar corona as an isothermal vertically stratified atmosphere with a uniform vertical magnetic field. We represent the plasma condensation as a density enhancement described by a 2D Gaussian profile. We analyse the temporal evolution of the descending plasma and study its dependence on such parameters as density and magnetic field strength.
Results: We confirm previous findings that indicate that the pressure gradient is the main force that opposes the action of gravity and slows down the blob descent, and that larger densities require larger pressure gradients to reach the constant speed phase. We find that the shape of a condensation with a horizontal variation of density is distorted during its fall because the denser parts of the blob fall faster than the lighter ones. This is explained by the fact that the duration of the initial acceleration phase and, therefore, the maximum falling speed attained by the plasma, increases with the ratio of blob to coronal density. We also find that the magnetic field plays a fundamental role in the evolution of the descending condensations. A strong enough magnetic field (greater than 10 G in our simulations) forces each plasma element to follow the path given by a particular field line, which allows for the description of the evolution of each vertical slice of the blob in terms of 1D dynamics, without the influence of the adjacent slices. In addition, under the typical conditions of the coronal rain events, the magnetic field prevents the development of Kelvin-Helmholtz instability.

Movies associated to Figs. 1, 8 and 10 are available at https://www.aanda.org Title: An introductory guide to fluid models with anisotropic temperatures. Part 1. CGL description and collisionless fluid hierarchy Authors: Hunana, P.; Tenerani, A.; Zank, G. P.; Khomenko, E.; Goldstein, M. L.; Webb, G. M.; Cally, P. S.; Collados, M.; Velli, M.; Adhikari, L. Bibcode: 2019JPlPh..85f2002H Altcode: 2019arXiv190109354H We present a detailed guide to advanced collisionless fluid models that incorporate kinetic effects into the fluid framework, and that are much closer to the collisionless kinetic description than traditional magnetohydrodynamics. Such fluid models are directly applicable to modelling the turbulent evolution of a vast array of astrophysical plasmas, such as the solar corona and the solar wind, the interstellar medium, as well as accretion disks and galaxy clusters. The text can be viewed as a detailed guide to Landau fluid models and it is divided into two parts. Part 1 is dedicated to fluid models that are obtained by closing the fluid hierarchy with simple (non-Landau fluid) closures. Part 2 is dedicated to Landau fluid closures. Here in Part 1, we discuss the fluid model of Chew-Goldberger-Low (CGL) in great detail, together with fluid models that contain dispersive effects introduced by the Hall term and by the finite Larmor radius corrections to the pressure tensor. We consider dispersive effects introduced by the non-gyrotropic heat flux vectors. We investigate the parallel and oblique firehose instability, and show that the non-gyrotropic heat flux strongly influences the maximum growth rate of these instabilities. Furthermore, we discuss fluid models that contain evolution equations for the gyrotropic heat flux fluctuations and that are closed at the fourth-moment level by prescribing a specific form for the distribution function. For the bi-Maxwellian distribution, such a closure is known as the `normal' closure. We also discuss a fluid closure for the bi-kappa distribution. Finally, by considering one-dimensional Maxwellian fluid closures at higher-order moments, we show that such fluid models are always unstable. The last possible non Landau fluid closure is therefore the `normal' closure, and beyond the fourth-order moment, Landau fluid closures are required. Title: An introductory guide to fluid models with anisotropic temperatures. Part 2. Kinetic theory, Padé approximants and Landau fluid closures Authors: Hunana, P.; Tenerani, A.; Zank, G. P.; Goldstein, M. L.; Webb, G. M.; Khomenko, E.; Collados, M.; Cally, P. S.; Adhikari, L.; Velli, M. Bibcode: 2019JPlPh..85f2003H Altcode: 2019arXiv190109360H In Part 2 of our guide to collisionless fluid models, we concentrate on Landau fluid closures. These closures were pioneered by Hammett and Perkins and allow for the rigorous incorporation of collisionless Landau damping into a fluid framework. It is Landau damping that sharply separates traditional fluid models and collisionless kinetic theory, and is the main reason why the usual fluid models do not converge to the kinetic description, even in the long-wavelength low-frequency limit. We start with a brief introduction to kinetic theory, where we discuss in detail the plasma dispersion function Z(ζ), and the associated plasma response function R(ζ)=1+ζZ(ζ)=-Z^' }(ζ)/2. We then consider a one-dimensional (1-D) (electrostatic) geometry and make a significant effort to map all possible Landau fluid closures that can be constructed at the fourth-order moment level. These closures for parallel moments have general validity from the largest astrophysical scales down to the Debye length, and we verify their validity by considering examples of the (proton and electron) Landau damping of the ion-acoustic mode, and the electron Landau damping of the Langmuir mode. We proceed by considering 1-D closures at higher-order moments than the fourth order, and as was concluded in Part 1, this is not possible without Landau fluid closures. We show that it is possible to reproduce linear Landau damping in the fluid framework to any desired precision, thus showing the convergence of the fluid and collisionless kinetic descriptions. We then consider a 3-D (electromagnetic) geometry in the gyrotropic (long-wavelength low-frequency) limit and map all closures that are available at the fourth-order moment level. In appendix Ae provide comprehensive tables with Padé approximants of R(ζ) up to the eighth-pole order, with many given in an analytic form. Title: Science Requirement Document (SRD) for the European Solar Telescope (EST) (2nd edition, December 2019) Authors: Schlichenmaier, R.; Bellot Rubio, L. R.; Collados, M.; Erdelyi, R.; Feller, A.; Fletcher, L.; Jurcak, J.; Khomenko, E.; Leenaarts, J.; Matthews, S.; Belluzzi, L.; Carlsson, M.; Dalmasse, K.; Danilovic, S.; Gömöry, P.; Kuckein, C.; Manso Sainz, R.; Martinez Gonzalez, M.; Mathioudakis, M.; Ortiz, A.; Riethmüller, T. L.; Rouppe van der Voort, L.; Simoes, P. J. A.; Trujillo Bueno, J.; Utz, D.; Zuccarello, F. Bibcode: 2019arXiv191208650S Altcode: The European Solar Telescope (EST) is a research infrastructure for solar physics. It is planned to be an on-axis solar telescope with an aperture of 4 m and equipped with an innovative suite of spectro-polarimetric and imaging post-focus instrumentation. The EST project was initiated and is driven by EAST, the European Association for Solar Telescopes. EAST was founded in 2006 as an association of 14 European countries. Today, as of December 2019, EAST consists of 26 European research institutes from 18 European countries. The Preliminary Design Phase of EST was accomplished between 2008 and 2011. During this phase, in 2010, the first version of the EST Science Requirement Document (SRD) was published. After EST became a project on the ESFRI roadmap 2016, the preparatory phase started. The goal of the preparatory phase is to accomplish a final design for the telescope and the legal governance structure of EST. A major milestone on this path is to revisit and update the Science Requirement Document (SRD). The EST Science Advisory Group (SAG) has been constituted by EAST and the Board of the PRE-EST EU project in November 2017 and has been charged with the task of providing with a final statement on the science requirements for EST. Based on the conceptual design, the SRD update takes into account recent technical and scientific developments, to ensure that EST provides significant advancement beyond the current state-of-the-art. The present update of the EST SRD has been developed and discussed during a series of EST SAG meetings. The SRD develops the top-level science objectives of EST into individual science cases. Identifying critical science requirements is one of its main goals. Those requirements will define the capabilities of EST and the post-focus instrument suite. The technical requirements for the final design of EST will be derived from the SRD. Title: Fast-to-Alfvén Mode Conversion and Ambipolar Heating in Structured Media. I. Simplified Cold Plasma Model Authors: Cally, Paul S.; Khomenko, Elena Bibcode: 2019ApJ...885...58C Altcode: Linear fast magnetoacoustic waves are introduced into a cold stratified plasma model made up of a doubly periodic ensemble of straight diminished-Alfvén-speed tubes. Coupling between fast and Alfvén waves is produced by stratification and the tube structures, and scattering is strong for wavelengths comparable to the inter-tube separation. Ambipolar diffusion is found to be enhanced by the structuring and is potentially significant at high frequencies. The production of kink waves is discussed and modeled. It is found that the tube structure significantly alters the wave energy reaching the corona and the form that it takes, even for moderate fast-slow tube contrast, with Alfvén waves becoming prominent. Title: Fast-to-Alfvén Mode Conversion and Ambipolar Heating in Structured Media. II. Numerical Simulation Authors: Khomenko, Elena; Cally, Paul S. Bibcode: 2019ApJ...883..179K Altcode: This paper studies the effectiveness of production of Alfvén waves in the solar atmosphere through the processes of mode conversion, taking into account several new effects that have not been considered before. We perform simulations of wave propagation and conversion from the photosphere to the corona. Both magnetic field and plasma parameters are structured in the form of small-scale flux tubes with characteristic scale significantly below the wavelength of the waves. The waves are allowed to dissipate through the ambipolar diffusion (AD) mechanism. We use an analytical magneto-static equilibrium model, which provides the AD coefficient values at the lower end of what is expected for the quiet solar regions. This work extends the simplified study of mode conversion by Cally and Cally & Khomenko to the case of warm, partially ionized, and structured plasma. We conclude that interaction of waves with the flux tube ensemble produces a discrete spectrum of high-order harmonics. The scattering is a linear process: however, the nonlinear effects have considerable influence upon the amplitudes of the harmonics. The magnetic Poynting flux reaching the corona is enhanced by about 35% and the reflection of waves at the transition region is decreased by about 50% when the flux tubes structure is present, relative to the horizontally homogeneous case. The energy flux of Alfvén waves exceeds that of acoustic waves at coronal heights. Ambipolar diffusion decreases the magnetic Poynting flux in the corona because the fast waves entering the transformation region at chromospheric heights are degraded and have lower amplitudes. The effect of the enhancement of Alfvén wave production due to interaction with flux tubes is independent of the numerical resolution, while the effect of the AD is resolution-dependent and is not converged at the 10 km resolution of our best simulations. Title: Two-fluid simulations of waves in the solar chromosphere. II. Propagation and damping of fast magneto-acoustic waves and shocks Authors: Popescu Braileanu, B.; Lukin, V. S.; Khomenko, E.; de Vicente, Á. Bibcode: 2019A&A...630A..79P Altcode: 2019arXiv190805262P Waves and shocks traveling through the solar chromospheric plasma are influenced by its partial ionization and weak collisional coupling, and may become susceptible to multi-fluid effects, similar to interstellar shock waves. In this study, we consider fast magneto-acoustic shock wave formation and propagation in a stratified medium, that is permeated by a horizontal magnetic field, with properties similar to that of the solar chromosphere. The evolution of plasma and neutrals is modeled using a two-fluid code that evolves a set of coupled equations for two separate fluids. We observed that waves in neutrals and plasma, initially coupled at the upper photosphere, become uncoupled at higher heights in the chromosphere. This decoupling can be a consequence of either the characteristic spatial scale at the shock front, that becomes similar to the collisional scale, or the change in the relation between the wave frequency, ion cyclotron frequency, and the collisional frequency with height. The decoupling height is a sensitive function of the wave frequency, wave amplitude, and the magnetic field strength. We observed that decoupling causes damping of waves and an increase in the background temperature due to the frictional heating. The comparison between analytical and numerical results allows us to separate the role of the nonlinear effects from the linear ones on the decoupling and damping of waves. Title: Two-fluid simulations of waves in the solar chromosphere. I. Numerical code verification Authors: Popescu Braileanu, B.; Lukin, V. S.; Khomenko, E.; de Vicente, Á. Bibcode: 2019A&A...627A..25P Altcode: 2019arXiv190503559P Solar chromosphere consists of a partially ionized plasma, which makes modeling the solar chromosphere a particularly challenging numerical task. Here we numerically model chromospheric waves using a two-fluid approach with a newly developed numerical code. The code solves two-fluid equations of conservation of mass, momentum, and energy, together with the induction equation for the case of the purely hydrogen plasma with collisional coupling between the charged and neutral fluid components. The implementation of a semi-implicit algorithm allows us to overcome the numerical stability constraints due to the stiff collisional terms. We test the code against analytical solutions of acoustic and Alfvén wave propagation in uniform medium in several regimes of collisional coupling. The results of our simulations are consistent with the analytical estimates, and with other results described in the literature. In the limit of a large collisional frequency, the waves propagate with a common speed of a single fluid. In the other limit of a vanishingly small collisional frequency, the Alfvén waves propagate with an Alfvén speed of the charged fluid only, while the perturbation in neutral fluid is very small. The acoustic waves in these limits propagate with the sound speed corresponding to either the charges or the neutrals, while the perturbation in the other fluid component is negligible. Otherwise, when the collision frequency is similar to the real part of the wave frequency, the interaction between charges and neutrals through momentum-transfer collisions cause alterations of the waves frequencies and damping of the wave amplitudes. Title: Spiral-shaped wavefronts in a sunspot umbra Authors: Felipe, T.; Kuckein, C.; Khomenko, E.; Thaler, I. Bibcode: 2019A&A...621A..43F Altcode: 2018arXiv181011257F Context. Solar active regions show a wide variety of oscillatory phenomena. The presence of the magnetic field leads to the appearance of several wave modes whose behavior is determined by the sunspot thermal and magnetic structure.
Aims: We aim to study the relation between the umbral and penumbral waves observed at the high photosphere and the magnetic field topology of the sunspot.
Methods: Observations of the sunspot in active region NOAA 12662 obtained with the GREGOR telescope (Observatorio del Teide, Tenerife, Spain) were acquired on 2017 June 17. The data set includes a temporal series in the Fe I 5435 Å line obtained with the imaging spectrograph GREGOR Fabry-Pérot Interferometer (GFPI) and a spectropolarimetric raster map acquired with the GREGOR Infrared Spectrograph (GRIS) in the 10 830 Å spectral region. The Doppler velocity deduced from the restored Fe I 5435 Å line has been determined, and the magnetic field vector of the sunspot has been inferred from spectropolarimetric inversions of the Ca I 10 839 Å and the Si I 10 827 Å lines.
Results: A two-armed spiral wavefront has been identified in the evolution of the two-dimensional velocity maps from the Fe I 5435 Å line. The wavefronts initially move counterclockwise in the interior of the umbra, and develop into radially outward propagating running penumbral waves when they reach the umbra-penumbra boundary. The horizontal propagation of the wavefronts approximately follows the direction of the magnetic field, which shows changes in the magnetic twist with height and horizontal position.
Conclusions: The spiral wavefronts are interpreted as the visual pattern of slow magnetoacoustic waves which propagate upward along magnetic field lines. Their apparent horizontal propagation is due to their sequential arrival to different horizontal positions at the formation height of the Fe I 5435 Å line, as given by the inclination and orientation of the magnetic field.

The movie associated to Fig. 2 is available at https://www.aanda.org Title: Fast-to-Alfvén Mode Conversion Mediated by Hall Current. II. Application to the Solar Atmosphere Authors: González-Morales, P. A.; Khomenko, E.; Cally, P. S. Bibcode: 2019ApJ...870...94G Altcode: 2018arXiv181106565G Coupling between fast magnetoacoustic and Alfvén waves can be observed in fully ionized plasmas mediated by stratification and 3D geometrical effects. In Paper I, Cally & Khomenko have shown that in a weakly ionized plasma, such as the solar photosphere and chromosphere, the Hall current introduces a new coupling mechanism. The present study extends the results from Paper I to the case of warm plasma. We report on numerical experiments where mode transformation is studied using quasi-realistic stratification in thermodynamic parameters resembling the solar atmosphere. This redresses the limitation of the cold plasma approximation assumed in Paper I, in particular allowing the complete process of coupling between fast and slow magnetoacoustic modes and subsequent coupling of the fast mode to the Alfvén mode through the Hall current. Our results confirm the efficacy of the mechanism proposed in Paper I for the solar case. We observe that the efficiency of the transformation is a sensitive function of the angle between the wave propagation direction and the magnetic field, and of the wave frequency. The efficiency increases when the field direction and the wave direction are aligned for increasing wave frequencies. After scaling our results to typical solar values, the maximum amplitude of the transformed Alfvén waves, for a frequency of 1 Hz, corresponds to an energy flux (measured above the height of peak Hall coupling) of ∼103 W m-2, based on an amplitude of 500 m s-1 at β = 1, which is sufficient to play a major role in both quiet and active region coronal heating. Title: Three-dimensional simulations of solar magneto-convection including effects of partial ionization Authors: Khomenko, E.; Vitas, N.; Collados, M.; de Vicente, A. Bibcode: 2018A&A...618A..87K Altcode: 2018arXiv180701061K In recent decades, REALISTIC three-dimensional radiative-magnetohydrodynamic simulations have become the dominant theoretical tool for understanding the complex interactions between the plasma and magnetic field on the Sun. Most of such simulations are based on approximations of magnetohydrodynamics, without directly considering the consequences of the very low degree of ionization of the solar plasma in the photosphere and bottom chromosphere. The presence of a large amount of neutrals leads to a partial decoupling of the plasma and magnetic field. As a consequence, a series of non-ideal effects, i.e., the ambipolar diffusion, Hall effect, and battery effect, arise. The ambipolar effect is the dominant in the solar chromosphere. We report on the first three-dimensional realistic simulations of magneto-convection including ambipolar diffusion and battery effects. The simulations are carried out using the newly developed MANCHA3Dcode. Our results reveal that ambipolar diffusion causes measurable effects on the amplitudes of waves excited by convection in the simulations, on the absorption of Poynting flux and heating, and on the formation of chromospheric structures. We provide a low limit on the chromospheric temperature increase owing to the ambipolar effect using the simulations with battery-excited dynamo fields.

