Author name code: khomenko
ADS astronomy entries on 2022-09-14
author:"Khomenko, Elena V."
------------------------------------------------------------------------
Title: Observational and numerical characterization of a recurrent
arc-shaped front propagating along a coronal fan
Authors: Sieyra, M. V.; Krishna Prasad, S.; Stenborg, G.; Khomenko,
E.; Van Doorsselaere, T.; Costa, A.; Esquivel, A.; Riedl, J. M.
Bibcode: 2022arXiv220810857S
Altcode:
Recurrent, arc-shaped intensity disturbances were detected by EUV
channels in an active region. The fronts were observed to propagate
along a coronal loop bundle rooted in a small area within a sunspot
umbra. Previous works have linked these intensity disturbances to slow
magnetoacoustic waves that propagate from the lower atmosphere to
the corona along the magnetic field. The slow magnetoacoustic waves
propagate at the local cusp speed. However, the measured propagation
speeds from the intensity images are usually smaller as they are
subject to projection effects due to the inclination of the magnetic
field with respect to the line-of-sight. Here, we aim to understand
the effect of projection by comparing observed speeds with those
from a numerical model. Using multi-wavelength data we determine the
periods present in the observations at different heights of the solar
atmosphere through Fourier analysis. We calculate the plane-of-sky
speeds along one of the loops from the cross-correlation time lags
obtained as a function of distance along the loop. We perform a 2D
ideal MHD simulation of an active region embedded in a stratified
atmosphere. We drive slow waves from the photosphere with a 3 minutes
periodicity. Synthetic time-distance maps are generated from the
forward-modelled intensities in coronal wavelengths and the projected
propagation speeds are calculated. The intensity disturbances show
a dominant period between [2-3] minutes at different heights of the
atmosphere. The apparent propagation speeds calculated for coronal
channels exhibit an accelerated pattern with values increasing from
40 to 120 km/s as the distance along the loop rises. The propagation
speeds obtained from the synthetic time-distance maps also exhibit
accelerated profiles within a similar range of speeds. We conclude
that the accelerated propagation in our observations is due to the
projection effect.
Title: Coupling between different atmospheric layers: waves and
energy transfer
Authors: Khomenko, Elena
Bibcode: 2022cosp...44.2545K
Altcode:
Solar atmosphere is nowadays viewed as an entire coupled system, where
the dynamical events observed in a certain layer can be consequences
or causes of those detected in the layers above or below. Photospheric
flows, interacting with magnetic structures, help energy propagation to
the chromosphere and above in the form of waves or vortex flows. These
processes are frequency dependent, and there are indications that
the high-frequency end of the spectrum is important for energizing
the solar atmosphere. Nevertheless, going towards high frequencies is
challenging both from the instrumental and modeling point of view. In
the case of the latter, new physical aspects are being included, such as
interaction between neutrals and plasma. In this talk I will review the
recent progress in observational and theoretical works on high-frequency
waves, shocks and vorticity propagation through the solar atmosphere,
with an emphasis onto multi-fluid modeling of these dynamical features.
Title: Acoustic-gravity wave propagation characteristics in 3D
radiation hydrodynamic simulations of the solar atmosphere
Authors: Fleck, Bernhard; Khomenko, Elena; Carlsson, Mats; Rempel,
Matthias; Steiner, Oskar; Riva, Fabio; Vigeesh, Gangadharan
Bibcode: 2022cosp...44.2503F
Altcode:
There has been tremendous progress in the degree of realism of
three-dimensional radiation magneto-hydrodynamic simulations of the
solar atmosphere in the past decades. Four of the most frequently
used numerical codes are Bifrost, CO5BOLD, MANCHA3D, and MURaM. Here
we test and compare the wave propagation characteristics in model
runs from these four codes by measuring the dispersion relation
of acoustic-gravity waves at various heights. We find considerable
differences between the various models.
Title: Modeling the thermal conduction in the solar atmosphere with
the code MANCHA3D
Authors: Navarro, A.; Khomenko, E.; Modestov, M.; Vitas, N.
