explanation      blue bibcodes open ADS page with paths to full text
Author name code: khomenko
ADS astronomy entries on 2022-09-14
author:"Khomenko, Elena V." 

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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.
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.

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Title: Coupling between different atmospheric layers: waves and
    energy transfer
Authors: Khomenko, Elena
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.

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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
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.

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Title: Modeling the thermal conduction in the solar atmosphere with
    the code MANCHA3D
Authors: Navarro, A.; Khomenko, E.; Modestov, M.; Vitas, N.
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. <BR /> 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. <BR /> 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. <BR /> 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.

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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
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.

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Title: Doppler-velocity Drifts Detected in a Solar Prominence
Authors: Zapiór, Maciej; Heinzel, Petr; Khomenko, Elena
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.

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Title: Numerical simulations of large-amplitude oscillations in
    solar prominences triggered by an eruptive event
Authors: Liakh, Valeriia; Khomenko, Elena; Luna, Manuel
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.

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Title: Transverse kink oscillations of inhomogeneous prominence
    threads
Authors: Martínez-Gómez, David; Terradas, Jaume; Soler, Roberto;
   Khomenko, Elena
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.

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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.
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.

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Title: Transverse kink oscillations of inhomogeneous prominence
threads: Numerical analysis and Hα forward modelling
Authors: Martínez-Gómez, David; Soler, Roberto; Terradas, Jaume;
   Khomenko, Elena
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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <P />Movies associated to Figs. 11-14 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202141968/olm">https://www.aanda.org</A>

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Title: Ambipolar Diffusion in the Lower Solar Atmosphere: MHD
    Simulations of a Sunspot
Authors: MacBride, Conor; Jess, David; Khomenko, Elena
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.

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Title: Magnetic field amplification and structure formation by the
    Rayleigh-Taylor instability
Authors: Popescu Braileanu, B.; Lukin, V. S.; Khomenko, E.
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.

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Title: Large-amplitude longitudinal oscillations in solar prominences
    simulated with different resolutions
Authors: Liakh, V.; Luna, M.; Khomenko, E.
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. <BR
  /> Aims: In this study, we investigate the attenuation and
  amplification of LALOs using high-resolution numerical simulations
  with progressively increasing spatial resolutions. <BR /> 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. <BR /> 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. <BR />
  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.

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Title: Simulations of the Biermann battery mechanism in two-fluid
    partially ionised plasmas
Authors: Martínez-Gómez, D.; Popescu Braileanu, B.; Khomenko, E.;
   Hunana, P.
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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <P />Movies associated to Figs. 2 and A.2 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202039113/olm">https://www.aanda.org</A>

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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.
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.

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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.
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.

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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
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
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
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
  χ<SUP>2</SUP>-values equal to 1.03 between the observed and synthesized
  umbral line profiles. <P />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.
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.
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. <P />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.
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. <P />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
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.
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 10<SUP>2</SUP> s.

---------------------------------------------------------
Title: Numerical simulations of large-amplitude oscillations in flux
    rope solar prominences
Authors: Liakh, V.; Luna, M.; Khomenko, E.
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.
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.
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.
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<SUP>-1</SUP> and can
  be triggered by the external perturbations such as Moreton or EIT
  waves. <BR /> Aims: Our aim is to analyze the properties of LAOs using
  realistic prominence models and the triggering mechanism by external
  disturbances. <BR /> 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. <BR /> 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. <BR /> 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.
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. <BR /> 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. <BR /> 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. <BR /> 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. <P
  />Movies associated to Figs. 1, 8 and 10 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201937078/olm">https://www.aanda.org</A>

---------------------------------------------------------
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.
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.
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.
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
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.
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 &amp; 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.
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.
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.
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. <BR /> 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. <BR />
  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. <BR
  /> 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. <BR /> 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. <P />The movie associated to Fig. 2 is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201834367/olm">https://www.aanda.org</A>

---------------------------------------------------------
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.
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 &amp; 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 ∼10<SUP>3</SUP> W m<SUP>-2</SUP>, based on an amplitude of 500
  m s<SUP>-1</SUP> 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.
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. <P
  />The movies associated to Figs. 16 and 17 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201833048/olm">https://www.aanda.org</A>