The movies associated to Figs. 16 and 17 are available at https://www.aanda.org Title: MHDSTS: a new explicit numerical scheme for simulations of partially ionised solar plasma Authors: González-Morales, P. A.; Khomenko, E.; Downes, T. P.; de Vicente, A. Bibcode: 2018A&A...615A..67G Altcode: 2018arXiv180304891G The interaction of plasma with magnetic field in the partially ionised solar atmosphere is frequently modelled via a single-fluid approximation, which is valid for the case of a strongly coupled collisional media, such as solar photosphere and low chromosphere. Under the single-fluid formalism the main non-ideal effects are described by a series of extra terms in the generalised induction equation and in the energy conservation equation. These effects are: Ohmic diffusion, ambipolar diffusion, the Hall effect, and the Biermann battery effect. From the point of view of the numerical solution of the single-fluid equations, when ambipolar diffusion or Hall effects dominate can introduce severe restrictions on the integration time step and can compromise the stability of the numerical scheme. In this paper we introduce two numerical schemes to overcome those limitations. The first of them is known as super time-stepping (STS) and it is designed to overcome the limitations imposed when the ambipolar diffusion term is dominant. The second scheme is called the Hall diffusion scheme (HDS) and it is used when the Hall term becomes dominant. These two numerical techniques can be used together by applying Strang operator splitting. This paper describes the implementation of the STS and HDS schemes in the single-fluid code MANCHA3D. The validation for each of these schemes is provided by comparing the analytical solution with the numerical one for a suite of numerical tests. Title: Partially Ionized Plasmas in Astrophysics Authors: Ballester, José Luis; Alexeev, Igor; Collados, Manuel; Downes, Turlough; Pfaff, Robert F.; Gilbert, Holly; Khodachenko, Maxim; Khomenko, Elena; Shaikhislamov, Ildar F.; Soler, Roberto; Vázquez-Semadeni, Enrique; Zaqarashvili, Teimuraz Bibcode: 2018SSRv..214...58B Altcode: 2017arXiv170707975B Partially ionized plasmas are found across the Universe in many different astrophysical environments. They constitute an essential ingredient of the solar atmosphere, molecular clouds, planetary ionospheres and protoplanetary disks, among other environments, and display a richness of physical effects which are not present in fully ionized plasmas. This review provides an overview of the physics of partially ionized plasmas, including recent advances in different astrophysical areas in which partial ionization plays a fundamental role. We outline outstanding observational and theoretical questions and discuss possible directions for future progress. Title: Fast-to-Alfvén Mode Conversion in the Presence of Ambipolar Diffusion Authors: Cally, Paul S.; Khomenko, Elena Bibcode: 2018ApJ...856...20C Altcode: It is known that fast magnetohydrodynamic waves partially convert to upward and/or downward propagating Alfvén waves in a stratified atmosphere where Alfvén speed increases with height. This happens around the fast wave reflection height, where the fast wave’s horizontal phase speed equals the Alfvén speed (in a low-β plasma). Typically, this takes place in the mid to upper solar chromosphere for low-frequency waves in the few-millihertz band. However, this region is weakly ionized and thus susceptible to nonideal MHD processes. In this article, we explore how ambipolar diffusion in a zero-β plasma affects fast waves injected from below. Classical ambipolar diffusion is far too weak to have any significant influence at these low frequencies, but if enhanced by turbulence (in the quiet-Sun chromosphere but not in sunspot umbrae) or the production of sufficiently small-scale structure, can substantially absorb waves for turbulent ambipolar Reynolds numbers of around 20 or less. In that case, it is found that the mode conversion process is not qualitatively altered from the ideal case, though conversion to Alfvén waves is reduced because the fast wave flux reaching the conversion region is degraded. It is also found that any upward propagating Alfvén waves generated in this process are almost immune to further ambipolar attenuation, thereby reducing local ambipolar heating compared to cases without mode conversion. In that sense, mode conversion provides a form of “Alfvén cooling.” Title: Rayleigh-Taylor instabilities with sheared magnetic fields in partially ionised plasmas Authors: Ruderman, M. S.; Ballai, I.; Khomenko, E.; Collados, M. Bibcode: 2018A&A...609A..23R Altcode: 2017A&A...609A..23R
Aims: In the present study we investigate the nature of the magnetic Rayleigh-Taylor instability appearing at a tangential discontinuity in a partially ionised plasma when the effect of magnetic shear is taken into account.
Methods: The partially ionised character of the plasma is described by the ambipolar diffusion in the induction equation. The dynamics of the plasma is investigated in a single-fluid approximation. After matching the solutions on both sides of the interface we derive a dispersion equation and calculate the instability increment using analytical methods for particular cases of parameters, and numerical investigation for a wide range of parameters.
Results: We calculated the dependence of the instability increment on the perturbation wavenumber. We also calculated the dependence of the maximum instability increment on the shear angle of the magnetic field for various values of the ionisation degree.
Conclusions: Our results show that the Rayleigh-Taylor instability becomes sensitive to the degree of plasma ionisation only for plasmas with small values of plasma beta and in a very weakly ionised state. Perturbations are unstable only for those wavenumbers that are below a cut-off value. Title: Signatures of the impact of flare-ejected plasma on the photosphere of a sunspot light bridge Authors: Felipe, T.; Collados, M.; Khomenko, E.; Rajaguru, S. P.; Franz, M.; Kuckein, C.; Asensio Ramos, A. Bibcode: 2017A&A...608A..97F Altcode: 2017arXiv170806133F
Aims: We investigate the properties of a sunspot light bridge, focusing on the changes produced by the impact of a plasma blob ejected from a C-class flare.
Methods: We observed a sunspot in active region NOAA 12544 using spectropolarimetric raster maps of the four Fe I lines around 15 655 Å with the GREGOR Infrared Spectrograph, narrow-band intensity images sampling the Fe I 6173 Å line with the GREGOR Fabry-Pérot Interferometer, and intensity broad-band images in G-band and Ca II H-band with the High-resolution Fast Imager. All these instruments are located at the GREGOR telescope at the Observatorio del Teide, Tenerife, Spain. The data cover the time before, during, and after the flare event. The analysis is complemented with Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager data from the Solar Dynamics Observatory. The physical parameters of the atmosphere at differents heights were inferred using spectral-line inversion techniques.
Results: We identify photospheric and chromospheric brightenings, heating events, and changes in the Stokes profiles associated with the flare eruption and the subsequent arrival of the plasma blob to the light bridge, after traveling along an active region loop.
Conclusions: The measurements suggest that these phenomena are the result of reconnection events driven by the interaction of the plasma blob with the magnetic field topology of the light bridge.

Movies attached to Figs. 1 and 3 are available at http://www.aanda.org Title: Flux Tube: Solar model Authors: Khomenko, E. Bibcode: 2017ascl.soft12010K Altcode: Flux Tube is a nonlinear, two-dimensional, numerical simulation of magneto-acoustic wave propagation in the photosphere and chromosphere of small-scale flux tubes with internal structure. Waves with realistic periods of three to five minutes are studied, after horizontal and vertical oscillatory perturbations are applied to the equilibrium model. Spurious reflections of shock waves from the upper boundary are minimized by a special boundary condition. Title: Numerical simulations of quiet Sun magnetic fields seeded by the Biermann battery Authors: Khomenko, E.; Vitas, N.; Collados, M.; de Vicente, A. Bibcode: 2017A&A...604A..66K Altcode: 2017arXiv170606037K The magnetic fields of the quiet Sun cover at any time more than 90% of its surface and their magnetic energy budget is crucial to explain the thermal structure of the solar atmosphere. One of the possible origins of these fields is the action of the local dynamo in the upper convection zone of the Sun. Existing simulations of the local solar dynamo require an initial seed field and sufficiently high spatial resolution in order to achieve the amplification of the seed field to the observed values in the quiet Sun. Here we report an alternative model of seeding based on the action of the Bierman battery effect. This effect generates a magnetic field due to the local imbalances in electron pressure in the partially ionized solar plasma. We show that the battery effect self-consistently creates from zero an initial seed field of a strength of the order of micro G, and together with dynamo amplification allows the generation of quiet Sun magnetic fields of a similar strength to those from solar observations. Title: High-frequency waves in the corona due to null points Authors: Santamaria, I. C.; Khomenko, E.; Collados, M.; de Vicente, A. Bibcode: 2017A&A...602A..43S Altcode: 2017arXiv170406551S This work aims to understand the behavior of non-linear waves in the vicinity of a coronal null point. In previous works we have shown that high-frequency waves are generated in such a magnetic configuration. This paper studies those waves in detail in order to provide a plausible explanation of their generation. We demonstrate that slow magneto-acoustic shock waves generated in the chromosphere propagate through the null point and produce a train of secondary shocks that escape along the field lines. A particular combination of the shock wave speeds generates waves at a frequency of 80 mHz. We speculate that this frequency may be sensitive to the atmospheric parameters in the corona and therefore can be used to probe the structure of this solar layer.

Movies attached to Figs 2 and 4 are available at http://www.aanda.org Title: Dependence of sunspot photospheric waves on the depth of the source of solar p-modes Authors: Felipe, T.; Khomenko, E. Bibcode: 2017A&A...599L...2F Altcode: 2017arXiv170200997F Photospheric waves in sunspots moving radially outward at speeds faster than the characteristic wave velocities have been recently detected. It has been suggested that they are the visual pattern of p-modes excited around 5 Mm beneath the sunspot's surface. Using numerical simulations, we performed a parametric study of the waves observed at the photosphere and higher layers that were produced by sources located at different depths beneath the sunspot's surface. The observational measurements are consistent with waves driven between approximately 1 Mm and 5 Mm below the sunspot's surface. Title: On the effects of ion-neutral interactions in solar plasmas Authors: Khomenko, Elena Bibcode: 2017PPCF...59a4038K Altcode: 2016arXiv161106063K Solar photosphere and chromosphere are composed of weakly ionized plasma for which collisional coupling decreases with height. This implies a breakdown of some hypotheses underlying magnetohydrodynamics at low altitudes and gives rise to non-ideal MHD effects such as ambipolar diffusion, Hall effect, etc. Recently, there has been progress in understanding the role of these effects for the dynamics and energetics of the solar atmosphere. There are evidences that phenomena such as wave propagation and damping, magnetic reconnection, formation of stable magnetic field concentrations, magnetic flux emergence, etc can be affected. This paper reviews the current state-of-the-art of multi-fluid MHD modeling of the coupled solar atmosphere. 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: Three-dimensional structure of a sunspot light bridge Authors: Felipe, T.; Collados, M.; Khomenko, E.; Kuckein, C.; Asensio Ramos, A.; Balthasar, H.; Berkefeld, T.; Denker, C.; Feller, A.; Franz, M.; Hofmann, A.; Joshi, J.; Kiess, C.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar, A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016A&A...596A..59F Altcode: 2016arXiv161104803F Context. Active regions are the most prominent manifestations of solar magnetic fields; their generation and dissipation are fundamental problems in solar physics. Light bridges are commonly present during sunspot decay, but a comprehensive picture of their role in the removal of the photospheric magnetic field is still lacking.
Aims: We study the three-dimensional configuration of a sunspot, and in particular, its light bridge, during one of the last stages of its decay.
Methods: We present the magnetic and thermodynamical stratification inferred from full Stokes inversions of the photospheric Si I 10 827 Å and Ca I 10 839 Å lines obtained with the GREGOR Infrared Spectrograph of the GREGOR telescope at the Observatorio del Teide, Tenerife, Spain. The analysis is complemented by a study of continuum images covering the disk passage of the active region, which are provided by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory.
Results: The sunspot shows a light bridge with penumbral continuum intensity that separates the central umbra from a smaller umbra. We find that in this region the magnetic field lines form a canopy with lower magnetic field strength in the inner part. The photospheric light bridge is dominated by gas pressure (high-β), as opposed to the surrounding umbra, where the magnetic pressure is higher. A convective flow is observed in the light bridge. This flow is able to bend the magnetic field lines and to produce field reversals. The field lines merge above the light bridge and become as vertical and strong as in the surrounding umbra. We conclude that this occurs because two highly magnetized regions approach each other during the sunspot evolution.

Movies associated to Figs. 2 and 13 are available at http://www.aanda.org Title: Tracing p-mode Waves from the Photosphere to the Corona in Active Regions Authors: Zhao, Junwei; Felipe, Tobías; Chen, Ruizhu; Khomenko, Elena Bibcode: 2016ApJ...830L..17Z Altcode: Atmosphere above sunspots is abundant with different types of waves. Among these waves are running penumbral waves in the chromosphere, quasi-periodic oscillations in the lower coronal loops, and recently reported running waves in sunspots’ photosphere, all of which were interpreted as magnetoacoustic waves by some authors. Are these waves in different atmospheric layers related to each other, what is the nature of these waves, and where are the ultimate sources of these waves? Applying a time-distance helioseismic analysis over a suite of multi-wavelength observations above a sunspot, we demonstrate that the helioseismic p-mode waves are able to channel up from the photosphere through the chromosphere and transition region into the corona, and that the magnetoacoustic waves observed in different atmospheric layers are a same wave originating from the photosphere but exhibiting differently under different physical conditions. We also show waves of different frequencies travel along different paths, which can be used to derive the physical properties of the atmosphere above sunspots. Our numerical simulation of traveling of waves from a subphotospheric source qualitatively resembles the observed properties of the waves and offers an interpretation of the shapes of the wavefronts above the photosphere. Title: Numerical simulations of magnetic Kelvin-Helmholtz instability at a twisted solar flux tube Authors: Murawski, K.; Chmielewski, P.; Zaqarashvili, T. V.; Khomenko, E. Bibcode: 2016MNRAS.459.2566M Altcode: 2016MNRAS.tmp..632M; 2016MNRAS.tmp..596M The paper aims to study the response of a solar small-scale and weak magnetic flux tube to photospheric twisting motions. We numerically solve three-dimensional ideal magnetohydrodynamic equations to describe the evolution of the perturbation within the initially static flux tube, excited by twists in the azimuthal component of the velocity. These twists produce rotation of the magnetic field lines. Perturbation of magnetic field lines propagates upwardly, driving vertical and azimuthal flow as well as plasma compressions and rarefactions in the form of eddies. We conclude that these eddies result from the sheared azimuthal flow which seeds Kelvin-Helmholtz instability (KHI) between the flux tube and the ambient medium. Numerically obtained properties of the KHI confirm the analytical predictions for the occurrence of the instability. Title: Observational Detection of Drift Velocity between Ionized and Neutral Species in Solar Prominences Authors: Khomenko, Elena; Collados, Manuel; Díaz, Antonio J. Bibcode: 2016ApJ...823..132K Altcode: 2016arXiv160401177K We report the detection of differences in the ion and neutral velocities in prominences using high-resolution spectral data obtained in 2012 September at the German Vacuum Tower Telescope (Observatorio del Teide, Tenerife). A time series of scans of a small portion of a solar prominence was obtained simultaneously with high cadence using the lines of two elements with different ionization states, namely, Ca II 8542 Å and He I 10830 Å. The displacements, widths, and amplitudes of both lines were carefully compared to extract dynamical information about the plasma. Many dynamical features are detected, such as counterstreaming flows, jets, and propagating waves. In all of the cases, we find a very strong correlation between the parameters extracted from the lines of both elements, confirming that both lines trace the same plasma. Nevertheless, we also find short-lived transients where this correlation is lost. These transients are associated with ion-neutral drift velocities of the order of several hundred m s-1. The patches of non-zero drift velocity show coherence in time-distance diagrams. Title: Vortex waves in sunspots Authors: López Ariste, A.; Centeno, R.; Khomenko, E. Bibcode: 2016A&A...591A..63L Altcode: Context. Waves in the magnetized solar atmosphere are one of the favourite means of transferring and depositing energy into the solar corona. The study of waves brings information not just on the dynamics of the magnetized plasma, but also on the possible ways in which the corona is heated.
Aims: The identification and analysis of the phase singularities or dislocations provide us with a complementary approach to the magnetoacoustic and Aflvén waves propagating in the solar atmosphere. They allow us to identify individual wave modes, shedding light on the probability of excitation or the nature of the triggering mechanism.
Methods: We use a time series of Doppler shifts measured in two spectral lines, filtered around the three-minute period region. The data show a propagating magnetoacoustic slow mode with several dislocations and, in particular, a vortex line. We study under what conditions the different wave modes propagating in the umbra can generate the observed dislocations.
Results: The observed dislocations can be fully interpreted as a sequence of sausage and kink modes excited sequentially on average during 15 min. Kink and sausage modes appear to be excited independently and sequentially. The transition from one to the other lasts less than three minutes. During the transition we observe and model the appearance of superoscillations inducing large phase gradients and phase mixing.