Bibcode: 2022A&A...663A..96N
Altcode: 2022arXiv220508846N
Context. Thermal conductivity is one of the important mechanisms of
heat transfer in the solar corona. In the limit of strongly magnetized
plasma, it is typically modeled by Spitzer's expression where the
heat flux is aligned with the magnetic field.
Aims: This paper
describes the implementation of the heat conduction into the code
MANCHA3D with an aim of extending single-fluid MHD simulations from
the upper convection zone into the solar corona.
Methods:
Two different schemes to model heat conduction are implemented:
(1) a standard scheme where a parabolic term is added to the energy
equation, and (2) a scheme where the hyperbolic heat flux equation is
solved.
Results: The first scheme limits the time step due to the
explicit integration of a parabolic term, which makes the simulations
computationally expensive. The second scheme solves the limitations
on the time step by artificially limiting the heat conduction speed
to computationally manageable values. The validation of both schemes
is carried out with standard tests in one, two, and three spatial
dimensions. Furthermore, we implement the model for heat flux derived by
Braginskii (1965, Reviews of Plasma Physics, 205) in its most general
form, when the expression for the heat flux depends on the ratio of
the collisional to cyclotron frequencies of the plasma, and, therefore
on the magnetic field strength. Additionally, our implementation
takes into account the heat conduction in parallel, perpendicular,
and transverse directions, and provides the contributions from ions
and electrons separately. The model recovers Spitzer's expression
for parallel thermal conductivity in the strongly magnetized limit
but also transitions smoothly between field-aligned conductivity
and isotropic conductivity for regions with a low or null magnetic
field. We describe the details of the implementation of Braginskii's
thermal conductivity using a combination of the first scheme for the
perpendicular and transverse directions and the second scheme for the
parallel component. We estimate thermal conductivities in a quiet-Sun
model. In this model, we find that the perpendicular and transverse
components for electrons and ions and the parallel component for
ions might have some significance below the transition region. Above
the transition region only the parallel component for ions might
be important. Finally, we present a two-dimensional test for heat
conduction using realistic values of the solar atmosphere where we
prove the robustness of the two schemes implemented and show that our
adaptation of the hyperbolic treatment offers a great advantage over
the computational cost of the simulations.
Title: The European Solar Telescope
Authors: Quintero Noda, C.; Schlichenmaier, R.; Bellot Rubio, L. R.;
Löfdahl, M. G.; Khomenko, E.; Jurcak, J.; Leenaarts, J.; Kuckein,
C.; González Manrique, S. J.; Gunar, S.; Nelson, C. J.; de la Cruz
Rodríguez, J.; Tziotziou, K.; Tsiropoula, G.; Aulanier, G.; Collados,
M.; the EST team
Bibcode: 2022arXiv220710905Q
Altcode:
The European Solar Telescope (EST) is a project aimed at studying
the magnetic connectivity of the solar atmosphere, from the deep
photosphere to the upper chromosphere. Its design combines the knowledge
and expertise gathered by the European solar physics community during
the construction and operation of state-of-the-art solar telescopes
operating in visible and near-infrared wavelengths: the Swedish 1m Solar
Telescope (SST), the German Vacuum Tower Telescope (VTT) and GREGOR,
the French Télescope Héliographique pour l'Étude du Magnétisme
et des Instabilités Solaires (THÉMIS), and the Dutch Open Telescope
(DOT). With its 4.2 m primary mirror and an open configuration, EST will
become the most powerful European ground-based facility to study the Sun
in the coming decades in the visible and near-infrared bands. EST uses
the most innovative technological advances: the first adaptive secondary
mirror ever used in a solar telescope, a complex multi-conjugate
adaptive optics with deformable mirrors that form part of the optical
design in a natural way, a polarimetrically compensated telescope design
that eliminates the complex temporal variation and wavelength dependence
of the telescope Mueller matrix, and an instrument suite containing
several (etalon-based) tunable imaging spectropolarimeters and several
integral field unit spectropolarimeters. This publication summarises
some fundamental science questions that can be addressed with the
telescope, together with a complete description of its major subsystems.