---------------------------------------------------------
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.
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
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
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.
2018A&A...609A..23R    Altcode: 2017A&A...609A..23R
  <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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.
2017A&A...608A..97F    Altcode: 2017arXiv170806133F
  <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <P />Movies attached to Figs. 1 and 3 are available at <A
  href="http://www.aanda.org/10.1051/0004-6361/201731374/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Flux Tube: Solar model
Authors: Khomenko, E.
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.
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.
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. <P />Movies attached to Figs 2 and 4 are available at <A
  href="http://www.aanda.org/10.1051/0004-6361/201629729/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Dependence of sunspot photospheric waves on the depth of the
    source of solar p-modes
Authors: Felipe, T.; Khomenko, E.
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
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.
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.
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. <BR />
  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. <BR /> 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. <BR /> 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. <P />Movies associated to Figs. 2 and 13
  are available at <A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: Tracing p-mode Waves from the Photosphere to the Corona in
    Active Regions
Authors: Zhao, Junwei; Felipe, Tobías; Chen, Ruizhu; Khomenko, Elena
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.
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.
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<SUP>-1</SUP>. 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.
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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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.
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
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.
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. <P />Movies are available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201628515/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Heating of the Partially Ionized Solar Chromosphere by Waves
    in Magnetic Structures
Authors: Shelyag, S.; Khomenko, E.; de Vicente, A.; Przybylski, D.
2016ApJ...819L..11S    Altcode: 2016arXiv160203373S
  In this paper, we show a “proof of concept” of the heating
  mechanism of the solar chromosphere due to wave dissipation caused by
  the effects of partial ionization. Numerical modeling of non-linear wave
  propagation in a magnetic flux tube, embedded in the solar atmosphere,
  is performed by solving a system of single-fluid quasi-MHD equations,
  which take into account the ambipolar term from the generalized Ohm’s
  law. It is shown that perturbations caused by magnetic waves can be
  effectively dissipated due to ambipolar diffusion. The energy input
  by this mechanism is continuous and shown to be more efficient than
  dissipation of static currents, ultimately leading to chromospheric
  temperature increase in magnetic structures.

---------------------------------------------------------
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.
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}}}<SUP>-1</SUP>. 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.
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<SUP>-1</SUP> 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
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
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<SUP>-4</SUP>. In galactic star-forming regions, f can be
  10<SUP>-10</SUP> or lower. Under these circumstances, the Hall current
  can couple low-frequency Alfvén and magnetoacoustic waves via the
  dimensionless Hall parameter ɛ =ω /{{{Ω }}}<SUB>{{i</SUB>}}f,
  where ω is the wave frequency and {{{Ω }}}<SUB>{{i</SUB>}} 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
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.
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? <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <P />Appendix A is available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201424340/olm">http://www.aanda.org</A>