Conclusions: The analysis of the observed wave dislocations leads us to the identification of the propagating wave modes in umbrae. The identification in the data of superoscillatory regions during the transition from one mode to the other may be an important indicator of the location of wave dissipation. Title: The possible origin of facular brightness in the solar atmosphere Authors: Kostik, R.; Khomenko, E. Bibcode: 2016A&A...589A...6K Altcode: 2016arXiv160203369K This paper studies the dependence of the Ca II H line core brightness on the strength and inclination of the photospheric magnetic field, and on the parameters of convective and wave motions in a facular region at the center of the solar disc. We use three simultaneous data sets that were obtained at the German Vacuum Tower Telescope (Observatorio del Teide, Tenerife): (1) spectra of Ba II 4554 Å line, registered with the instrument TESOS to measure the variations of intensity and velocity through the photosphere up to the temperature minimum; (2) spectropolarimetric data in Fe I 1.56 μm lines (registered with the instrument TIP II) to measure photospheric magnetic fields; (3) filtergrams in Ca II H that give information about brightness fluctuations in the chromosphere. The results show that the Ca II H brightness in the facula strongly depends on the power of waves with periods in the 5-min range, which propagate upwards, and also on the phase shift between velocity oscillations at the bottom photosphere and around the temperature minimum height that is measured from Ba II line. The Ca II H brightness is maximum at locations where the phase shift between temperature and velocity oscillations lies within 0°-100°. There is an indirect influence of convective motions on the Ca II H brightness. The higher the amplitude of convective velocities is and the greater the height is where they change their direction of motion, the brighter the facula. In summary, our results lead to conclusions that facular regions appear bright not only because of the Wilson depression in magnetic structures, but also owing to real heating. Title: Tracing Helioseismic Waves from the Photosphere to the Corona Authors: Zhao, Junwei; Felipe, Tobias; Chen, Ruizhu; Khomenko, Elena Bibcode: 2016SPD....4730307Z Altcode: Can p-mode waves in sunspots propagate to the chromosphere and the corona? And what are their counterparts in different atmospheric heights? In order to study the connection between the photospheric p-mode waves and the waves observed above the photosphere, we use a helioseismic analysis technique, namely time-distance helioseismology, and analyze multi-height observations from different instruments. We find clear evidences that some p-mode waves in the photosphere, running penumbral waves in the chromosphere, and the periodic disturbances in the coronal fan structures are actually same magnetoacoustic waves that exhibit differently at the different atmospheric heights. The 6-mHz waves, with inclined wavefronts, propagate slantingly upward along magnetic field lines. The 3-mHz waves, forming backward-'C'-shape wavefronts, propagate mostly horizontally. Through numerical simulations, we demonstrate that these p-mode waves that can travel upward to the corona, possibly originate from sources located a few megameters beneath sunspots' surface. Title: Simulated interaction of magnetohydrodynamic shock waves with a complex network-like region Authors: Santamaria, I. C.; Khomenko, E.; Collados, M.; de Vicente, A. Bibcode: 2016A&A...590L...3S Altcode: 2016arXiv160408783S We provide estimates of the wave energy reaching the solar chromosphere and corona in a network-like magnetic field topology, including a coronal null point. The waves are excited by an instantaneous strong subphotospheric source and propagate through the subphotosphere, photosphere, chromosphere, transition region, and corona with the plasma beta and other atmospheric parameters varying by several orders of magnitude. We compare two regimes of the wave propagation: a linear and nonlinear regime. While the amount of energy reaching the corona is similar in both regimes, this energy is transmitted at different frequencies. In both cases the dominant periods of waves at each height strongly depend on the local magnetic field topology, but this distribution is only in accordance with observations in the nonlinear case.

Movies are available in electronic form at http://www.aanda.org 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: On the Robustness of the Pendulum Model for Large-amplitude Longitudinal Oscillations in Prominences Authors: Luna, M.; Terradas, J.; Khomenko, E.; Collados, M.; de Vicente, A. Bibcode: 2016ApJ...817..157L Altcode: 2015arXiv151205125L Large-amplitude longitudinal oscillations (LALOs) in prominences are spectacular manifestations of solar activity. In such events nearby energetic disturbances induce periodic motions on filaments with displacements comparable to the size of the filaments themselves and with velocities larger than 20 {km} {{{s}}}-1. The pendulum model, in which the gravity projected along a rigid magnetic field is the restoring force, was proposed to explain these events. However, it can be objected that in a realistic situation where the magnetic field reacts to the mass motion of the heavy prominence, the simplified pendulum model could be no longer valid. We have performed nonlinear time-dependent numerical simulations of LALOs considering a dipped magnetic field line structure. In this work we demonstrate that for even relatively weak magnetic fields the pendulum model works very well. We therefore validate the pendulum model and show its robustness, with important implications for prominence seismology purposes. With this model it is possible to infer the geometry of the dipped field lines that support the prominence. Title: Evershed flow observed in neutral and singly ionized iron lines Authors: Khomenko, E.; Collados, M.; Shchukina, N.; Díaz, A. Bibcode: 2015A&A...584A..66K Altcode: 2015arXiv151000334K The amplitudes of the Evershed flow are measured using pairs of carefully selected Fe i and Fe ii spectral lines that are close in wavelength and registered simultaneously. A sunspot belonging to the NOAA 11582 group was scanned using the spectrograph of the German Vacuum Tower Telescope (Observatorio del Teide, Tenerife). Velocities were extracted from intensity profiles using the λ-meter technique. The formation heights of the observed spectral lines were calculated using semi-empirical models of a bright and dark penumbral filament taking into account the sunspot location at the limb. Our objective is to compare azimuthally averaged amplitudes of the Evershed flow extracted from neutral and ion lines. We find measurable differences in the radial component of the flow. All five pairs of lines show the same tendency; the flow measured from the Fe i lines has an amplitude that is a few hundred ms-1 larger than that of the Fe ii lines. This tendency is preserved at all photospheric heights and radial distances in the penumbra. We discuss the possible origin of this effect. Title: Oscillations and Waves in Sunspots Authors: Khomenko, Elena; Collados, Manuel Bibcode: 2015LRSP...12....6K Altcode: A magnetic field modifies the properties of waves in a complex way. Significant advances have been made recently in our understanding of the physics of sunspot waves with the help of high-resolution observations, analytical theories, as well as numerical simulations. We review the current ideas in the field, providing the most coherent picture of sunspot oscillations as by present understanding. Title: Fast-to-Alfvén Mode Conversion Mediated by the Hall Current. I. Cold Plasma Model Authors: Cally, Paul S.; Khomenko, Elena Bibcode: 2015ApJ...814..106C Altcode: 2015arXiv151003927C The photospheric temperature minimum in the Sun and solar-like stars is very weakly ionized, with an ionization fraction f as low as 10-4. In galactic star-forming regions, f can be 10-10 or lower. Under these circumstances, the Hall current can couple low-frequency Alfvén and magnetoacoustic waves via the dimensionless Hall parameter ɛ =ω /{{{Ω }}}{{i}}f, where ω is the wave frequency and {{{Ω }}}{{i}} is the mean ion gyrofrequency. This is analyzed in the context of a cold (zero-β) plasma and in less detail for a warm plasma. It is found that Hall coupling preferentially occurs where the wavevector is nearly field-aligned. In these circumstances, Hall coupling in theory produces a continual oscillation between fast and Alfvén modes as the wave passes through the weakly ionized region. At low frequencies (mHz), characteristic of solar and stellar normal modes, ɛ is probably too small for more than a fraction of one oscillation to occur. On the other hand, the effect may be significant at the far higher frequencies (Hz) associated with magnetic reconnection events. In another context, characteristic parameters for star-forming gas clouds suggest that {O}(1) or more full oscillations may occur in one cloud crossing. This mechanism is not expected to be effective in sunspots, due to their high ion gyrofrequencies and Alfvén speeds, since the net effect depends inversely on both and therefore inverse quadratically on field strength. Title: On the Source of Propagating Slow Magnetoacoustic Waves in Sunspots Authors: Krishna Prasad, S.; Jess, D. B.; Khomenko, Elena Bibcode: 2015ApJ...812L..15K Altcode: 2015arXiv151003275K; 2015ApJ...812L..15P Recent high-resolution observations of sunspot oscillations using simultaneously operated ground- and space-based telescopes reveal the intrinsic connection between different layers of the solar atmosphere. However, it is not clear whether these oscillations are externally driven or generated in situ. We address this question by using observations of propagating slow magnetoacoustic waves along a coronal fan loop system. In addition to the generally observed decreases in oscillation amplitudes with distance, the observed wave amplitudes are also found to be modulated with time, with similar variations observed throughout the propagation path of the wave train. Employing multi-wavelength and multi-instrument data, we study the amplitude variations with time as the waves propagate through different layers of the solar atmosphere. By comparing the amplitude modulation period in different layers, we find that slow magnetoacoustic waves observed in sunspots are externally driven by photospheric p-modes, which propagate upward into the corona before becoming dissipated. Title: On the nature of transverse coronal waves revealed by wavefront dislocations Authors: López Ariste, A.; Luna, M.; Arregui, I.; Khomenko, E.; Collados, M. Bibcode: 2015A&A...579A.127L Altcode: 2015arXiv150503348L Context. Coronal waves are an important aspect of the dynamics of the plasma in the corona. Wavefront dislocations are topological features of most waves in nature and also of magnetohydrodynamic waves. Are there dislocations in coronal waves?
Aims: The finding and explanation of dislocations may shed light on the nature and characteristics of the propagating waves, their interaction in the corona, and in general on the plasma dynamics.
Methods: We positively identify dislocations in coronal waves observed by the Coronal Multi-channel Polarimeter (CoMP) as singularities in the Doppler shifts of emission coronal lines. We study the possible singularities that can be expected in coronal waves and try to reproduce the observed dislocations in terms of localization and frequency of appearance.
Results: The observed dislocations can only be explained by the interference of a kink and sausage wave modes propagating with different frequencies along the coronal magnetic field. In the plane transverse to the propagation, the cross-section of the oscillating plasma must be smaller than the spatial resolution, and the two waves result in net longitudinal and transverse velocity components that are mixed through projection onto the line of sight. Alfvén waves can be responsible for the kink mode, but a magnetoacoustic sausage mode is necessary in all cases. Higher (flute) modes are excluded. The kink mode has a pressure amplitude that is less than the pressure amplitude of the sausage mode, though its observed velocity is higher. This concentrates dislocations on the top of the loop.
Conclusions: To explain dislocations, any model of coronal waves must include the simultaneous propagation and interference of kink and sausage wave modes of comparable but different frequencies with a sausage wave amplitude much smaller than the kink one.

Appendix A is available in electronic form at http://www.aanda.org Title: Magnetohydrodynamic wave propagation from the subphotosphere to the corona in an arcade-shaped magnetic field with a null point Authors: Santamaria, I. C.; Khomenko, E.; Collados, M. Bibcode: 2015A&A...577A..70S Altcode: 2015arXiv150303094S
Aims: The aim of this work is to study the energy transport by means of Magnetohydrodynamic (MHD) waves propagating in quiet-Sun magnetic topology from layers below the surface to the corona. Upwardly propagating waves find obstacles, such as the equipartition layer with plasma β = 1, the transition region, and null points, and they get transmitted, converted, reflected, and refracted. Understanding the mechanisms by which MHD waves can reach the corona can give us information about the solar atmosphere and the magnetic structures.
Methods: We carried out two-dimensional numerical simulations of wave propagation in a magnetic field structure that consists of two vertical flux tubes with the same polarity separated by an arcade-shaped magnetic field. This configuration contains a null point in the corona, which significantly modifies the behavior of the waves as they pass near it.
Results: We describe in detail the wave propagation through the atmosphere under different driving conditions. We also present the spatial distribution of the mean acoustic and magnetic energy fluxes for the cases where these calculations are possible, as well as the spatial distribution of the dominant frequencies in the whole domain.
Conclusions: We conclude that the energy reaches the corona preferably along almost vertical magnetic fields, that is, inside the vertical flux tubes. This energy is acoustic in nature. Most of the magnetic energy stays concentrated below the transition region owing to the refraction of the magnetic waves and the continuous conversion of acoustic-like waves into fast magnetic waves in the equipartition layer located in the photosphere where plasma β = 1. However, part of the magnetic energy reaches the low corona when propagating in the region where the arcades are located, but waves are sent back downward into the lower atmosphere at the null-point surroundings. This phenomenon, together with the reflection and refraction of waves in the TR and the lower turning point, act as a re-feeding of the atmosphere, which keeps oscillating during all the simulation time even if a driver with a single pulse was used as initial perturbation. In the frequency distribution, we find that high frequency waves can reach the corona outside the vertical flux tubes.

Movies related to Figs. 3, 7, and 11 are available in electronic form at http://www.aanda.org Title: Beyond MHD: modeling and observation of partially ionized solar plasma processes Authors: Khomenko, E. Bibcode: 2015hsa8.conf..677K Altcode: 2015arXiv150401578K The temperature and density conditions in the magnetized photosphere and chromosphere of the Sun lead to a very small degree of atomic ionization. At particular heights, the magnetic field may be strong enough to give rise to a cyclotron frequency larger than the collisional frequency for some species. These circumstances influence the collective behavior of the particles and some of the hypotheses of magnetohydrodynamics may be relaxed, giving rise to non-ideal MHD effects. In this paper we discuss our recent developments in modeling non-ideal plasma effects, as well as their observational consequences. Title: Synthetic Observations of Wave Propagation in a Sunspot Umbra Authors: Felipe, T.; Socas-Navarro, H.; Khomenko, E. Bibcode: 2014ApJ...795....9F Altcode: 2014arXiv1408.6565F Spectropolarimetric temporal series from Fe I λ6301.5 Å and Ca II infrared triplet lines are obtained by applying the Stokes synthesis code NICOLE to a numerical simulation of wave propagation in a sunspot umbra from MANCHA code. The analysis of the phase difference between Doppler velocity and intensity core oscillations of the Fe I λ6301.5 Å line reveals that variations in the intensity are produced by opacity fluctuations rather than intrinsic temperature oscillations, except for frequencies between 5 and 6.5 mHz. On the other hand, the photospheric magnetic field retrieved from the weak field approximation provides the intrinsic magnetic field oscillations associated to wave propagation. Our results suggest that this is due to the low magnetic field gradient of our sunspot model. The Stokes parameters of the chromospheric Ca II infrared triplet lines show striking variations as shock waves travel through the formation height of the lines, including emission self-reversals in the line core and highly abnormal Stokes V profiles. Magnetic field oscillations inferred from the Ca II infrared lines using the weak field approximation appear to be related with the magnetic field strength variation between the photosphere and the chromosphere. Title: Fluid description of multi-component solar partially ionized plasma Authors: Khomenko, E.; Collados, M.; Díaz, A.; Vitas, N. Bibcode: 2014PhPl...21i2901K Altcode: 2014arXiv1408.1871K We derive self-consistent formalism for the description of multi-component partially ionized solar plasma, by means of the coupled equations for the charged and neutral components for an arbitrary number of chemical species, and the radiation field. All approximations and assumptions are carefully considered. Generalized Ohm's law is derived for the single-fluid and two-fluid formalism. Our approach is analytical with some order-of-magnitude support calculations. After general equations are developed, we particularize to some frequently considered cases as for the interaction of matter and radiation. Title: Rayleigh-Taylor instability in prominences from numerical simulations including partial ionization effects Authors: Khomenko, E.; Díaz, A.; de Vicente, A.; Collados, M.; Luna, M. Bibcode: 2014A&A...565A..45K Altcode: 2014arXiv1403.4530K We study the Rayleigh-Taylor instability (RTI) at a prominence-corona transition region in a non-linear regime. Our aim is to understand how the presence of neutral atoms in the prominence plasma influences the instability growth rate, as well as the evolution of velocity, magnetic field vector, and thermodynamic parameters of turbulent drops. We perform 2.5D numerical simulations of the instability initiated by a multi-mode perturbation at the corona-prominence interface using a single-fluid magnetohydrodynamic (MHD) approach including a generalized Ohm's law. The initial equilibrium configuration is purely hydrostatic and contains a homogeneous horizontal magnetic field forming an angle with the direction in which the plasma is perturbed. We analyze simulations with two different orientations of the magnetic field. For each field orientation we compare two simulations, one for the pure MHD case, and one including the ambipolar diffusion in Ohm's law (AD case). Other than that, both simulations for each field orientation are identical. The numerical results in the initial stage of the instability are compared with the analytical linear calculations. We find that the configuration is always unstable in the AD case. The growth rate of the small-scale modes in the non-linear regime is up to 50% larger in the AD case than in the purely MHD case and the average velocities of flows are a few percentage points higher. Significant drift momenta are found at the interface between the coronal and the prominence material at all stages of the instability, produced by the faster downward motion of the neutral component with respect to the ionized component. The differences in temperature of the bubbles between the ideal and non-ideal case are also significant, reaching 30%. There is an asymmetry between large rising bubbles and small-scale down flowing fingers, favoring the detection of upward velocities in observations. Title: Rayleigh-Taylor instability in partially ionized compressible plasmas: One fluid approach Authors: Díaz, A. J.; Khomenko, E.; Collados, M. Bibcode: 2014A&A...564A..97D Altcode: 2014arXiv1401.5388D
Aims: We study the modification of the classical criterion for the linear onset and growth rate of the Rayleigh-Taylor instability (RTI) in a partially ionized (PI) plasma in the one-fluid description by considering a generalized induction equation.
Methods: The governing linear equations and appropriate boundary conditions, including gravitational terms, are derived and applied to the case of the RTI in a single interface between two partially ionized plasmas. The boundary conditions lead to an equation for the frequencies in which some have positive complex parts, marking the appearance of the RTI. We study the ambipolar term alone first, extending the result to the full induction equation later.
Results: The configuration is always unstable because of the presence of a neutral species. In the classical stability regime, the growth rate is small, since the collisions prevent the neutral fluid to fully develop the RTI. For parameters in the classical instability regime, the growth rate is lowered, but the differences with the compressible MHD case are small for the considered theoretical values of the collision frequencies and diffusion coefficients for solar prominences.