Title: Doppler-velocity Drifts Detected in a Solar Prominence
Authors: Zapiór, Maciej; Heinzel, Petr; Khomenko, Elena
Bibcode: 2022ApJ...934...16Z
Altcode:
We analyzed multiline observations of a quiescent prominence from the
slit spectrograph located at the Ondřejov Observatory. Dopplergrams
and integrated intensity maps of the whole prominence were obtained
from observations in six spectral lines: Ca II H, Hϵ, Hβ, He I D3,
Hα, and Ca II IR. By combining integrated intensity maps with non-LTE
radiative-transfer modeling, we carefully identified areas in an
optically thin regime. The comparison of the Doppler-velocity maps and
scatterplots from different lines shows the existence of differences
in the velocity of ions and neutrals called velocity drift. The drift
is of a local nature, present mostly at prominence edges in the area
with a large velocity gradient, as can be tentatively expected based on
multifluid MHD models. We could not explore the time evolution of the
drift, since our data set consists of a single scan only. Our paper
brings another contribution to a rather controversial problem of the
detection of multifluid effects in solar prominences.
Title: Numerical simulations of large-amplitude oscillations in
solar prominences triggered by an eruptive event
Authors: Liakh, Valeriia; Khomenko, Elena; Luna, Manuel
Bibcode: 2022cosp...44.2541L
Altcode:
Large-amplitude oscillations (LAOs) in solar prominences are very
spectacular phenomena. Their activation is often associated with
external energetic disturbance, such as waves from the flare or
eruption, nearby magnetic reconnection, jets. This study represents
the first attempt to understand how the perturbations produced by
the eruptive event trigger LAOs in the solar prominences using the
capabilities of numerical simulations. In these numerical experiments,
the prominence model consists of the flux rope magnetic structure
formed from the sheared arcade using the footpoints driving and the
prominence plasma loaded into the bottom helical part of the pre-formed
flux rope. The main source of the disturbances is represented by the
eruptive flux rope formed similarly from the sheared arcade with a
combination of the converging and shearing motions at the base of
the magnetic field lines. On the one hand, we have investigated the
possibility of the excitation of Moreton and EIT waves by the eruptive
flux rope. On the other hand, we have studied the triggering of LAOs by
these energetic waves. In the second phase of the numerical experiment,
the tearing instability develops in the elongated current sheet below
the erupting flux rope. This phase is characterized by the chaotic
formation of the magnetic islands in the current sheet. When merging
with post-reconnection loops, the formed plasmoids bring additional
perturbations in the velocity field.
Title: Transverse kink oscillations of inhomogeneous prominence
threads
Authors: Martínez-Gómez, David; Terradas, Jaume; Soler, Roberto;
Khomenko, Elena
Bibcode: 2022cosp...44.2556M
Altcode:
Observations of transverse oscillations in solar prominence threads
are usually interpreted as the fundamental kink mode, while the
detection of the first harmonic remains elusive. It is known that the
properties of these oscillations are greatly affected by the density
distribution along the magnetic flux tube. In this talk, we present the
results of a numerical study on how the density inhomogeneity in the
longitudinal and radial directions modify the periods and damping times
of kink oscillations, and how this effect would be reflected in the
observations. We have solved the ideal magnetohydrodynamics equations
through two different methods: a) performing 3D numerical simulations,
and b) solving a 2D generalised eigenvalue problem. We have studied the
dependence of the periods, damping times and amplitudes of transverse
kink oscillations on the ratio between the densities at the centre
and at the ends of the tube, and also on the average density. Then,
we have applied forward modelling on our 3D simulations and computed
synthetic H-alpha profiles to obtain the observational signatures of
several linear and non-linear features of the oscillations. Our results
confirm that the ratio of the period of the fundamental oscillation
mode to the period of the first harmonic increases as the ratio
of the central density to the footpoint density is increased or as
the average density of the tube is decreased. The damping times due
to resonant absorption decrease as the central to footpoint density
increases. Contrary to the case of longitudinally homogeneous threads,
we found that the damping time to period ratio also increases as the
density ratio is increases or the average density is reduced. The
analysis of the synthetic profiles shows that the H-alpha emission can
be used to detect the fundamental mode of oscillation, but the first
harmonic is barely detectable in H-alpha, which may explain the lack
of observations of this mode. Therefore, a combination of different
spectral lines is required to get information about the period ratio
and to use it to infer physical properties of the threads.