---------------------------------------------------------
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.
2015A&A...577A..70S    Altcode: 2015arXiv150303094S
  <BR /> 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. <BR
  /> 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. <BR /> 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. <BR /> 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. <P />Movies
  related to Figs. 3, 7, and 11 are available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201424701/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Beyond MHD: modeling and observation of partially ionized
    solar plasma processes
Authors: Khomenko, E.
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.
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.
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.
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.
2014A&A...564A..97D    Altcode: 2014arXiv1401.5388D
  <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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.
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
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.
2013A&A...559A.107K    Altcode: 2013arXiv1310.0184K
  <BR /> 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. <BR />
  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. <BR
  /> 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.
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.
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<SUP>−1</SUP> was detected. In the photosphere, predominantly upward
  motions were found, varying from −3.1 km s<SUP>−1</SUP> upflows
  to 1.4 km s<SUP>−1</SUP> 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
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.
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.
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.
2012A&A...548A..35F    Altcode: 2012arXiv1209.2771F
  <BR /> 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. <BR />
  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 ⟨ B<SUB>vert</SUB> ⟩ = 0,50,100, and
  200 G. For the relevant calculations we used the Copenhagen Stagger
  code. <BR /> 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)<SUB>⊙</SUB>|
  ~ 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. <BR /> 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.
2012A&A...545A..22K    Altcode: 2012arXiv1207.4340K
  <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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.
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.
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.
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. <BR /> Aims: We analyze
  the robustness of circular polarization asymmetries to phase-diversity
  image reconstruction techniques. <BR /> 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. <BR />
  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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
  epsilon<SUB>sun</SUB>(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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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. <BR
  />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. <BR />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. <BR />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. <P
  />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.
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. <BR />Methods:
  We use numerical simulations of magneto-acoustic wave propagation in
  a magnetostatic sunspot model. <BR />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.
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. <BR />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. <BR />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.
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). <BR />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). <BR />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. <BR />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). <BR />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.
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.
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
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.
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.
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.
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. <P />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.
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.
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 v<SUB>A</SUB>&lt;c<SUB>S</SUB>
  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.
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.
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.
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.
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. <BR />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. <BR />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. <BR />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.
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.
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.
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.
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 &lt;&lt; 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.
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.,
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.
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 c<SUB>S</SUB> and the
  Alfvén speed v<SUB>A</SUB>, 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 c<SUB>S</SUB>&lt;v<SUB>A</SUB> does not reach the
  chromosphere and reflects back to the photosphere at a somewhat higher
  layer than the c<SUB>S</SUB>=v<SUB>A</SUB> 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.
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.
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.
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.
2005A&A...442.1059K    Altcode:
  Realistic solar magneto-convection simulations including the
  photospheric layers are used to study the polarization of the Fe i
  Zeeman-sensitive spectral lines at 6301.5, 6302.5, 15 648 and 15 652
  Å. The Stokes spectra are synthesized in a series of snapshots with
  a mixed-polarity magnetic field whose average unsigned strength varies
  from &lt; B &gt; = 10 to 140 G. The effects of spatial resolution and
  of the amount of magnetic flux in the simulation box on the profiles
  shapes, amplitudes and shifts are discussed. The synthetic spectra show
  many properties in common with those observed in quiet solar regions. In
  particular, the simulations reproduce the width and depth of spatially
  averaged Stokes I profiles, the basic classes of the Stokes V profiles
  and their amplitude and area asymmetries, as well as the abundance of
  the irregular-shaped Stokes V profiles. It is demonstrated that the
  amplitudes of the 1.56 μm lines observed in the inter-network are
  consistent with a "true" average unsigned magnetic field strength of
  20 G. We show that observations using these and visible lines, carried
  out under different seeing conditions (e.g., simultaneous observations
  at different telescopes), may result in different asymmetries and
  even opposite polarities of the profiles in the two spectral regions
  observed at the same spatial point.

---------------------------------------------------------
Title: 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.
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, &lt; B &gt;,
  between 10 and 30 G. The distribution of Stokes V amplitudes depends
  sensitively on &lt; B &gt;. 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 &lt;
  B &gt; = 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.
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.
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.
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.
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.
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.
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.
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<SUP>-3</SUP> (in units
  of continuum intensity I<SUB>c</SUB>), which is significantly above
  the noise level of 2-3 x 10<SUP>-4</SUP>. This implies that we detect
  fluxes as low as 2 x 10<SUP>15</SUP> Mx/px. We find evidence that we
  have detected most of the net flux that is in principle detectable at
  1<SUP>”</SUP> 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 20<SUP>o</SUP>. 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. <P />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.
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.
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.
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.
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<SUP>-3</SUP>. 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
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<SUP>-1</SUP> 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<SUP>-1</SUP> and has a downflow-to-upflow
  velocity difference of about 7 km s<SUP>-1</SUP> initially and some
  4 km s<SUP>-1</SUP> 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.
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.
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{<SUP>”</SUP>}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.

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Title: Comparision of Observed and Theoretical Amplitudes of
    Oscillations above granules and intergranular lanes
Authors: Khomenko, E. V.
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.
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.
2001ASPC..223..680K    Altcode: 2001csss...11..680K
  No abstract at ADS

---------------------------------------------------------
Title: Wave propagation in the solar atmosphere
Authors: Khomenko, E. V.
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.
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.
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.
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.
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.
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.