Conclusions: The PI modifies some aspects of the linear RTI instability, since it takes into account that neutrals do not feel the stabilizing effect of the magnetic field. For the set of parameters representative for solar prominences, our model gives the resulting timescale comparable to observed lifetimes of RTI plumes. Title: Rayleigh-Taylor instability in partially ionized prominence plasma Authors: Khomenko, E.; Díaz, A.; de Vicente, A.; Collados, M.; Luna, M. Bibcode: 2014IAUS..300...90K Altcode: 2013arXiv1310.7016K We study Rayleigh-Taylor instability (RTI) at the coronal-prominence boundary by means of 2.5D numerical simulations in a single-fluid MHD approach including a generalized Ohm's law. The initial configuration includes a homogeneous magnetic field forming an angle with the direction in which the plasma is perturbed. For each field inclination we compare two simulations, one for the pure MHD case, and one including the ambipolar diffusion in the Ohm's law, otherwise identical. We find that the configuration containing neutral atoms is always unstable. The growth rate of the small-scale modes in the non-linear regime is larger than in the purely MHD case. Title: 3D simulations of Rayleigh-Taylor instability in prominences including partial ionization effects Authors: Khomenko, Elena; Collados, Manuel; De Vicente, Angel; Luna, Manuel; Diaz, Antonio Bibcode: 2014cosp...40E1476K Altcode: We study the Rayleigh-Taylor instability (RTI) at a prominence-corona transition region in a non-linear regime. Our aim is to understand how the presence of neutral atoms in the prominence plasma influences the instability growth rate, and the evolution of velocity, magnetic field vector and thermodynamic parameters of turbulent drops. We perform 3D numerical simulations of the instability initiated by a multi-mode perturbation at the corona-prominence interface using a single-fluid MHD approach including a generalized Ohm's law. Pairs of simulations are compared, one of them done under ideal MHD conditions, and others include ambipolar diffusion (AD) in the Ohm's law. Other than that, the simulations of each pair are identical in their magnetic field orientation and thermal parameters. The numerical results in the initial stage of the instability are compared with the analytical linear calculations. We find that the configuration is always unstable in the AD case. The growth rate of the small-scale modes in the non-linear regime is up to 50% larger in the AD case than in the purely MHD case and the average velocities of flows are a few percent larger. Significant drift momenta are found at the interface between the coronal and the prominence material at all stages of the instability, produced by the faster downward motion of the neutral component with respect to the ionized component. The differences in temperature of the bubbles between the ideal and non-ideal case are also significant. Title: Properties of oscillatory motions in a facular region Authors: Kostik, R.; Khomenko, E. Bibcode: 2013A&A...559A.107K Altcode: 2013arXiv1310.0184K
Aims: We study the properties of waves in a facular region of moderate strength in the photosphere and chromosphere. Our aim is to statistically analyse the wave periods, power, and phase relations as a function of the magnetic field strength and inclination.
Methods: Our work is based on observations obtained at the German Vacuum Tower Telescope (Observatorio del Teide, Tenerife) using two different instruments: the Triple Etalon SOlar Spectrometer (TESOS) in the Ba ii 4554 Å line to measure velocity and intensity variations through the photosphere and, simultaneously, the Tenerife Infrared Polarimeter (TIP-II), in the Fe i 1.56 μm lines to measure the Stokes parameters and magnetic field strength in the lower photosphere. Additionally, we use the simultaneous broad-band filtergrams in the Ca ii H line to obtain information about intensity oscillations in the chromosphere.
Results: We find several clear trends in the oscillation behaviour: (i) the period of oscillation increases by 15-20% with the magnetic field increasing from 500 to 1500 G. (ii) The temperature-velocity phase shifts show a strikingly different distribution in the facular region compared to the quiet region, a significant number of cases in the range from - 180° to 180° is detected in the facula. (iii) The most powerful chromospheric Ca ii H intensity oscillations are observed at locations with strong magnetic fields (1.3-1.5 kG) inclined by 10-12 degrees, as a result of upward propagating waves with rather low phase speeds, and temperature-velocity phase shifts between 0° and 90°. (iv) The power of the photospheric velocity oscillations from the Ba ii line increases linearly with decreasing magnetic field inclination, reaching its maximum at strong field locations. Title: Dislocations in Magnetohydrodynamic Waves in a Stellar Atmosphere Authors: López Ariste, A.; Collados, M.; Khomenko, E. Bibcode: 2013PhRvL.111h1103L Altcode: 2013arXiv1308.0145L We describe the presence of wave front dislocations in magnetohydrodynamic waves in stratified stellar atmospheres. Scalar dislocations such as edges and vortices can appear in Alfvén waves, as well as in general magnetoacoustic waves. We detect those dislocations in observations of magnetohydrodynamic waves in sunspots in the solar chromosphere. Through the measured charge of all the dislocations observed, we can give for the first time estimates of the modal contribution in the waves propagating along magnetic fields in solar sunspots. Title: Atmosphere Dynamics of the Active Region NOAA 11024 Authors: Kondrashova, N. N.; Pasechnik, M. N.; Chornogor, S. N.; Khomenko, E. V. Bibcode: 2013SoPh..284..499K Altcode: 2012arXiv1212.1307K We present results of the study of chromospheric and photospheric line-of-sight velocity fields in the young active region NOAA 11024. Multi-layer, multi-wavelength observational data were used for the analysis of the emerging flux in this active region. Spectropolarimetric observations were carried out with the telescope THEMIS on Tenerife (Canary Islands) on 4 July 2009. In addition, space-borne data from SOHO/MDI, STEREO and GOES were also considered. The combination of data from ground- and space-based telescopes allowed us to study the dynamics of the lower atmosphere of the active region with high spatial, spectral, and temporal resolutions. THEMIS spectra show strong temporal variations of the velocity in the chromosphere and photosphere for various activity features: two pores, active and quiet plage regions, and two surges. The range of variations of the chromospheric line-of-sight velocity at the heights of the formation of the Hα core was extremely large. Both upward and downward motions were observed in these layers. In particular, a surge with upward velocities up to −73 km s−1 was detected. In the photosphere, predominantly upward motions were found, varying from −3.1 km s−1 upflows to 1.4 km s−1 downflows in different structures. The velocity variations at different levels in the lower atmosphere are compatible with the emergence of magnetic flux. Title: Magnetohydrodynamic waves driven by p-modes Authors: Khomenko, Elena; Calvo Santamaria, Irantzu Bibcode: 2013JPhCS.440a2048K Altcode: 2013arXiv1302.4351K Waves are observed at all layers of the solar atmosphere and the magnetic field plays a key role in their propagation. While deep down in the atmosphere the p-modes are almost entirely of acoustic nature, in the upper layers magnetic forces are dominating, leading to a large variety of new wave modes. Significant advances have been made recently in our understanding of the physics of waves interaction with magnetic structures, with the help of analytical theories, numerical simulations, as well as high-resolution observations. In this contribution, we review recent observational findings and current theoretical ideas in the field, with an emphasis on the following questions: (i) Peculiarities of the observed wave propagation in network, plage and facular regions; (ii) Role of the mode transformation and observational evidences of this process: (iii) Coupling of the photosphere, chromosphere, and above by means of waves propagating in magnetic structures. Title: MHD wave propagation in the solar network Authors: Calvo Santamaria, I.; Khomenko, E.; Cally, P. S.; Collados, M. Bibcode: 2013hsa7.conf..806C Altcode: Magneto-acoustic and Alfvénic waves are ubiquitous in solar coronal loops, possibly being excited by photospheric motions. It is not clear, though, how these waves get so high, having obstacles such as the acoustic cut-off frequency, reflection and refraction of fast MHD waves and also the strongly reflecting transition region. In this contribution we report on 2D numerical modelling of waves in magnetic arcade structures extending from photospheric layers through the transition region to the corona. Waves in the arcade are excited by sub-photospheric p-modes. We discuss the behaviour of waves, their conversion and propagation properties and possible mechanisms allowing their escape through the transition region. Title: Simulations of Chromospheric Heating by Ambipolar Diffusion Authors: Khomenko, E.; Collados Vera, M. Bibcode: 2012ASPC..463..281K Altcode: 2012arXiv1202.2252K We propose a mechanism for efficient heating of the solar chromosphere based on non-ideal plasma effects. Three ingredients are needed for the work of this mechanism: (1) presence of neutral atoms; (2) presence of a non-potential magnetic field; (3) decrease of the collisional coupling of the plasma. Due to the decrease of collisional coupling, a net relative motion appears between the neutral and ionized components, usually referred to as “ambipolar diffusion.” This results in a significant enhancement of current dissipation as compared to the classical MHD case. We propose that the current dissipation in this situation is able to provide enough energy to heat the chromosphere by several kK on the time scale of minutes, or even seconds. In this paper, we show that this energy supply might be sufficient to balance the radiative energy losses of the chromosphere. Title: Solar Fe abundance and magnetic fields. Towards a consistent reference metallicity Authors: Fabbian, D.; Moreno-Insertis, F.; Khomenko, E.; Nordlund, Å. Bibcode: 2012A&A...548A..35F Altcode: 2012arXiv1209.2771F
Aims: We investigate the impact on Fe abundance determination of including magnetic flux in series of 3D radiation-magnetohydrodynamics (MHD) simulations of solar convection, which we used to synthesize spectral intensity profiles corresponding to disc centre.
Methods: A differential approach is used to quantify the changes in theoretical equivalent width of a set of 28 iron spectral lines spanning a wide range in wavelength, excitation potential, oscillator strength, Landé factor, and formation height. The lines were computed in local thermodynamic equilibrium (LTE) using the spectral synthesis code LILIA. We used input magnetoconvection snapshots covering 50 min of solar evolution and belonging to series having an average vertical magnetic flux density of ⟨ Bvert ⟩ = 0,50,100, and 200 G. For the relevant calculations we used the Copenhagen Stagger code.
Results: The presence of magnetic fields causes both a direct (Zeeman-broadening) effect on spectral lines with non-zero Landé factor and an indirect effect on temperature-sensitive lines via a change in the photospheric T - τ stratification. The corresponding correction in the estimated atomic abundance ranges from a few hundredths of a dex up to |Δlog ɛ(Fe)| ~ 0.15 dex, depending on the spectral line and on the amount of average magnetic flux within the range of values we considered. The Zeeman-broadening effect gains relatively more importance in the IR. The largest modification to previous solar abundance determinations based on visible spectral lines is instead due to the indirect effect, i.e., the line-weakening caused by a warmer stratification as seen on an optical depth scale. Our results indicate that the average solar iron abundance obtained when using magnetoconvection models can be ~ 0.03-0.11 dex higher than when using the simpler hydrodynamics (HD) convection approach.
Conclusions: We demonstrate that accounting for magnetic flux is important in state-of-the-art solar photospheric abundance determinations based on 3D convection simulations. Title: Properties of convective motions in facular regions Authors: Kostik, R.; Khomenko, E. V. Bibcode: 2012A&A...545A..22K Altcode: 2012arXiv1207.4340K
Aims: We study the properties of solar granulation in a facular region from the photosphere up to the lower chromosphere. Our aim is to investigate the dependence of granular structure on magnetic field strength.
Methods: We used observations obtained at the German Vacuum Tower Telescope (Observatorio del Teide, Tenerife) using two different instruments: the Triple Etalon SOlar Spectrometer (TESOS) to measure velocity and intensity variations along the photosphere in the Ba ii 4554 Å line; and, simultaneously, the Tenerife Infrared Polarimeter (TIP-II) to the measure Stokes parameters and the magnetic field strength at the lower photosphere in the Fe i 1.56 μm lines.
Results: We find that the convective velocities of granules in the facular area decrease with magnetic field while the convective velocities of intergranular lanes increase with the field strength. Similar to the quiet areas, there is a contrast and velocity sign reversal taking place in the middle photosphere. The reversal heights depend on the magnetic field strength and are, on average, about 100 km higher than in the quiet regions. The correlation between convective velocity and intensity decreases with magnetic field at the bottom photosphere, but increases in the upper photosphere. The contrast of intergranular lanes observed close to the disk center is almost independent of the magnetic field strength.
Conclusions: The strong magnetic field of the facular area seems to stabilize the convection and to promote more effective energy transfer in the upper layers of the solar atmosphere, since the convective elements reach greater heights. Title: First Results from the SUNRISE Mission Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.; Gandorfer, A.; Hirzberger, J.; Jafarzadeh, S.; Lagg, A.; Riethmüller, T. L.; Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; González, M. J. M.; Pillet, V. M.; Khomenko, E.; Yelles Chaouche, L.; Iniesta, J. C. d. T.; Domingo, V.; Palacios, J.; Knölker, M.; González, N. B.; Borrero, J. M.; Berkefeld, T.; Franz, M.; Roth, M.; Schmidt, W.; Steiner, O.; Title, A. M. Bibcode: 2012ASPC..455..143S Altcode: The SUNRISE balloon-borne solar observatory consists of a 1m aperture Gregory telescope, a UV filter imager, an imaging vector polarimeter, an image stabilization system, and further infrastructure. The first science flight of SUNRISE yielded high-quality data that reveal the structure, dynamics, and evolution of solar convection, oscillations, and magnetic fields at a resolution of around 100 km in the quiet Sun. Here we describe very briefly the mission and the first results obtained from the SUNRISE data, which include a number of discoveries. Title: Heating of the Magnetized Solar Chromosphere by Partial Ionization Effects Authors: Khomenko, E.; Collados, M. Bibcode: 2012ApJ...747...87K Altcode: 2011arXiv1112.3374K In this paper, we study the heating of the magnetized solar chromosphere induced by the large fraction of neutral atoms present in this layer. The presence of neutrals, together with the decrease with height of the collisional coupling, leads to deviations from the classical magnetohydrodynamic behavior of the chromospheric plasma. A relative net motion appears between the neutral and ionized components, usually referred to as ambipolar diffusion. The dissipation of currents in the chromosphere is enhanced by orders of magnitude due to the action of ambipolar diffusion, as compared with the standard ohmic diffusion. We propose that a significant amount of magnetic energy can be released to the chromosphere just by existing force-free 10-40 G magnetic fields there. As a consequence, we conclude that ambipolar diffusion is an important process that should be included in chromospheric heating models, as it has the potential to rapidly heat the chromosphere. We perform analytical estimations and numerical simulations to prove this idea. Title: Influence of phase-diversity image reconstruction techniques on circular polarization asymmetries Authors: Asensio Ramos, A.; Martínez González, M. J.; Khomenko, E.; Martínez Pillet, V. Bibcode: 2012A&A...539A..42A Altcode: 2011arXiv1111.2496A Context. Full Stokes filter-polarimeters are key instruments for investigating the rapid evolution of magnetic structures on the solar surface. To this end, the image quality is routinely improved using a-posteriori image reconstruction methods.
Aims: We analyze the robustness of circular polarization asymmetries to phase-diversity image reconstruction techniques.
Methods: We used snapshots of magneto-hydrodynamical simulations carried out with different initial conditions to synthesize spectra of the magnetically sensitive Fe i line at 5250.2 Å. We degraded the synthetic profiles spatially and spectrally to simulate observations with the IMaX full Stokes filter-polarimeter. We also simulated the focused/defocused pairs of images used by the phase-diversity algorithm for reconstruction and the polarimetric modulation scheme. We assume that standard optimization methods are able to infer the projection of the wavefront on the Zernike polynomials with 10% precision. We also consider the less favorable case of 25% precision. We obtain reconstructed monochromatic modulated images that are later demodulated and compared with the original maps.