Title: Generalized Fluid Models of the Braginskii Type
Authors: Hunana, P.; Passot, T.; Khomenko, E.; Martínez-Gómez, D.;
Collados, M.; Tenerani, A.; Zank, G. P.; Maneva, Y.; Goldstein, M. L.;
Webb, G. M.
Bibcode: 2022ApJS..260...26H
Altcode: 2022arXiv220111561H
Several generalizations of the well-known fluid model of Braginskii
(1965) are considered. We use the Landau collisional operator and the
moment method of Grad. We focus on the 21-moment model that is analogous
to the Braginskii model, and we also consider a 22-moment model. Both
models are formulated for general multispecies plasmas with arbitrary
masses and temperatures, where all of the fluid moments are described
by their evolution equations. The 21-moment model contains two "heat
flux vectors" (third- and fifth-order moments) and two "viscosity
tensors" (second- and fourth-order moments). The Braginskii model
is then obtained as a particular case of a one ion-electron plasma
with similar temperatures, with decoupled heat fluxes and viscosity
tensors expressed in a quasistatic approximation. We provide all of
the numerical values of the Braginskii model in a fully analytic form
(together with the fourth- and fifth-order moments). For multispecies
plasmas, the model makes the calculation of the transport coefficients
straightforward. Formulation in fluid moments (instead of Hermite
moments) is also suitable for implementation into existing numerical
codes. It is emphasized that it is the quasistatic approximation that
makes some Braginskii coefficients divergent in a weakly collisional
regime. Importantly, we show that the heat fluxes and viscosity tensors
are coupled even in the linear approximation, and that the fully
contracted (scalar) perturbations of the fourth-order moment, which
are accounted for in the 22-moment model, modify the energy exchange
rates. We also provide several appendices, which can be useful as a
guide for deriving the Braginskii model with the moment method of Grad.
Title: Transverse kink oscillations of inhomogeneous prominence
threads: Numerical analysis and Hα forward modelling
Authors: Martínez-Gómez, David; Soler, Roberto; Terradas, Jaume;
Khomenko, Elena
Bibcode: 2022A&A...658A.106M
Altcode: 2021arXiv211109036M
Context. Prominence threads are very long and thin flux tubes that
are partially filled with cold plasma. Observations have shown that
transverse oscillations are frequent in these solar structures. The
observations are usually interpreted as the fundamental kink mode,
while the detection of the first harmonic remains elusive.
Aims:
The properties of oscillations in threads are greatly affected by the
density distribution along the flux tube. Here, we aim to study how the
density inhomogeneities in the longitudinal and radial directions modify
the periods and damping times of kink oscillations and how this effect
would be reflected in the observations.
Methods: We solved the
ideal magnetohydrodynamics equations using two different methods: (a)
performing 3D numerical simulations and (b) solving a 2D generalised
eigenvalue problem. We studied the dependence of the periods, damping
times, and amplitudes of transverse kink oscillations on the ratio
between the densities at the centre and at the ends of the tube,
and also on the average density. We applied forward modelling to our
3D simulations to compute synthetic Hα profiles.
Results: We
confirm that the ratio of the period of the fundamental oscillation
mode to the period of the first harmonic increases as the ratio of
the central density to the footpoint density is increased, or as the
averaged density of the tube is decreased. We find that the damping
times due to resonant absorption decrease as the central-to-footpoint
density ratio increases. Contrary to the case of longitudinally
homogeneous tubes, we find that the damping-time-to-period ratio also
increases as the density ratio is increased or the average density
is reduced. We present snapshots and time-distance diagrams of the
emission in the Hα line.
Conclusions: The results presented
here have implications for the field of prominence seismology. While
the Hα emission can be used to detect the fundamental mode,
the first harmonic is barely detectable in Hα. This may explain
the lack of detections of the first harmonic. A combination of
different spectral lines is required to obtain information about
the period ratio and to use it to infer physical properties of the
threads.
Movies associated to Figs. 11-14 are available at https://www.aanda.org
Title: Ambipolar Diffusion in the Lower Solar Atmosphere: MHD
Simulations of a Sunspot
Authors: MacBride, Conor; Jess, David; Khomenko, Elena
Bibcode: 2021AGUFMSH25A2065M
Altcode:
Behind MHD is the assumption that there is a low degree of plasma
ionization. However, in the upper photosphere and chromosphere a reduced
temperature often results in the partial ionization of the plasma. The
interaction between the decoupled neutral and ionized components
of such a partially ionized plasma produce ambipolar diffusion. To
investigate the role of ambipolar diffusion in energy transfer and
dissipation in the chromosphere, we analyze a 2D numerical model of
magnetoacoustic waves propagating through the chromosphere above the
umbra of a sunspot. We solve the ideal MHD equations for perturbations
to the magnetostatic equilibrium using the Mancha code. The effect of
ambipolar diffusion is isolated by varying the inclusion of additional
terms in the MHD equations which account for this process. Data-driven
perturbations are introduced in the photosphere using observational
SDO/HMI umbral velocity time series. Using energy spectra, we analyze
the chromospheric dynamics of our simulation and gather insights into
the role of ambipolar diffusion, and partial ionization, in energy
transfer and dissipation in the lower solar atmosphere.
Title: Magnetic field amplification and structure formation by the
Rayleigh-Taylor instability
Authors: Popescu Braileanu, B.; Lukin, V. S.; Khomenko, E.
Bibcode: 2021arXiv211213043P
Altcode:
We report on results of high resolution two fluid non-linear simulations
of the Rayleigh Taylor Instability (RTI) at the interface between
a solar prominence and the corona. These follow results reported
earlier by Popescu Braileanu et al. (2021a,b) on linear and early
non-linear RTI dynamics in this environment. The simulations use a
two fluid model that includes collisions between neutrals and charges,
including ionization/recombination, energy and momentum transfer, and
frictional heating. High resolution 2.5D magnetized RTI simulations
with the magnetic field dominantly normal to and slightly sheared
with respect to the prominence plane demonstrate that in a fully
developed state of RTI a large fraction of the gravitational energy
of a prominence thread can be converted into quasi-turbulent energy
of the magnetic field. RTI magnetic energy generation is further
accompanied by magnetic and plasma density structure formation,
including dynamic formation, break-up, and merging of current sheets
and plasmoid sub-structures. The simulations show the role of flow
decoupling and ionization/recombination reactions between the neutrals
and charges on the structure formation in magnetized RTI. We provide
a careful examination of sources and form of numerical dissipation of
the evolving magnetic field structures.
Title: Large-amplitude longitudinal oscillations in solar prominences
simulated with different resolutions
Authors: Liakh, V.; Luna, M.; Khomenko, E.
Bibcode: 2021A&A...654A.145L
Altcode: 2021arXiv210801143L
Context. Large-amplitude longitudinal oscillations (LALOs) in solar
prominences have been widely studied in recent decades. However, their
damping and amplification mechanisms are not well understood.
Aims: In this study, we investigate the attenuation and
amplification of LALOs using high-resolution numerical simulations
with progressively increasing spatial resolutions.
Methods:
We performed time-dependent numerical simulations of LALOs using the
2D magnetic configuration that contains a dipped region. After the
prominence mass loading in the magnetic dips, we triggered LALOs by
perturbing the prominence mass along the magnetic field. We performed
the experiments with four values of spatial resolution.
Results:
In the simulations with the highest resolution, the period shows good
agreement with the pendulum model. The convergence experiment revealed
that the damping time saturates at the bottom prominence region with
increasing resolution, indicating the existence of a physical reason for
the damping of oscillations. At the prominence top, the oscillations are
amplified during the first minutes and are then slowly attenuated. The
characteristic time suggests more significant amplification in the
experiments with the highest spatial resolution. The analysis revealed
that the energy exchange between the bottom and top prominence regions
is responsible for the attenuation and amplification of LALOs.