Results: Although asymmetries are often difficult to define in the quiet Sun due to the complexity of the Stokes V profiles, we show how asymmetries are degraded with spatial and spectral smearing. The results indicate that, although image reconstruction techniques reduce the spatial smearing, they can modify the asymmetries of the profiles, which are mainly caused by the appearance of spatially-correlated noise. Title: Beyond single fluid MHD: multi-fluid modeling of the coupled solar atmosphere Authors: Khomenko, Elena Bibcode: 2012decs.confE..26K Altcode: The particular temperature and density conditions in the magnetized photosphere and chromosphere of the Sun usually lead to a very small degree of atomic ionization. In addition, at particular heights, the magnetic field may be strong enough to give rise to a cyclotron frequency larger than the collisional frequency for some species, while for others the opposite may happen. These circumstances can influence the collective behaviour of the particles and some of the hypotheses of magnetohydrodynamics may be relaxed, giving rise to non-ideal MHD effects. These effects are potentially important for the dynamics and energy exchange in the solar photosphere and, especially, chromosphere. In particular, there are evidences that such phenomena as wave propagation and damping, magnetic reconnection, formation of stable magnetic field concentrations, magnetic flux emergence, etc. can be affected. In this contribution, I will discuss the current state-of-the-art of multi-fluid MHD modelling of the coupled solar atmosphere. I will revise the major issues, physical mechanisms and assumptions of the MHD approach, and discuss future simulations that would be required to address some unresolved topics. I will present the first results of numerical simulations using the modified MHD equations, showing that the chromosphere can be effectively heated due to non-ideal MHD effects. Title: Numerical Simulations of Conversion to Alfvén Waves in Sunspots Authors: Khomenko, E.; Cally, P. S. Bibcode: 2012ApJ...746...68K Altcode: 2011arXiv1111.2851K We study the conversion of fast magnetoacoustic waves to Alfvén waves by means of 2.5D numerical simulations in a sunspot-like magnetic configuration. A fast, essentially acoustic, wave of a given frequency and wave number is generated below the surface and propagates upward through the Alfvén/acoustic equipartition layer where it splits into upgoing slow (acoustic) and fast (magnetic) waves. The fast wave quickly reflects off the steep Alfvén speed gradient, but around and above this reflection height it partially converts to Alfvén waves, depending on the local relative inclinations of the background magnetic field and the wavevector. To measure the efficiency of this conversion to Alfvén waves we calculate acoustic and magnetic energy fluxes. The particular amplitude and phase relations between the magnetic field and velocity oscillations help us to demonstrate that the waves produced are indeed Alfvén waves. We find that the conversion to Alfvén waves is particularly important for strongly inclined fields like those existing in sunspot penumbrae. Equally important is the magnetic field orientation with respect to the vertical plane of wave propagation, which we refer to as "field azimuth." For a field azimuth less than 90° the generated Alfvén waves continue upward, but above 90° downgoing Alfvén waves are preferentially produced. This yields negative Alfvén energy flux for azimuths between 90° and 180°. Alfvén energy fluxes may be comparable to or exceed acoustic fluxes, depending upon geometry, though computational exigencies limit their magnitude in our simulations. Title: Magneto-acoustic wave energy in sunspots: observations and numerical simulations Authors: Felipe, T.; Khomenko, E.; Collados, M.; Beck, C. Bibcode: 2011hsa6.conf..630F Altcode: We have reproduced some sunspot wave signatures obtained from spectropolarimetric observations through 3D MHD numericalsimulations. The results of the simulations arecompared with the oscillations observed simultaneously at different heights from the SiI lambda10827Å line, HeI lambda10830Å line, the CaII H core and the FeI blends at the wings of the CaII H line. The simulations show a remarkable agreement with the observations, and we have used them to quantify the energy contribution of the magneto-acoustic waves to the chromospheric heating in sunspots. Our findings indicate that the energy supplied by these waves is 5-10 times lower than the amount needed to balance the chromospheric radiative losses. Title: The Sun at high resolution: first results from the Sunrise mission Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.; Gandorfer, A.; Hirzberger, J.; Lagg, A.; Riethmüller, T. L.; Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; Pillet, V. Martínez; Khomenko, E.; del Toro Iniesta, J. C.; Domingo, V.; Palacios, J.; Knölker, M.; González, N. Bello; Borrero, J. M.; Berkefeld, T.; Franz, M.; Roth, M.; Schmidt, W.; Steiner, O.; Title, A. M. Bibcode: 2011IAUS..273..226S Altcode: The Sunrise balloon-borne solar observatory consists of a 1m aperture Gregory telescope, a UV filter imager, an imaging vector polarimeter, an image stabilization system and further infrastructure. The first science flight of Sunrise yielded high-quality data that reveal the structure, dynamics and evolution of solar convection, oscillations and magnetic fields at a resolution of around 100 km in the quiet Sun. Here we describe very briefly the mission and the first results obtained from the Sunrise data, which include a number of discoveries. Title: Magnetoacoustic Wave Energy from Numerical Simulations of an Observed Sunspot Umbra Authors: Felipe, T.; Khomenko, E.; Collados, M. Bibcode: 2011ApJ...735...65F Altcode: 2011arXiv1104.4138F We aim at reproducing the height dependence of sunspot wave signatures obtained from spectropolarimetric observations through three-dimensional MHD numerical simulations. A magnetostatic sunspot model based on the properties of the observed sunspot is constructed and perturbed at the photosphere, introducing the fluctuations measured with the Si I λ10827 line. The results of the simulations are compared with the oscillations observed simultaneously at different heights from the He I λ10830 line, the Ca II H core, and the Fe I blends in the wings of the Ca II H line. The simulations show a remarkable agreement with the observations. They reproduce the velocity maps and power spectra at the formation heights of the observed lines, as well as the phase and amplification spectra between several pairs of lines. We find that the stronger shocks at the chromosphere are accompanied with a delay between the observed signal and the simulated one at the corresponding height, indicating that shocks shift the formation height of the chromospheric lines to higher layers. Since the simulated wave propagation matches very well the properties of the observed one, we are able to use the numerical calculations to quantify the energy contribution of the magnetoacoustic waves to the chromospheric heating in sunspots. Our findings indicate that the energy supplied by these waves is too low to balance the chromospheric radiative losses. The energy contained at the formation height of the lowermost Si I λ10827 line in the form of slow magnetoacoustic waves is already insufficient to heat the higher layers, and the acoustic energy which reaches the chromosphere is around 3-9 times lower than the required amount of energy. The contribution of the magnetic energy is even lower. Title: Sunspot Models Authors: Khomenko, E. Bibcode: 2011ascl.soft05007K Altcode: These IDL codes create a thick magneto-static structure with parameters of a typical sunspot in a solar like photosphere - chromosphere. The variable parameters are field strength on the axis, radius, and Wilson depression (displacement of the atmosphere on the axis with respect to the field-free atmosphere). Output are magnetic field vector, pressure and density distributions with radius and height. The structure has azimuthal symmetry. The codes are relatively self explanatory and the download packages contain README files. Title: Unnoticed Magnetic Field Oscillations in the Very Quiet Sun Revealed by SUNRISE/IMaX Authors: Martínez González, M. J.; Asensio Ramos, A.; Manso Sainz, R.; Khomenko, E.; Martínez Pillet, V.; Solanki, S. K.; López Ariste, A.; Schmidt, W.; Barthol, P.; Gandorfer, A. Bibcode: 2011ApJ...730L..37M Altcode: 2011arXiv1103.0145M We present observational evidence for oscillations of magnetic flux density in the quiet areas of the Sun. The majority of magnetic fields on the solar surface have strengths of the order of or lower than the equipartition field (300-500 G). This results in a myriad of magnetic fields whose evolution is largely determined by the turbulent plasma motions. When granules evolve they squash the magnetic field lines together or pull them apart. Here, we report on the periodic deformation of the shapes of features in circular polarization observed at high resolution with SUNRISE. In particular, we note that the area of patches with a constant magnetic flux oscillates with time, which implies that the apparent magnetic field intensity oscillates in antiphase. The periods associated with this oscillatory pattern are compatible with the granular lifetime and change abruptly, which suggests that these oscillations might not correspond to characteristic oscillatory modes of magnetic structures, but to the forcing by granular motions. In one particular case, we find three patches around the same granule oscillating in phase, which means that the spatial coherence of these oscillations can reach 1600 km. Interestingly, the same kind of oscillatory phenomenon is also found in the upper photosphere. Title: Numerical simulations of conversion to Alfvén waves in solar active regions Authors: Khomenko, E.; Cally, P. S. Bibcode: 2011JPhCS.271a2042K Altcode: 2010arXiv1009.4575K We study the coupling of magneto-acoustic waves to Alvén waves using 2.5D numerical simulations. In our experiment, a fast magnetoacoustic wave of a given frequency and wavenumber is generated below the surface. The magnetic field in the domain is assumed homogeneous and inclined. The efficiency of the conversion to Alfvén waves near the layer of equal acoustic and Alfven speeds is measured calculating their energy flux. The particular amplitude and phase relations between the oscillations of magnetic field and velocity help us to demonstrate that the waves produced after the transformation and reaching upper atmosphere are indeed Alfvén waves. We find that the conversion from fast magneto-acoustic waves to Alfvén waves is particularly important for the inclination θ and azimuth phi angles of the magnetic field between 55 and 65 degrees, with the maximum shifted to larger inclinations for lower frequency waves. The maximum Alfvén flux transmitted to the upper atmosphere is about 2-3 times lower than the corresponding acoustic flux. Title: Magneto-acoustic waves in sunspots from observations and numerical simulations Authors: Felipe, T.; Khomenko, E.; Collados, M.; Beck, C. Bibcode: 2011JPhCS.271a2040F Altcode: 2010arXiv1009.5512F We study the propagation of waves from the photosphere to the chromosphere of sunspots. From time series of cospatial Ca II H (including its line blends) intensity spectra and polarimetric spectra of Si I λ 1082.7 nm and He I λ 1083.0 nm we retrieve the line-of-sight velocity at several heights. The analysis of the phase difference and amplification spectra shows standing waves for frequencies below 4 mHz and propagating waves for higher frequencies, and allows us to infer the temperature and height where the lines are formed. Using these observational data, we have constructed a model of sunspot, and we have introduced the velocity measured with the photospheric Si I λ 1082.7 nm line as a driver. The numerically propagated wave pattern fits reasonably well with the observed using the lines formed at higher layers, and the simulations reproduce many of the observed features. The observed waves are slow MHD waves propagating longitudinally along field lines. Title: Solar Abundance Corrections Derived Through Three-dimensional Magnetoconvection Simulations Authors: Fabbian, D.; Khomenko, E.; Moreno-Insertis, F.; Nordlund, Å. Bibcode: 2010ApJ...724.1536F Altcode: 2010arXiv1006.0231F We explore the effect of the magnetic field when using realistic three-dimensional convection experiments to determine solar element abundances. By carrying out magnetoconvection simulations with a radiation-hydro code (the Copenhagen stagger code) and through a posteriori spectral synthesis of three Fe I lines, we obtain evidence that moderate amounts of mean magnetic flux cause a noticeable change in the derived equivalent widths compared with those for a non-magnetic case. The corresponding Fe abundance correction for a mean flux density of 200 G reaches up to ~0.1 dex in magnitude. These results are based on space- and time-averaged line profiles over a time span of 2.5 solar hours in the statistically stationary regime of the convection. The main factors causing the change in equivalent widths, namely the Zeeman broadening and the modification of the temperature stratification, act in different amounts and, for the iron lines considered here, in opposite directions; yet, the resulting |Δlog epsilonsun(Fe)| coincides within a factor of 2 in all of them, even though the sign of the total abundance correction is different for the visible and infrared lines. We conclude that magnetic effects should be taken into account when discussing precise values of the solar and stellar abundances and that an extended study is warranted. Title: Where the Granular Flows Bend Authors: Khomenko, E.; Martínez Pillet, V.; Solanki, S. K.; del Toro Iniesta, J. C.; Gandorfer, A.; Bonet, J. A.; Domingo, V.; Schmidt, W.; Barthol, P.; Knölker, M. Bibcode: 2010ApJ...723L.159K Altcode: 2010arXiv1008.0517K Based on IMaX/SUNRISE data, we report on a previously undetected phenomenon in solar granulation. We show that in a very narrow region separating granules and intergranular lanes, the spectral line width of the Fe I 5250.2 Å line becomes extremely small. We offer an explanation of this observation with the help of magneto-convection simulations. These regions with extremely small line widths correspond to the places where the granular flows bend from upflow in granules to downflow in intergranular lanes. We show that the resolution and image stability achieved by IMaX/SUNRISE are important requisites to detect this interesting phenomenon. Title: Numerical simulation of propagation of the MHD waves in sunspots Authors: Parchevsky, K.; Kosovichev, A.; Khomenko, E.; Olshevsky, V.; Collados, M. Bibcode: 2010HiA....15..354P Altcode: We present results of numerical 3D simulation of propagation of MHD waves in sunspots. We used two self consistent magnetohydrostatic background models of sunspots. There are two main differences between these models: (i) the topology of the magnetic field and (ii) dependence of the horizontal profile of the sound speed on depth. The model with convex shape of the magnetic field lines near the photosphere has non-zero horizorntal perturbations of the sound speed up to the depth of 7.5 Mm (deep model). In the model with concave shape of the magnetic field lines near the photosphere Δ c/c is close to zero everywhere below 2 Mm (shallow model). Strong Alfven wave is generated at the wave source location in the deep model. This wave is almost unnoticeable in the shallow model. Using filtering technique we separated magnetoacoustic and magnetogravity waves. It is shown, that inside the sunspot magnetoacoustic and magnetogravity waves are not spatially separated unlike the case of the horizontally uniform background model. The sunspot causes anisotropy of the amplitude distribution along the wavefront and changes the shape of the wavefront. The amplitude of the waves is reduced inside the sunspot. This effect is stronger for the magnetogravity waves than for magnetoacoustic waves. The shape of the wavefront of the magnetogravity waves is distorted stronger as well. The deep model causes bigger anisotropy for both mgnetoacoustic and magneto gravity waves than the shallow model. Title: Modeling the Subsurface Structure of Sunspots Authors: Moradi, H.; Baldner, C.; Birch, A. C.; Braun, D. C.; Cameron, R. H.; Duvall, T. L.; Gizon, L.; Haber, D.; Hanasoge, S. M.; Hindman, B. W.; Jackiewicz, J.; Khomenko, E.; Komm, R.; Rajaguru, P.; Rempel, M.; Roth, M.; Schlichenmaier, R.; Schunker, H.; Spruit, H. C.; Strassmeier, K. G.; Thompson, M. J.; Zharkov, S. Bibcode: 2010SoPh..267....1M Altcode: 2009arXiv0912.4982M; 2010SoPh..tmp..171M While sunspots are easily observed at the solar surface, determining their subsurface structure is not trivial. There are two main hypotheses for the subsurface structure of sunspots: the monolithic model and the cluster model. Local helioseismology is the only means by which we can investigate subphotospheric structure. However, as current linear inversion techniques do not yet allow helioseismology to probe the internal structure with sufficient confidence to distinguish between the monolith and cluster models, the development of physically realistic sunspot models are a priority for helioseismologists. This is because they are not only important indicators of the variety of physical effects that may influence helioseismic inferences in active regions, but they also enable detailed assessments of the validity of helioseismic interpretations through numerical forward modeling. In this article, we provide a critical review of the existing sunspot models and an overview of numerical methods employed to model wave propagation through model sunspots. We then carry out a helioseismic analysis of the sunspot in Active Region 9787 and address the serious inconsistencies uncovered by Gizon et al. (2009a, 2009b). We find that this sunspot is most probably associated with a shallow, positive wave-speed perturbation (unlike the traditional two-layer model) and that travel-time measurements are consistent with a horizontal outflow in the surrounding moat. Title: Multi-layer Study of Wave Propagation in Sunspots Authors: Felipe, T.; Khomenko, E.; Collados, M.; Beck, C. Bibcode: 2010ApJ...722..131F Altcode: 2010arXiv1008.4004F We analyze the propagation of waves in sunspots from the photosphere to the chromosphere using time series of co-spatial Ca II H intensity spectra (including its line blends) and polarimetric spectra of Si I λ10,827 and the He I λ10,830 multiplet. From the Doppler shifts of these lines we retrieve the variation of the velocity along the line of sight at several heights. Phase spectra are used to obtain the relation between the oscillatory signals. Our analysis reveals standing waves at frequencies lower than 4 mHz and a continuous propagation of waves at higher frequencies, which steepen into shocks in the chromosphere when approaching the formation height of the Ca II H core. The observed nonlinearities are weaker in Ca II H than in He I lines. Our analysis suggests that the Ca II H core forms at a lower height than the He I λ10,830 line: a time delay of about 20 s is measured between the Doppler signal detected at both wavelengths. We fit a model of linear slow magnetoacoustic wave propagation in a stratified atmosphere with radiative losses according to Newton's cooling law to the phase spectra and derive the difference in the formation height of the spectral lines. We show that the linear model describes well the wave propagation up to the formation height of Ca II H, where nonlinearities start to become very important. Title: Magneto-acoustic Waves in Sunspots: First Results From a New Three-dimensional Nonlinear Magnetohydrodynamic Code Authors: Felipe, T.; Khomenko, E.; Collados, M. Bibcode: 2010ApJ...719..357F Altcode: 2010arXiv1006.2998F Waves observed in the photosphere and chromosphere of sunspots show complex dynamics and spatial patterns. The interpretation of high-resolution sunspot wave observations requires modeling of three-dimensional (3D) nonlinear wave propagation and mode transformation in the sunspot upper layers in realistic spot model atmospheres. Here, we present the first results of such modeling. We have developed a 3D nonlinear numerical code specially designed to calculate the response of magnetic structures in equilibrium to an arbitrary perturbation. The code solves the 3D nonlinear MHD equations for perturbations; it is stabilized by hyper-diffusivity terms and is fully parallelized. The robustness of the code is demonstrated by a number of standard tests. We analyze several simulations of a sunspot perturbed by pulses of different periods at a subphotospheric level, from short periods, introduced for academic purposes, to longer and realistic periods of 3 and 5 minutes. We present a detailed description of the 3D mode transformation in a non-trivial sunspot-like magnetic field configuration, including the conversion between fast and slow magneto-acoustic waves and the Alfvén wave, by calculation of the wave energy fluxes. Our