Conclusions: High-resolution experiments are crucial when studying
the periods and the damping mechanism of LALOs. The period agrees
with the pendulum model only when using a sufficiently high spatial
resolution. The results suggest that numerical diffusion in simulations
with insufficient spatial resolution can hide important physical
mechanisms, such as amplification of oscillations.
Title: Simulations of the Biermann battery mechanism in two-fluid
partially ionised plasmas
Authors: Martínez-Gómez, D.; Popescu Braileanu, B.; Khomenko, E.;
Hunana, P.
Bibcode: 2021A&A...650A.123M
Altcode: 2021arXiv210406956M
Context. In the absence of an initial seed, the Biermann battery term
of a non-ideal induction equation acts as a source that generates
weak magnetic fields. These fields are then amplified via a dynamo
mechanism. The Kelvin-Helmholtz instability is a fluid phenomenon that
takes place in many astrophysical scenarios and can trigger the action
of the Biermann battery and dynamo processes.
Aims: We aim to
investigate the effect of the ionisation degree of the plasma and the
interaction between the charged and neutral species on the generation
and amplification of magnetic fields during the different stages of the
instability.
Methods: We use the two-fluid model implemented in
the numerical code MANCHA-2F. We perform 2D simulations starting from a
configuration with no initial magnetic field and which is unstable due
to a velocity shear. We vary the ionisation degree of the plasma and
we analyse the role that the different collisional terms included in
the equations of the model play on the evolution of the instability and
the generation of magnetic field.
Results: We find that when no
collisional coupling is considered between the two fluids, the effect
of the Biermann battery mechanism does not depend on the ionisation
degree. However, when elastic collisions are taken into account, the
generation of magnetic field is increased as the ionisation degree
is reduced. This behaviour is slightly enhanced if the process of
charge-exchange is also considered. We also find a dependence on the
total density of the plasma related to the dependence on the coupling
degree between the two fluids. As the total density is increased,
the results from the two-fluid model converge to the predictions of
single-fluid models.
Conclusions: The charged-neutral interaction
in a partially ionised plasmas has a non-negligible effect on the
Biermann battery mechanism and it effectively enhances the generation
of a magnetic field. In addition, single-fluid models, which assume
a very strong coupling between the two species, may overestimate
the contribution of this interaction in comparison with two-fluid
models. Movies associated to Figs. 2 and A.2 are available at https://www.aanda.org
Title: Two-fluid simulations of Rayleigh-Taylor instability in a
magnetized solar prominence thread. II. Effects of collisionality
Authors: Popescu Braileanu, B.; Lukin, V. S.; Khomenko, E.; de
Vicente, Á.
Bibcode: 2021A&A...650A.181P
Altcode: 2021arXiv210112731P
Solar prominences are formed by partially ionized plasma
with inter-particle collision frequencies generally warranting
magnetohydrodynamic treatment. In this work we explore the dynamical
impacts and observable signatures of two-fluid effects in the parameter
regimes when ion-neutral collisions do not fully couple the neutral
and charged fluids. We perform 2.5D two-fluid (charges-neutrals)
simulations of the Rayleigh-Taylor instability (RTI) at a smoothly
changing interface between a solar prominence thread and the corona. The
purpose of this study is to deepen our understanding of the RTI and
the effects of the partial ionization on the development of RTI using
nonlinear two-fluid numerical simulations. Our two-fluid model takes
into account neutral viscosity, thermal conductivity, and collisional
interaction between neutrals and charges: ionization-recombination,
energy and momentum transfer, and frictional heating. In this paper, the
sensitivity of the RTI dynamics to collisional effects for different
magnetic field configurations supporting the prominence thread
is explored. This is done by artificially varying, or eliminating,
effects of both elastic and inelastic collisions by modifying the model
equations. We find that ionization and recombination reactions between
ionized and neutral fluids do not substantially impact the development
of the primary RTI. However, such reactions can impact the development
of secondary structures during the mixing of the cold prominence and
hotter surrounding coronal material. We find that collisionality
within and between ionized and neutral particle populations plays
an important role in both linear and nonlinear development of RTI;
ion-neutral collision frequency is the primary determining factor in
development or damping of small-scale structures. We also observe that
the degree and signatures of flow decoupling between ion and neutral
fluids can depend on the inter-particle collisionality and on the
magnetic field configuration of the prominence thread.
Title: Chromospheric Heating by Magnetohydrodynamic Waves and
Instabilities
Authors: Srivastava, A. K.; Ballester, J. L.; Cally, P. S.; Carlsson,
M.; Goossens, M.; Jess, D. B.; Khomenko, E.; Mathioudakis, M.;
Murawski, K.; Zaqarashvili, T. V.
Bibcode: 2021JGRA..12629097S
Altcode: 2021arXiv210402010S
The importance of the chromosphere in the mass and energy transport
within the solar atmosphere is now widely recognized. This review
discusses the physics of magnetohydrodynamic waves and instabilities
in large-scale chromospheric structures as well as in magnetic flux
tubes. We highlight a number of key observational aspects that have
helped our understanding of the role of the solar chromosphere
in various dynamic processes and wave phenomena, and the heating
scenario of the solar chromosphere is also discussed. The review
focuses on the physics of waves and invokes the basics of plasma
instabilities in the context of this important layer of the solar
atmosphere. Potential implications, future trends and outstanding
questions are also delineated.
Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope
(DKIST)
Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio,
Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart;
Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa,
Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez
Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler,
Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun,
Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres,
Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.;
Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini,
Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena;
Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor;
Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael;
Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli,
Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys,
Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.;
Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis,
Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David
E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson,
Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.;
Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.;
Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava,
Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas
A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas,
Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST
Instrument Scientists; DKIST Science Working Group; DKIST Critical
Science Plan Community
Bibcode: 2021SoPh..296...70R
Altcode: 2020arXiv200808203R
The National Science Foundation's Daniel K. Inouye Solar Telescope
(DKIST) will revolutionize our ability to measure, understand,
and model the basic physical processes that control the structure
and dynamics of the Sun and its atmosphere. The first-light DKIST
images, released publicly on 29 January 2020, only hint at the
extraordinary capabilities that will accompany full commissioning of
the five facility instruments. With this Critical Science Plan (CSP)
we attempt to anticipate some of what those capabilities will enable,
providing a snapshot of some of the scientific pursuits that the DKIST
hopes to engage as start-of-operations nears. The work builds on the
combined contributions of the DKIST Science Working Group (SWG) and
CSP Community members, who generously shared their experiences, plans,
knowledge, and dreams. Discussion is primarily focused on those issues
to which DKIST will uniquely contribute.
Title: Modeling of 3d Atmospheres of Cool Stars with the Mancha Code
Authors: Perdomo, Andrea; Vitas, Nikola; Khomenko, Elena; Collados,
Manuel
Bibcode: 2021csss.confE.129P
Altcode:
The first results of the application of the MANCHA code to the case of
stars beyond the solar case are presented: hydrodynamical simulations
of stars of spectral type K0V and M0V compared with the solar case.
Title: Accurately constraining velocity information from spectral
imaging observations using machine learning techniques
Authors: MacBride, Conor D.; Jess, David B.; Grant, Samuel D. T.;
Khomenko, Elena; Keys, Peter H.; Stangalini, Marco
Bibcode: 2021RSPTA.37900171M
Altcode: 2020arXiv200707904M
Determining accurate plasma Doppler (line-of-sight) velocities from
spectroscopic measurements is a challenging endeavour, especially
when weak chromospheric absorption lines are often rapidly evolving
and, hence, contain multiple spectral components in their constituent
line profiles. Here, we present a novel method that employs machine
learning techniques to identify the underlying components present
within observed spectral lines, before subsequently constraining
the constituent profiles through single or multiple Voigt fits. Our
method allows active and quiescent components present in spectra to
be identified and isolated for subsequent study. Lastly, we employ
a Ca ɪɪ 8542 Å spectral imaging dataset as a proof-of-concept
study to benchmark the suitability of our code for extracting
two-component atmospheric profiles that are commonly present
in sunspot chromospheres. Minimization tests are employed to
validate the reliability of the results, achieving median reduced
χ2-values equal to 1.03 between the observed and synthesized
umbral line profiles. This article is part of the Theo Murphy
meeting issue `High-resolution wave dynamics in the lower solar
atmosphere'.