main findings are as follows: (1) the conversion from acoustic to the Alfvén mode is only observed if the driving pulse is located out of the sunspot axis, but this conversion is energetically inefficient; (2) as a consequence of the cutoff effects and refraction of the fast magneto-acoustic mode, the energy of the evanescent waves with periods around 5 minutes remains almost completely below the level β = 1; (3) waves with frequencies above the cutoff propagate field aligned to the chromosphere and their power becomes dominating over that of evanescent 5 minute oscillations, in agreement with observations. Title: Mode transformation and frequency change with height in 3D numerical simulations of magneto-acoustic wave propagation in sunspots Authors: Felipe, T.; Khomenko, E.; Collados, M. Bibcode: 2010arXiv1005.3684F Altcode: Three-dimensional numerical simulations of magnetoacoustic wave propagation are performed in a sunspot atmosphere with a computational domain covering from the photosphere to the chromosphere. The wave source, with properties resembling the solar spectrum, is located at different distances from the axis of the sunspot for each simulation. These results are compared with the theory of mode transformation and also with observational features. Simulations show that the dominant oscillation frequency in the chromosphere decreases with the radial distance from the sunspot axis. The energy flux of the different wave modes involved, including de Alfvén mode, is evaluated and discussed. Title: Towards pulsation mode identification in 3-D: theoretical simulations of line profile variations in roAp stars Authors: Kochukhov, O.; Khomenko, E. Bibcode: 2010arXiv1004.0139K Altcode: Time-resolved spectroscopic observations of rapidly oscillating Ap (roAp) stars show a complex picture of propagating magneto-acoustic pulsation waves, with amplitude and phase strongly changing as a function of atmospheric height. We have recently conducted numerical, non-linear MHD simulations to get an insight into the complex atmospheric dynamics of magnetic pulsators. Here we use the resulting time-dependent atmospheric structure and velocity field to predict line profile variations for roAp stars. These calculations use realistic atmospheric structure, account for vertical chemical stratification and treat the line formation in pulsating stellar atmosphere without relying on the simplistic single-layer approximation universally adopted for non-radial pulsators. The new theoretical calculations provide an essential tool for interpreting the puzzling complexity of the spectroscopic pulsations in roAp stars. Title: Numerical Simulation of Excitation and Propagation of Helioseismic MHD Waves in Magnetostatic Models of Sunspots Authors: Parchevsky, K.; Kosovichev, A.; Khomenko, E.; Olshevsky, V.; Collados, M. Bibcode: 2010arXiv1002.1117P Altcode: We present comparison of numerical simulations of propagation of MHD waves,excited by subphotospheric perturbations, in two different ("deep" and "shallow") magnetostatic models of the sunspots. The "deep" sunspot model distorts both the shape of the wavefront and its amplitude stronger than the "shallow" model. For both sunspot models, the surface gravity waves (f-mode) are affected by the sunspots stronger than the acoustic p-modes. The wave amplitude inside the sunspot depends on the photospheric strength of the magnetic field and the distance of the source from the sunspot axis. For the source located at 9 Mm from the center of the sunspot, the wave amplitude increases when the wavefront passes through the central part of the sunspot. For the source distance of 12 Mm, the wave amplitude inside the sunspot is always smaller than outside. For the same source distance from the sunspot center but for the models with different strength of the magnetic field, the wave amplitude inside the sunspot increases with the strength of the magnetic field. The simulations show that unlike the case of the uniform inclined background magnetic field, the p- and f-mode waves are not spatially separated inside the sunspot where the magnetic field is strongly non-uniform. These properties have to be taken into account for interpretation of observations of MHD waves traveling through sunspot regions. Title: Magnetic Fingerprints of Solar and Stellar Oscillations Authors: Khomenko, Elena Bibcode: 2010ASSP...14...51K Altcode: 2010hsa5.conf...51K; 2008arXiv0812.0042K Waves connect all the layers of the Sun, from its interior to the upper atmosphere. It is becoming clear now the important role of magnetic field on the wave propagation. Magnetic field modifies propagation speed of waves, thus affecting the conclusions of helioseismological studies. It can change the direction of the wave propagation, help channeling them straight up to the corona, extending the dynamic and magnetic couplings between all the layers. Modern instruments provide measurements of complex patterns of oscillations in solar active regions and of tiny effects such as temporal oscillations of the magnetic field. The physics of oscillations in a variety of magnetic structures of the Sun is similar to that of pulsations of stars that posses strong magnetic fields, such as roAp stars. All these arguments point toward a need of systematic self-consistent modeling of waves in magnetic structures that is able to take into account the complexity of the magnetic field configurations. In this paper, we describe simulations of this kind, summarize our recent findings and bring together results from the theory and observations. Title: Simulations of Waves in Sunspots Authors: Khomenko, E. Bibcode: 2009ASPC..416...31K Altcode: 2008arXiv0812.0040K A magnetic field modifies the properties of waves in a complex way. Significant advances have been made recently in our understanding of the physics of waves in solar active regions with the help of analytical theories, numerical simulations, and high-resolution observations. In this contribution we review the current ideas in the field, with the emphasis on theoretical models of waves in sunspots. Title: NUMERICAL SIMULATION OF PROPAGATION AND SCATTERING OF THE MHD WAVES IN SUNSPOTS Authors: Parchevsky, K.; Kosovichev, A. G.; Khomenko, E.; Collados, M. Bibcode: 2009AGUFMSH23B1535P Altcode: We present comparison of numerical simulation results of MHD wave propagation in two different magnitostatic models of sunspots refferred to as "deep" and "shallow" models. The "deep" model has convex shape of magnetic field lines near the photosphere and non-zero horizorntal perturbations of the sound speed up to the bottom of the model (7.5 Mm). The "shallow" model has concave shape of the magnetic field lines near the photosphere and horizontally uniform sound speed below 2 Mm. Common feature of MHD waves behaviour in these two models is that for weak magnetic field (less than 1kG at the photosphere) waves reduce their amplitude when they reach the center of the sunspot and restore the amplitude when pass the center. For the "deep" model this effect is bigger than for the "shallow" model. The wave amplitude inside sunspots depends on the strength of the magnetic field. For the "shallow" model with photospheric magnetic field of 2.2 kG the wave amplitude inside the sunspot becomes bigger than outside (opposite to the weak magnetic field). The wave amplitude depends on the distance of the source from the sunspot center. For the "shallow" model and source distance of 9 Mm from the sunspot center the wave amplitude at some moment (when the wavefront passes the sunspot center) becomes bigger inside the sunspot than outside. For the source distance of 12 Mm the wave amplitude remains smaller inside the sunspot than outside for all moments of time. Using filtering technique we separated magnetoacoustic and magnetogravity waves. Simulations show that the sunspot changes the shape of the wave front and amplitude of the f-modes significantly stronger than the p-modes. It is shown, that inside the sunspot magnetoacoustic and magnetogravity waves are not spatially separated unlike the case of the horizontally uniform background model. Strong Alfven wave is generated at the wave source location in the "deep" model. This wave exists in the "shallow" model as well, but with much smaller amplitude. Title: The solar Ba{II} 4554 Å line as a Doppler diagnostic: NLTE analysis in 3D hydrodynamical model Authors: Shchukina, N. G.; Olshevsky, V. L.; Khomenko, E. V. Bibcode: 2009A&A...506.1393S Altcode: 2009arXiv0905.0985S Aims: The aim of this paper is to analyse the validity of the Dopplergram and λ-meter techniques for the Doppler diagnostics of solar photospheric velocities using the Ba II 4554 Å line.
Methods: Both techniques are evaluated by means of NLTE radiative transfer calculations of the Ba II 4554 Å line in a three-dimensional hydrodynamical model of solar convection. We consider the cases of spatially unsmeared profiles and the profiles smeared to the resolution of ground-based observations.
Results: We find that: (i) speckle-reconstructed Dopplergram velocities reproduce the “true” velocities well at heights around 300 km, except for intergranular lanes with strong downflows where the velocity can be overestimated; (ii) the λ-meter velocities give a good representation of the “true” velocities through the whole photosphere, both under the original and reduced spatial resolutions. The velocities derived from the inner wing of smeared Ba II 4554 Å line profiles are more reliable than those for the outer wing. Only under high spatial resolution does the inner wing velocities calculated in intergranular regions give an underestimate (or even a sign reversal) compared with the model velocities; (iii) NLTE effects should be taken into account in modelling the Ba II 4554 Å line profiles. Such effects are more pronounced in intergranular regions.
Conclusions: Our analysis supports the opinion that the Dopplergram technique applied to the Ba II 4554 Å line is a valuable tool for the Doppler diagnostics of the middle photosphere around 300 km. The λ-meter technique applied to this line gives us a good opportunity to “trace” the non-thermal motions along the whole photosphere up to the temperature minimum and lower chromosphere.

Appendix is only available in electronic form at http://www.aanda.org Title: Sunspot seismic halos generated by fast MHD wave refraction Authors: Khomenko, E.; Collados, M. Bibcode: 2009A&A...506L...5K Altcode: 2009arXiv0905.3060K Aims: We suggest an explanation for the high-frequency power excess surrounding active regions known as seismic halos.
Methods: We use numerical simulations of magneto-acoustic wave propagation in a magnetostatic sunspot model.
Results: We propose that seismic halos can be caused by the additional energy injected by high-frequency fast mode waves refracted in the higher atmosphere due to the rapid increase of the Alfvén speed. Our model qualitatively explains the magnitude of the halo and allows us to make predictions of its behavior that can be checked in future observations. Title: Solar granulation from photosphere to low chromosphere observed in Ba II 4554 Å line Authors: Kostik, R.; Khomenko, E.; Shchukina, N. Bibcode: 2009A&A...506.1405K Altcode: 2009arXiv0909.1210K Aims: The purpose of this paper is to characterize the statistical properties of solar granulation in the photosphere and low chromosphere up to 650 km.
Methods: We use velocity and intensity variations obtained at different atmospheric heights from observations in Ba II 4554 Å. The observations were done during good seeing conditions at the VTT at the Observatorio del Teide on Tenerife. The line core forms rather high in the atmosphere and allows granulation properties to be studied at heights that have been not accessed before in similar studies. In addition, we analyze the synthetic profiles of the Ba II 4554 Å line by the same method computed taking NLTE effects into account in the 3D hydrodynamical model atmosphere.
Results: We suggest a 16-column model of solar granulation depending on the direction of motion and on the intensity contrast measured in the continuum and in the uppermost layer. We calculate the heights of intensity contrast sign reversal and velocity sign reversal. We show that both parameters depend strongly on the granulation velocity and intensity at the bottom photosphere. The larger the two parameters, the higher the reversal takes place in the atmosphere. On average, this happens at about 200-300 km. We suggest that this number also depends on the line depth of the spectral line used in observations. Despite the intensity and velocity reversal, about 40% of the column structure of granulation is preserved up to heights around 650 km. Title: The energy of waves in the photosphere and lower chromosphere. I. Velocity statistics Authors: Beck, C.; Khomenko, E.; Rezaei, R.; Collados, M. Bibcode: 2009A&A...507..453B Altcode: 2009arXiv0905.1011B Context: Acoustic waves are one of the primary suspects besides magnetic fields for the chromospheric heating process to temperatures above radiative equilibrium (RE).
Aims: We derived the mechanical wave energy as seen in line-core velocities on disc centre to obtain a measure of mechanical energy flux with height for a comparison with the energy requirements in a semi-empirical atmosphere model, the Harvard-Smithsonian reference atmosphere (HSRA).
Methods: We analyzed a 1-hour time series and a large-area map of Ca II H spectra on the traces of propagating waves. We analyzed the velocity statistics of several spectral lines in the wing of Ca II H, and the line-core velocity of Ca II H. We converted the velocity amplitudes into volume (∝ ρ v^2) and mass energy densities (∝ v^2). For comparison, we used the increase of internal energy (∝ R ρ Δ T) necessary to lift a RE atmosphere to the HSRA temperature stratification.
Results: We find that the velocity amplitude grows in agreement with linear wave theory and thus slower with height than predicted from energy conservation. The mechanical energy of the waves above around z ~ 500 km is insufficient to maintain on a long-term average the chromospheric temperature rise in the semi-empirical HSRA model. The intensity variations of the Ca line core (z ~ 1000 km) can, however, be traced back to the velocity variations of the lowermost forming spectral line considered (z ~ 250 km).
Conclusions: The chromospheric intensity, and hence, (radiation) temperature variations are seen to be induced by passing waves originating in the photosphere. The wave energy is found to be insufficient to maintain the temperature stratification of the semi-empirical HSRA model above 500 km. We will in a following paper of this series investigate the energy contained in the intensity variations to see if the semi-empirical model is appropriate for the spectra. Title: Simulations of Magnetoacoustic Pulsations in Atmospheres of Rapidly Oscillating Ap Stars Authors: Khomenko, E.; Kochukhov, O. Bibcode: 2009ApJ...704.1218K Altcode: 2009arXiv0909.1214K Rapidly oscillating Ap (roAp) stars exhibit an astrophysically interesting combination of strong, dipolar-like magnetic fields and high-overtone p-mode pulsations similar to the Sun. Recent time-resolved spectroscopy of these stars unravelled a complex picture of propagating magnetoacoustic pulsation waves, with amplitude and phase strongly changing as a function of atmospheric height. To interpret these observations and gain a new insight into the atmospheric dynamics of roAp stars we have carried out two-dimensional time-dependent, non-linear magnetohydrodynamical simulations of waves for a realistic atmospheric stratification of a cool Ap star. We explore a grid of simulations in a wide parameter space, treating oscillations of the velocity, magnetic field, and thermodynamic quantities in a self-consistent manner. Our simulations foster a new understanding of the influence of the atmosphere and the magnetic field on the propagation and reflection properties of magnetoacoustic waves, formation of node surfaces, and relative variation of different quantities. Our simulations reproduce all main features of the observed pulsational behavior of roAp stars. We show, for the first time, that the overall depth dependence of the pulsations in roAp atmospheres is strongly influenced by the density inversion at the photospheric base. Title: Observational Signatures of Numerically Simulated MHD Waves in Small-scale Flux Sheets Authors: Khomenko, E.; Collados, M.; Felipe, T. Bibcode: 2009ASPC..405..183K Altcode: 2008arXiv0801.3966K We present some results obtained from the synthesis of Stokes profiles in small-scale flux sheets with propagating MHD waves. To that aim, 2D flux sheets showing internal structure have been excited with 5 min period drivers, allowing non-linear waves to propagate inside the magnetic structure. The observational signatures of these waves in Stokes profiles of several spectral lines that are commonly used in spectropolarimetric measurements are discussed. Title: Simulations of magneto-hydrodynamic waves in atmospheres of roAp stars Authors: Khomenko, Elena; Kochukhov, Oleg Bibcode: 2009IAUS..259..409K Altcode: 2009arXiv0901.1204K We report 2D time-dependent non-linear magneto-hydrodynamical simulations of waves in the atmospheres of roAp stars. We explore a grid of simulations in a wide parameter space. The aim of our study is to understand the influence of the atmosphere and the magnetic field on the propagation and reflection properties of magneto-acoustic waves, formation of shocks and node layers. Title: Theoretical Modeling of Propagation of Magnetoacoustic Waves in Magnetic Regions Below Sunspots Authors: Khomenko, E.; Kosovichev, A.; Collados, M.; Parchevsky, K.; Olshevsky, V. Bibcode: 2009ApJ...694..411K Altcode: 2008arXiv0809.0278K We use two-dimensional numerical simulations and eikonal approximation to study properties of magnetohydrodynamic (MHD) waves traveling below the solar surface through the magnetic structure of sunspots. We consider a series of magnetostatic models of sunspots of different magnetic field strengths, from 10 Mm below the photosphere to the low chromosphere. The purpose of these studies is to quantify the effect of the magnetic field on local helioseismology measurements by modeling waves excited by subphotospheric sources. Time-distance propagation diagrams and wave travel times are calculated for models of various field strengths and compared to the nonmagnetic case. The results clearly indicate that the observed time-distance helioseismology signals in sunspot regions correspond to fast MHD waves. The slow MHD waves form a distinctly different pattern in the time-distance diagram, which has not been detected in observations. The numerical results are in good agreement with the solution in the short-wavelength (eikonal) approximation, providing its validation. The frequency dependence of the travel times is in good qualitative agreement with observations. Title: Magnetohydrostatic Sunspot Models from Deep Subphotospheric to Chromospheric Layers Authors: Khomenko, E.; Collados, M. Bibcode: 2008ApJ...689.1379K Altcode: 2008arXiv0808.3571K In order to understand the influence of magnetic fields on the propagation properties of waves, as derived from different local helioseismology techniques, forward modeling of waves is required. Such calculations need a model in magnetohydrostatic equilibrium as an initial atmosphere through which to propagate oscillations. We provide a method to construct such a model in equilibrium for a wide range of parameters, for use in simulations of artificial helioseismologic data. The method combines the advantages of self-similar solutions and current-distributed models. A set of models is developed by numerical integration of magnetohydrostatic equations from the subphotospheric to chromospheric layers. Title: On the Possible Sources of Chromospheric Heating Authors: Beck, C.; Collados, M. Vera; Khomenko, E.; Rezaei, R. Bibcode: 2008ESPM...12.2.14B Altcode: The chromospheric temperature rise to values above the photospheric temperature cannot be due to radiative energy transport alone. We will outline different possibilities for the additional energy transport in the solar atmosphere by processes that require (or exclude) the presence of magnetic fields. We will discuss which of them could be identified and studied in detail using current data. To find the signature of the different heating processes and derive quantitative estimates of their efficiency, we analyzed simultaneous spectropolarimetric observations of photospheric magnetic fields (@630 nm) and intensity spectra of the chromospheric Ca II H line (396 nm). The mechanical energy flux at several height layers was derived from the velocity amplitudes of propagating acoustic waves seen in different spectral lines. The enhancement of chromospheric (radiation) temperature above the radiative equilibrium values was taken from an inversion of the Ca II H spectra with the SIR code assuming local thermal equilibrium (LTE) and complete redistribution (CRD). We compare the obtained energy values with each other and with the energy requirements demanded by theoretical/semi-empirical atmospheric models.

We find that the most important agent of chromospheric heating are propagating (magneto-)acoustic waves, which suffice to explain the brightenings in Ca II H spectra and their corresponding temperature enhancements. The energy contained in these intensity variations of the Ca II H line, however, is found to be insufficient to maintain a full-time and full-volume "hot" chromosphere. Additional energy transport mechanisms without a signature in the Ca II H spectra are thus necessary. Finally, we will outline which improvements are to be expected with future observations of higher quality (spatial resolution, enhanced polarimetric sensitivity, temporal cadence, other spectral lines) to be achieved with new ground-based telescopes like GREGOR or EST. Title: Multi-layer Study of Wave Propagation in Sunspots Authors: Felipe, T.; Khomenko, E.; Collados, M.; Beck, C. Bibcode: 2008ESPM...12.2.12F Altcode: Observations in different spectral lines give us information about the different layers of the solar atmosphere. Here we analyze the propagation of waves in sunspots from the photosphere to the chromosphere using time series of cospatial Ca II H intensity spectra and polarimetric spectra of Si I 10827 A and He I 10830 A multiplet. From the Doppler shifts of these lines we retrieve the temporal variations of the velocity along the line-of-sight at several heights. Phase spectra are used to get the relation between oscillatory signals measured at each spectral signature. Our analysis reveals standing waves for frequencies lower than 3.5 mHz and propagating waves for higher frequencies, which steepen into shocks in the chromosphere. Oscillations are detectable in Ca II H wings and they are propagated along line wing layers to the line core. Ca II H core forms at a lower height than the He I 10830 A line. A time delay of about 30 s is measured between the Doppler signals detected at both wavelengths. We also find that in "cold" sunspots the Si I 10827 A forms deeper than in the quiet sun. This type of measurements demonstrate the importance of simultaneous co-spatial observations at different wavelengths. Future infrastructures, such as GREGOR and EST, should include multi-wavelength capabilities to make possible the study of the photosphere-chromosphere connection with the highest spatial and temporal resolution. Title: Nonlinear Numerical Simulations of Magneto-Acoustic Wave Propagation in Small-Scale Flux Tubes Authors: Khomenko, E.; Collados, M.; Felipe, T. Bibcode: 2008SoPh..251..589K Altcode: 2008SoPh..tmp...32K; 2007arXiv0710.3335K We present results of nonlinear, two-dimensional, numerical simulations of magneto-acoustic wave propagation in the photosphere and chromosphere of small-scale flux tubes with internal structure. Waves with realistic periods of three to five minutes are studied, after horizontal and vertical oscillatory perturbations are applied to the equilibrium model. Spurious reflections of shock waves from the upper boundary are minimized by a special boundary condition. This has allowed us to increase the duration of the simulations and to make it long enough to perform a statistical analysis of oscillations. The simulations show that deep horizontal motions of the flux tube generate a slow (magnetic) mode and a surface mode. These modes are efficiently transformed into a slow (acoustic) mode in the vA<cS atmosphere. The slow (acoustic) mode propagates vertically along the field lines, forms shocks, and remains always within the flux tube. It might effectively deposit the energy of the driver into the chromosphere. When the driver oscillates with a high frequency, above the cutoff, nonlinear wave propagation occurs with the same dominant driver period at all heights. At low frequencies, below the cutoff, the dominant period of oscillations changes with height from that of the driver in the photosphere to its first harmonic (half period) in the chromosphere. Depending on the period and on the type of the driver, different shock patterns are observed. Title: Seismology of Sunspots: An Interplay between Temperature and Magnetic Field Structures Authors: Olshevsky, V.; Khomenko, E.; Collados, M. Bibcode: 2008ESPM...12..3.2O Altcode: Using a numerical three-dimensional MHD modelling of magneto-acoustic wave propagation in a realistic magnetostatic sunspot model we investigate the influence of the magnetic field on the parameters measured by local helioseismology. We find that the variations of temperature as well as the presence of the magnetic field cause important changes to the wave travel times. Magnetic field speeds up the waves to considerable amount, while the temperature depression within a sunspot causes the opposite action. The calculated travel time differences between the unmagnetized and magnetized atmospheres lie in the range typically obtained from local helioseismology correlation analysis. Our numerical results are also in agreement with the analytical calculations of the travel times applying WKB technique. Title: Channeling 5 Minute Photospheric Oscillations into the Solar Outer Atmosphere through Small-Scale Vertical Magnetic Flux Tubes Authors: Khomenko, E.; Centeno, R.; Collados, M.; Trujillo Bueno, J. Bibcode: 2008ApJ...676L..85K Altcode: 2008arXiv0802.0938K We report two-dimensional MHD simulations which demonstrate that photospheric 5 minute oscillations can leak into the chromosphere inside small-scale vertical magnetic flux tubes. The results of our numerical experiments are compatible with those inferred from simultaneous spectropolarimetric observations of the photosphere and chromosphere obtained with the Tenerife Infrared Polarimeter (TIP) at 10830 Å. We conclude that the efficiency of energy exchange by radiation in the solar photosphere can lead to a significant reduction of the cutoff frequency and may allow for the propagation of the 5 minute waves vertically into the chromosphere. Title: Multiline Spectropolarimetry of the Quiet Sun at 5250 and 6302 Å Authors: Socas-Navarro, H.; Borrero, J. M.; Asensio Ramos, A.; Collados, M.; Domínguez Cerdeña, I.; Khomenko, E. V.; Martínez González, M. J.; Martínez Pillet, V.; Ruiz Cobo, B.; Sánchez Almeida, J. Bibcode: 2008ApJ...674..596S Altcode: The reliability of quiet-Sun magnetic field diagnostics based on the Fe I lines at 6302 Å has been questioned by recent work. Here we present the results of a thorough study of high-resolution multiline observations taken with the new spectropolarimeter SPINOR, comprising the 5250 and 6302 Å spectral domains. The observations were analyzed using several inversion algorithms, including Milne-Eddington, LTE with 1 and 2 components, and MISMA codes. We find that the line-ratio technique applied to the 5250 Å lines is not sufficiently reliable to provide a direct magnetic diagnostic in the presence of thermal fluctuations and variable line broadening. In general, one needs to resort to inversion algorithms, ideally with realistic magnetohydrodynamic constrains. When this is done, the 5250 Å lines do not seem to provide any significant advantage over those at 6302 Å. In fact, our results point toward a better performance with the latter (in the presence of turbulent line broadening). In any case, for very weak flux concentrations, neither spectral region alone provides sufficient constraints to fully disentangle the intrinsic field strengths. Instead, we advocate for a combined analysis of both spectral ranges, which yields a better determination of the quiet-Sun magnetic properties. Finally, we propose the use of two other Fe I lines (at 4122 and 9000 Å) with identical line opacities that seem to work much better than the others. Title: Observations of a bright plume in solar granulation Authors: Kostik, R. I.; Khomenko, E. V. Bibcode: 2007A&A...476..341K Altcode: Aims:The aim of this paper is to analyze the thermal properties, oscillatory, and flow motions of a bright, long-lasting feature observed in solar granulation, which we call the plume.
Methods: We used the spectral observations of quiet granulation at solar disc center, including the two Fe II 5234 and Fe I 6393 Å lines recorded simultaneously at the German Vacuum Tower telescope in Tenerife. The recorded data revealed a stable, bright structure of 3-4 arcsec size present during the whole 2.5 h of observations. We compare the velocity fields extracted by means of a λ-meter method and temperature and pressure stratification obtained after inversion of the profiles related to granules, intergranular lanes, and the plume.
Results: The following results were obtained: (i) the correlation between variations in convective velocity and intensity in the plume is close to zero at all observed heights; (ii) the velocity flow in the plume changes from a downflow in the deep layers to an upflow in the upper layers; (iii) the brightness of the plume increases with height; (iv) the amplitudes of the five-minute oscillations of intensity and velocity are twice lower in the plume than outside, and vertically propagating waves are observed; (v) the plume is hotter and denser than the quiet Sun in the upper photosphere.
Conclusions: We conclude that the observed phenomenon has a non-convective origin. The decrease in the amplitudes of oscillations in the plume cannot be attributed to the higher density in comparison to the surrounding atmosphere. Along with other findings, this indicates the possible presence of magnetic field. Title: Multi-Line Quiet Sun Spectro-Polarimetry at 5250 and 6302 Å Authors: Socas-Navarro, H.; Borrero, J.; Asensio Ramos, A.; Collados, M.; Domínguez Cerdeña, I.; Khomenko, E. V.; Martínez González, M. J.; Martínez Pillet, V.; Ruiz Cobo, B.; Sánchez Almeida, J. Bibcode: 2007arXiv0710.1099S Altcode: The reliability of quiet Sun magnetic field diagnostics based on the \ion{Fe}{1} lines at 6302 Åhas been questioned by recent work. We present here the results of a thorough study of high-resolution multi-line observations taken with the new spectro-polarimeter SPINOR, comprising the 5250 and 6302 Åspectral domains. The observations were analyzed using several inversion algorithms, including Milne-Eddington, LTE with 1 and 2 components, and MISMA codes. We find that the line-ratio technique applied to the 5250 Ålines is not sufficiently reliable to provide a direct magnetic diagnostic in the presence of thermal fluctuations and variable line broadening. In general, one needs to resort to inversion algorithms, ideally with realistic magneto-hydrodynamical constrains. When this is done, the 5250 Ålines do not seem to provide any significant advantage over those at 6302 Å. In fact, our results point towards a better performance with the latter (in the presence of turbulent line broadening). In any case, for very weak flux concentrations, neither spectral region alone provides sufficient constraints to fully disentangle the intrinsic field strengths. Instead, we advocate for a combined analysis of both spectral ranges, which yields a better determination of the quiet Sun magnetic properties. Finally, we propose the use of two other \ion{Fe}{1} lines (at 4122 and 9000 Å) with identical line opacities that seem to work much better than the others. Title: On the Stokes V Amplitude Ratio as an Indicator of the Field Strength in the Solar Internetwork Authors: Khomenko, E.; Collados, M. Bibcode: 2007ApJ...659.1726K Altcode: The results of the determination of magnetic field strength from weak polarimetric signals in solar internetwork regions are contradictory. We investigate the origin of this contradiction with the help of MHD simulations. It is shown that the Stokes V amplitude ratio of the Fe I λλ15652-15648 lines is a good indicator of kG magnetic field concentrations, even for magnetic fields with a complex internal structure like those in MHD simulations. The Stokes V amplitude ratio of the Fe I λλ5247-5250 lines also shows a good correlation with magnetic field strength. However, in simulations with a flux level appropriate for the internetwork, it gives values corresponding to sub-kG fields. The reason is the rapid decrease of the field strength with height in kG magnetic field concentrations. These lines sample high regions of the atmosphere, where the field is already below kG levels. We also find that the Stokes V amplitude ratio of the Fe I λλ6301-6302 lines shows no correlation with the magnetic field strength. The reason lies in the large difference in the heights of formation of these two lines. The value of the magnetic field strength obtained from the Fe I λλ6301 and 6302 lines depends crucially on the treatment of gradients of the magnetic field, line-of-sight velocity, and temperature, even at a numerical spatial resolution of 20 km. Title: Line ratio method applied to inter-network magnetic fields Authors: Khomenko, E.; Collados, M. Bibcode: 2007msfa.conf..303K Altcode: We investigate the validity of the Stokes V amplitude ratio as an indicator of the magnetic field strength in solar inter-network regions with the help ofMHD simulations. We show that the Stokes V amplitude ratio of the Fe I 15652-15648 Å lines and Fe I 5247-5250 Å lines show a good correlation with the magnetic field strength even for magnetic fields with a complex internal structure like those in MHD simulations. However, in the latter case, the amplitude ratio sub-estimates the magnetic field strength, always revealing sub-kG values. The Stokes V amplitude ratio of the Fe I 6301-6302 Å lines shows no correlation with the magnetic field strength. The reasons of this behaviour are explained. Title: Numerical modeling of MHD wave propagation in sunspots: a 3D case Authors: Olshevsky, V.; Khomenko, E.; Collados, M. Bibcode: 2007msfa.conf..347O Altcode: We present the first results of a 3D numerical modeling of linear MHD wave propagation in a realistic sunspot model. In our simulations, a piston located at the base of the photosphere generates waves with a certain period. The ratio between the acoustic and the Alfven speed, cS /vA, decreases from much larger than one at the photosphere to much lower than one in the chromosphere in our simulation domain. Waves propagate through the region where cS << vA, where mode transformation is observed. At a somewhat higher region, where cS = vA, the fast (magnetic) mode reflects back to the photosphere due to the vertical and horizontal gradients of vA. The slow (acoustic) mode propagates to the upper layers and increases its velocity amplitude. Unlike the 2D simulations, the Alfven mode is also generated by the piston and experiences transformations at the cS = vA layer. The behaviour of this mode requires further study. Title: Magnetic field inversions from Stokes profiles generated by MHD simulations Authors: Khomenko, E.; Collados, M. Bibcode: 2007MmSAI..78..166K Altcode: We report tests of inversion methods applied to complex Stokes spectra generated by realistic MHD simulations. The average magnetic field strength of the simulations used is of 30 and 140 G, which we believe is representative of quiet solar regions. The behaviour of the Fe I at 1.56 mu m and 630 nm lines is analyzed. The tests have been done with the original resolution of simulations (20 km) and also with resolution of 0.6'' and 1.4'' (after having conveniently degraded the images). Title: On the Determination of Magnetic Field Strength and Flux in Inter-Network Authors: Khomenko, E.; Collados, M., Bibcode: 2006ASPC..358...42K Altcode: The results of the determination of magnetic field strength and flux from weak polarimetric signals in solar inter-network regions are contradictory. We investigate the origin of this contradiction with the help of MHD simulations. It is shown that the Stokes-V line ratio of the Fe I 5247/5250 Å and 15652/15648 Å line pairs is a good indicator of kG magnetic field concentrations, even for magnetic fields with a complex internal structure like those in MHD simulations. On the contrary, the Stokes-V line ratio of the Fe I 6301/6302 Å lines shows no correlation with magnetic field strength. The reason lies in the large difference in the heights of formation of these two lines. The value of the magnetic field strength obtained from the inversion of the Fe I 6301 Å and 6302 Å lines depends crucially on the treatment of gradients of magnetic field, LOS velocity, and temperature even at numerical spatial resolution of 20 km. Title: Numerical Modeling of Magnetohydrodynamic Wave Propagation and Refraction in Sunspots Authors: Khomenko, E.; Collados, M. Bibcode: 2006ApJ...653..739K Altcode: We present numerical simulations of magnetoacoustic wave propagation from the photosphere to the low chromosphere in a magnetic sunspot-like structure. A thick flux tube, with dimensions typical of a small sunspot, is perturbed by a vertical or horizontal velocity pulse at the photospheric level. The type of mode generated by the pulse depends on the ratio between the sound speed cS and the Alfvén speed vA, on the magnetic field inclination at the location of the driver, and on the shape of the pulse in the horizontal direction. Mode conversion is observed to occur in the region in which both characteristic speeds have similar values. The fast (magnetic) mode in the region cS<vA does not reach the chromosphere and reflects back to the photosphere at a somewhat higher layer than the cS=vA line. This behavior is due to wave refraction, caused primarily by the vertical and horizontal gradients of the Alfvén speed. The slow (acoustic) mode continues up to the chromosphere along the magnetic field lines with increasing amplitude. We show that this behavior is characteristic for waves in a wide range of periods generated at different distances from the sunspot axis. Since an important part of the energy of the pulse is returned back to the photosphere by the fast mode, the mechanism of energy transport from the photosphere to the chromosphere by waves in sunspots is rather ineffective. Title: Diagnostics of Quiet-Sun Magnetism Authors: Khomenko, E. Bibcode: 2006ASPC..354...63K Altcode: Most of our knowledge of solar surface magnetism comes from the analysis of polarization spectra. The Stokes spectra contain detailed information on the structure and dynamics of the magnetized photospheric plasma and its interaction with convection, i.e., magnetoconvection. The interpretation of high-resolution observations requires sophisticated techniques such as radiative transfer of polarized light in 3D model atmospheres. On the other hand, 3D magnetoconvection simulations include elaborate physics and are becoming sufficiently realistic to make predictions about the complex processes that take place in the Sun's magnetized atmosphere. This paper concentrates on the diagnostics of the magnetic fields in quiet solar photospheric regions outside sunspots and active regions. Until recently the influence of the magnetic field on the dynamics of these regions was considered unimportant. However, it turns out that a considerable amount of magnetic energy is probably stored in the ``quiet'' Sun. The issue of quiet solar magnetism remains open and is much debated in the literature. Title: Fine structure of wave motions in the solar photosphere: Observations and theory Authors: Kostyk, R. I.; Shchukina, N. G.; Khomenko, E. V. Bibcode: 2006ARep...50..588K Altcode: Spectral observations of the 639.361-nm FeI line at the center of the quiet solar disk with high spatial (0.4″) and temporal (10 s) resolution are used to investigate the behavior of local 5-min oscillations over granules and intergranular lanes. The power of the 5-min oscillations in the upper photosphere (at heights of H ≈ 490 km) is higher the faster the convective motions in the lower photosphere (H ≈ 10 km). This suggests that turbulent convection is responsible for the excitation of local solar oscillations. A statistical analysis of the oscillations shows that, on average, both the intensity and velocity of the oscillation amplitudes are greater over intergranular lanes. This difference in amplitudes is present throughout the studied heights in the photosphere (H = 0-490 km). The period at which the power spectrum of velocity oscillations reaches its maximum is longer over intergranules than over granules. Simulations of the propagation of acoustic-gravity waves in an atmosphere taking into account the convection pattern give a satisfactory explanation for the above observed effects. It is concluded that the atmospheric modulation of the 5-min oscillations is an additional or alternative mechanism responsible for differences between these oscillations over granules and intergranules. Title: Simulations of - Acoustic Waves in Sunspots Authors: Khomenko, E. V.; Collados, M. Bibcode: 2005ESASP.596E..40K Altcode: 2005ccmf.confE..40K No abstract at ADS 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: Magnetic flux in the internetwork quiet Sun Authors: Khomenko, E. V.; Martínez González, M. J.; Collados, M.; Vögler, A.; Solanki, S. K.; Ruiz Cobo, B.; Beck, C. Bibcode: 2005A&A...436L..27K Altcode: We report a direct comparison of the amplitudes of Stokes spectra of the Fe i 630 nm and 1.56 μm lines produced by realistic MHD simulations with simultaneous observations in the same spectral regions. The Stokes spectra were synthesized in snapshots with a mixed polarity magnetic field having a spatially averaged strength, < B >, between 10 and 30 G. The distribution of Stokes V amplitudes depends sensitively on < B >. A quiet inter-network region was observed at the German VTT simultaneously with TIP (1.56 μm) and POLIS (630 nm). We find that the Stokes V amplitudes of both infrared and visible observations are best reproduced by the simulation snapshot with < B > = 20 G. In observations with 1 resolution, up to 2/3 of the magnetic flux can remain undetected. Title: Bright features in the solar photosphere Authors: Kostik, R. I.; Khomenko, E. V. Bibcode: 2005KFNTS...5..141K Altcode: We report thermodynamical properties of so-called ``thermal plume'' observed near the solar disc centre in 2001. The spectral observations of two iron lines analysed were obtained with the use of the Vacuum Tower Telescope (Tenerife). Title: Helioseismology space and ground-based studies Authors: Kostik, R. I.; Osipov, S. N.; Khomenko, E. V.; Lebedev, N. I. Bibcode: 2005KFNTS...5..138K Altcode: We give a preliminary report on the observations of solar irradiance fluctuations with the DIFOS photometer aboard the Russian-Ukrainian satellite CORONAS-F launched in 2001. In addition, the parallel ground-based spectral observations (VTT, Tenerife) carried out with 20-day observing space campaign are described. Title: Convective and wave motions in a thermal plume Authors: Kostik, Roman I.; Khomenko, Elena V. Bibcode: 2004IAUS..223..271K Altcode: 2005IAUS..223..271K Here we report thermodynamical properties of a a so-called "thermal plume" observed near the solar disc center in 2001. The spectral observations of two iron lines analysed have been obtained using the Vacuum Tower Telescope (Tenerife). 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: Helioseismology space and ground based studies Authors: Kostik, R. I.; Osipov, S. N.; Khomenko, E. V.; Lebedev, N. I. Bibcode: 2004IAUS..223..273K Altcode: 2005IAUS..223..273K This is a preliminary report on the observations of solar irradiance fluctuations with the DIFOS photometer aboard the Russian-Ukrainian satellite CORONAS-F launched in 2001. In addition the parallel ground-based spectral observations (VTT, Tenerife) carried out with the first 20-days observing space campaign are described. 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: Quiet-Sun inter-network magnetic fields observed in the infrared Authors: Khomenko, E. V.; Collados, M.; Solanki, S. K.; Lagg, A.; Trujillo Bueno, J. Bibcode: 2003A&A...408.1115K Altcode: This paper presents the results of an investigation of the quiet Sun's magnetic field based on high-resolution infrared spectropolarimetric observations obtained with the Tenerife Infrared Polarimeter (TIP) at the German VTT of the Observatorio del Teide. We observed two very quiet regions at disc centre. The seeing was exceptionally good during both observing runs, being excellent during one of them. In both cases the network was intentionally avoided to the extent possible, to focus the analysis on the characteristics of the weak polarization signals of the inter-network regions. We find that the Stokes V profile of Fe I 15648 Å line in almost 50% of the pixels and Stokes Q and/or U in 20% of the pixels have a signal above 10-3 (in units of continuum intensity Ic), which is significantly above the noise level of 2-3 x 10-4. This implies that we detect fluxes as low as 2 x 1015 Mx/px. We find evidence that we have detected most of the net flux that is in principle detectable at 1'' resolution with the Zeeman effect. The observed linear polarization resulting from the transverse Zeeman effect indicates that the magnetic fields have a broad range of inclinations, although most of the pixels show polarization signatures which imply an inclination of about 20o. Nearly 30% of the selected V-profiles have irregular shapes with 3 or more lobes, suggesting mixed polarities with different LOS velocity within the resolution element. The profiles are classified using a single value decomposition approach. The spatial distribution of the magnetic signal shows that profiles of different classes (having different velocities, splitting, asymmetries) are clustered together and form patches, close to the spatial resolution in size. Most of the field is found to be located in intergranular lanes. The statistical properties of the mainly inter-network field sampled by these observations are presented, showing that most of the observed fields are weak with relatively few kG features. The field strength distribution peaks at 350 G and has a FWHM of 300 G. Other parameters, such as profile asymmetries, filling factors and line-of-sight velocities are also determined and discussed.

Based on observations with the German Vacuum Tower Telescope (VTT) operated by the Kiepenheuer-Institut für Sonnenphysik at the Spanish Observatorio del Teide of the Instituto de Astrofísica de Canarias (IAC). Title: Magnetoacoustic Waves in Sunspots Authors: Khomenko, E. V.; Collados, M.; Bellot Rubio, L. R. Bibcode: 2003ApJ...588..606K Altcode: Observed variations of the magnetic field strength in sunspot umbrae consist of intrinsic oscillations and ``false'' oscillations due to time-dependent opacity effects. Here we present an approach intended for the separation of these components. We develop a mathematical formalism based on the analytical solution of the MHD equations including gravity, inclination of the magnetic field, and effects of nonadiabaticity. The theoretical results are compared with observations in the near-infrared at 1.56 μm by Bellot Rubio and coworkers using the Tenerife Infrared Polarimeter. It is shown that part of the detected field strength variations can be intrinsic magnetic field oscillations caused by magnetoacoustic waves. Title: Local oscillations and their modification in inhomogeneous solar atmosphere Authors: Khomenko, E. V. Bibcode: 2002KFNT...18..559K Altcode: Properties of the local 5-minute oscillations caused by their interaction with convective motions are studied. We use a vast statistical material of SOHO/MDI observations. We confirm that oscillations above granules and intergranular lanes are different. Powerful oscillations occur not only above intergranular lanes but above granules as well. We show that the model for oscillations in the inhomogeneous atmosphere which takes into account the velocities of granular motions and reflection of waves allows the observed dependencies to be explained qualitatively without any assumption about different rate of excitation of oscillations in granules and lanes. Title: The Effect of Acoustic Waves on Spectral-Line Profiles in the Solar Atmosphere: Observations and Theory Authors: Kostyk, R. I.; Khomenko, E. V. Bibcode: 2002ARep...46..925K Altcode: The fine structure of the FeI λ 532.4185-nm line of neutral iron is studied with high spatial (0.5″) and temporal (9.3 s) resolution using observations of a quiet region at the center of the solar disk. The character of the line asymmetry depends strongly on the nature of the velocity field, i.e., on whether it is due to convective or wave motions. The magnitude of the asymmetry due to acoustic waves is comparable to that due to convective motions. The propagation of acoustic waves in moving granules and intergranular lanes is studied by solving a system of hydrodynamical equations in a three-dimensional model for the solar atmosphere. The temporal variations in the bisector of the line synthesized in a non-LTE approximation agree well with the observational data. Title: Statistical properties of magnetic fields in intranetwork Authors: Khomenko, E. V.; Collados, M.; Lagg, A.; Solanki, S. K.; Trujillo Bueno, J. Bibcode: 2002ESASP.505..445K Altcode: 2002IAUCo.188..445K; 2002solm.conf..445K We report a study of the quiet sun's magnetic field based on high-resolution infrared spectropolarimetric observations (TIP/VTT). We find that in almost 50% of the pixels Stokes V and in 15% the Stokes Q and/or U profiles have a signal above 10-3. The statistical properties of the mainly intranetwork field sampled by these observations are presented, showing that most of the observed fields are weak (the field strength distribution peaks at 350 G and has a FWHM of 300 G) with very few kG features. The magnetized regions occupy a very small fill fractions (about 2%). The field changes properties on granular spatial scales and the size of the patches formed by similar profiles is close to 1". Most of the parameters of the observed polarization profiles show correlations with granulation parameters. Title: Observation of Convective Collapse and Upward-moving Shocks in the Quiet Sun Authors: Bellot Rubio, Luis R.; Rodríguez Hidalgo, Inés; Collados, Manuel; Khomenko, Elena; Ruiz Cobo, Basilio Bibcode: 2001ApJ...560.1010B Altcode: We present spectropolarimetric evidence of convective collapse and destruction of magnetic flux by upward-moving fronts in the quiet Sun. The observational material consists of time series of the full Stokes vector of two infrared spectral lines emerging from regions associated with Ca II K network points. The amplitude of the circular polarization profiles of a particular spatial point is seen to increase while the profiles are redshifted. It then decreases during a much shorter phase characterized by large blueshifts. Inspection of the data indicates that the blueshift occurs because of the sudden appearance of a new, strongly displaced Stokes V profile of the same polarity. The amplification of the magnetic signal takes place in a time interval of about 13 minutes, while blueshifts and the concomitant decreasing Stokes V amplitudes last for only 2 minutes. An inversion code based on the thin flux-tube scenario has been applied to the data in order to derive the thermal, magnetic, and dynamic structures of the atmosphere. According to our results, the field strength undergoes a moderate increase from 400 to 600 G at z=0 km during the phase in which redshifts are present. The observed redshifts are produced by internal downflows of up to 6 km s-1 at z=0 km. After ~13 minutes, the material falling down inside the tube appears to bounce off in the deeper layers, originating an upward-propagating front whose manifestation on the Stokes V profiles is a large blueshift. The front moves with a speed of 2.3 km s-1 and has a downflow-to-upflow velocity difference of about 7 km s-1 initially and some 4 km s-1 after 2 minutes. It strongly weakens the magnetic field strength and may be responsible for the complete destruction of the magnetic feature. The observed behavior is in general agreement with theoretical predictions of flux expulsion, convective collapse, and development of shocks within magnetic flux tubes. Title: Phases of the 5-min Photospheric Oscillations above Granules and Intergranular Lines Authors: Khomenko, E. V. Bibcode: 2001ASSL..259..275K Altcode: 2001dysu.conf..275K In this work we re-examine the links between the 5-minute oscillations and granulation using observations of the Fe I 5324 Å line obtained with high spatial and temporal resolution. In contradiction to the previous studies we show that oscillations above the brightest granules as well as above the darkest intergranular lanes occur with the smaller amount of radiative energy losses. This causes the amplitude amplification with the contrast of granulation. Title: Five-minute oscillations above granules and intergranular lanes Authors: Khomenko, E. V.; Kostik, R. I.; Shchukina, N. G. Bibcode: 2001A&A...369..660K Altcode: We discuss the links between the photospheric 5-min oscillations and the granulation pattern using a 30-min time series of CCD spectrograms of solar granulation recorded with high spatial (0{''}5) and temporal (9.3 s) resolution. The observed images contain the Fe \sc{i 5324 Å spectral line with good height coverage from the low photosphere up to the temperature minimum region. Amplitudes, phases and periods of the 5-min oscillations are found to be different above granules and intergranular lanes. Strong oscillations occur well separated temporally and spatially. Many features of this different behaviour can be described in the frame of a relatively simple model of wave propagation in the solar atmosphere. To that aim, we have introduced oscillations into a 3D snapshot of a theoretical time dependent solar model atmosphere. NLTE synthesis of the time series of the Fe \sc{i 5324 Å line profiles was performed taking into account granular and oscillatory components of the velocity field. Both, observations and theoretical modeling, lead to similar results: (i) oscillations above granules and intergranular lanes occur with different periods; (ii) the most energetic intensity oscillations occur above intergranular lanes; the most energetic velocity oscillations occur above granules and lanes with maximum contrast, {i.e.} above the regions with maximum convective velocities; (iii) velocity oscillations at the lower layers of the atmosphere lead oscillations at the upper layers in intergranular lanes. In granules the phase shift is nearly zero. We conclude that differences in oscillations above granules and lanes are caused mainly by variations of the physical conditions in these structures. Title: Comparision of Observed and Theoretical Amplitudes of Oscillations above granules and intergranular lanes Authors: Khomenko, E. V. Bibcode: 2001IAUS..203..192K Altcode: Differences in the amplitudes, phases and periods of the five-minute oscillations above granules and intergranular lanes are found to be well-described in a frame of a relatively simple model. We utilized a 3D snapshot of the theoretically computed time depended solar model atmosphere. We considered a vertical monochromatic wave propagation in a moving, isothermal medium. NLTE synthesis of a time series of the FeI 5324 Å line profiles in the model atmospheres from a horizontal cut of the snapshot was performed taking into account granular and oscillatory components of the velocity field. Observations of the FeI 5324 Å line in a quite solar disk center and our theoretical modeling lead to the similar results: Periods of oscillations in intergranular lanes are lower than in granules; Amplitudes of the velocity oscillations grow with the contrast of granulation. Amplitudes of the intensity oscillations are larger in intergranular lanes than in granules; Velocity oscillations at the lower levels of the atmosphere lead oscillations at the outer layers in intergranular lanes. In granules this phase shift is nearly zero. The fact that our simple model describes the basics features of the oscillations above granules and intergranular lanes made us come to the conclusion that differences in those oscillations are caused mainly by variations of the physical conditions in these structures. Title: Simulation of the wave propagation in the 3D solar atmosphere Authors: Khomenko, E. V. Bibcode: 2001ESASP.464..589K Altcode: 2001soho...10..589K A theory of wave propagation in the solar atmosphere has been applied to study peculiarities in the wave behaviour above granules and intergranular lanes. We obtained analytical solution of the hydrodynamical equations considering a vertical monochromatic wave propagation in a moving isothermal medium. Effects of the wave reflection were taken into account. We introduced oscillations into a 3D snapshot of the theoretical time dependent solar model atmosphere. NLTE synthesis of the two solar lines was performed: the relatively strong Fe I 5324 Å which is formed over all the photosphere and a weaker Ni I 6768 Å line. The latter is widely used in helioseismology. Both granular and oscillatory components of the velocity field were taken into account. We compared our modeling with the high spatial resolution observations obtained from the ground using VTT on Tenerife and from space using SOHO/MDI. In general, amplitudes and phases of oscillations above granules and intergranular lanes are reproduced by the applied model. We conclude that many differences in oscillations above granules and lanes can be induced by variations of the physical conditions in these structures. Title: Simulation of Temporal Variations of the Solar Line Fe I 532. 4185 nm by the 5-min Oscillations (CD-ROM Directory: contribs/khomenko) Authors: Khomenko, E. V.; Shchukina, N. G. Bibcode: 2001ASPC..223..680K Altcode: 2001csss...11..680K No abstract at ADS Title: Wave propagation in the solar atmosphere Authors: Khomenko, E. V. Bibcode: 2000KFNTS...3..456K Altcode: The theory of a wave propagation in the solar atmosphere has been applied to study peculiarities in the wave behaviour above granules and intergranular lanes. We introduced oscillations into a 3D snapshot of the theoretical time dependent solar model atmosphere. NLTE synthesis of the two solar lines was performed: the relatively strong Fe I 5324 Å which is formed over all the photosphere and a weaker Ni I 6768 Å line. The latter is widely used in helioseismology. We compared our modeling with the high spatial resolution observations obtained from the ground using VTT on Tenerife and from space using SOHO/MDI. We conclude that many differences in oscillations above granules and lanes can be induced by variations of the physical conditions in these structures. Title: Granulation and five-minute oscillations Authors: Khomenko, E. V.; Kostik, R. I.; Shchukina, N. G. Bibcode: 2000KFNTS...3..431K Altcode: We discuss the links between the photospheric 5-min oscillations and granulation patterns using a 30-min time series of CCD spectral images of solar granulation recorded with high spatial (0.5'') and temporal (9.3 s) resolution. The observed images contain Fe I 5324 Å line. Our observations and theoretical modeling lead to the similar results: (i) period of oscillations varies above granules and lanes; (ii) amplitudes of the velocity oscillations grow with the contrast of granulation. Amplitudes of the intensity oscillations are larger in intergranular lanes than in granules. Our simple model describes the basic features of the oscillations above granules and intergranular lanes. We conclude that differences in these oscillations are caused mainly by variations of the physical conditions in these structures. Title: Interaction of Granulation with the 5-min Photospheric Oscillations Authors: Kostik, R. I.; Shchukina, N. G.; Khomenko, E. V. Bibcode: 1999ESASP.448..319K Altcode: 1999ESPM....9..319K; 1999mfsp.conf..319K No abstract at ADS Title: Formation and Destruction of a Weak Magnetic Feature in the Solar Photosphere Authors: Khomenko, E.; Collados, M.; Bellot Rubio, L. R.; Rodríguez Hidalgo, I.; Ruiz Cobo, B. Bibcode: 1999ESASP.448..307K Altcode: 1999mfsp.conf..307K; 1999ESPM....9..307K No abstract at ADS Title: Phase characteristics of the local five-minute oscillations. Authors: Khomenko, E. V. Bibcode: 1999KFNT...15..145K Altcode: 1999KNFT...15..145K Local five-minute oscillations were studied on the basis of observations with very high spatial and temporal resolution. Phase shifts between oscillations in the Doppler velocity, central residual intensity, and equivalent width of the line Fe I λ 532.4185 nm were analyzed. Phase shifts between velocity and temperature oscillations above different granulation structures in the middle and upper photosphere were found. The phase shift magnitude is shown to change depending on granulation brightness at the height of continuum formation. Oscillations above the brightest granules and the darkest intergranular lanes occur with smaller radiative energy losses as compared to the oscillations above the granular structures of lower contrast. Relaxation time of temperature inhomogeneities above granules and intergranular lanes was estimated with the phase shift obtained. Title: Phase characteristics of local five-minute oscillations of the Sun. Authors: Khomenko, E. V. Bibcode: 1999KPCB...15..109K Altcode: Local five-minute oscillations were studied using observations with high spatial and temporal resolution. Phase shifts between oscillations in the radial velocity, central residual intensity, and equivalent width of the Fe I λ532.4185 nm line were analyzed. Phase shifts between the velocity and temperature oscillations were found to exist in the middle and upper photosphere over various granulation structures. Their magnitude depends on granulation brightness at the height of the continuum formation. The oscillations over the brightest granules and the darkest intergranular lanes occur with smaller radiative energy losses as compared to the oscillations over the granular structures of lower contrast. Estimates were made for the relaxation time of temperature inhomogeneities over granules and dark lanes of different contrast.