Title: Two-fluid simulations of Rayleigh-Taylor instability in
a magnetized solar prominence thread. I. Effects of prominence
magnetization and mass loading
Authors: Popescu Braileanu, B.; Lukin, V. S.; Khomenko, E.; de
Vicente, Á.
Bibcode: 2021A&A...646A..93P
Altcode: 2020arXiv200715984P
Solar prominences are formed by partially ionized plasma with
inter-particle collision frequencies, which generally warrant
magnetohydrodynamic treatment. In this work, we explore the dynamical
impacts and observable signatures of two-fluid effects in the
parameter regimes when ion-neutral collisions do not fully couple
the neutral and charged fluids. We performed 2.5D two-fluid (charge -
neutrals) simulations of the Rayleigh-Taylor instability (RTI) at a
smoothly changing interface between a solar prominence thread and the
corona. The purpose of this study is to deepen our understanding of the
RTI and the effects of partial ionization on the development of the RTI
using nonlinear two-fluid numerical simulations. Our two-fluid model
takes into account viscosity, thermal conductivity, and collisional
interaction between neutrals and charge: ionization or recombination,
energy and momentum transfer, and frictional heating. In this paper,
we explore the sensitivity of the RTI dynamics to the prominence
equilibrium configuration, including the impact of the magnetic field
strength and shear supporting the prominence thread, and the amount of
prominence mass-loading. We show that at small scales, a realistically
smooth prominence-corona interface leads to qualitatively different
linear RTI evolution than that which is expected for a discontinuous
interface, while magnetic field shear has the stabilizing effect
of reducing the growth rate or eliminating the instability. In the
nonlinear phase, we observe that in the presence of field shear the
development of the instability leads to formation of coherent and
interacting 2.5D magnetic structures, which, in turn, can lead to
substantial plasma flow across magnetic field lines and associated
decoupling of the fluid velocities of charged particles and neutrals.
Title: Influence of ambipolar and Hall effects on vorticity in
three-dimensional simulations of magneto-convection
Authors: Khomenko, E.; Collados, M.; Vitas, N.; González-Morales,
P. A.
Bibcode: 2021RSPTA.37900176K
Altcode: 2020arXiv200909753K
This paper presents the results of the analysis of three-dimensional
simulations of solar magneto-convection that include the joint action of
the ambipolar diffusion and the Hall effect. Three simulation runs are
compared: one including both ambipolar diffusion and the Hall effect;
one including only ambipolar diffusion and one without any of these
two effects. The magnetic field is amplified from initial field to
saturation level by the action of turbulent local dynamo. In each of
these cases, we study 2 h of simulated solar time after the local
dynamo reaches the saturation regime. We analyse the power spectra
of vorticity, of magnetic field fluctuations and of the different
components of the magnetic Poynting flux responsible for the transport
of vertical or horizontal perturbations. Our preliminary results show
that the ambipolar diffusion produces a strong reduction of vorticity
in the upper chromospheric layers and that it dissipates the vortical
perturbations converting them into thermal energy. The Hall effect
acts in the opposite way, strongly enhancing the vorticity. When the
Hall effect is included, the magnetic field in the simulations becomes,
on average, more vertical and long-lived flux tube-like structures are
produced. We trace a single magnetic structure to study its evolution
pattern and the magnetic field intensification, and their possible
relation to the Hall effect. This article is part of the Theo
Murphy meeting issue `High-resolution wave dynamics in the lower
solar atmosphere'.
Title: Acoustic-gravity wave propagation characteristics in
three-dimensional radiation hydrodynamic simulations of the solar
atmosphere
Authors: Fleck, B.; Carlsson, M.; Khomenko, E.; Rempel, M.; Steiner,
O.; Vigeesh, G.
Bibcode: 2021RSPTA.37900170F
Altcode: 2020arXiv200705847F
There has been tremendous progress in the degree of realism of
three-dimensional radiation magneto-hydrodynamic simulations of the
solar atmosphere in the past decades. Four of the most frequently
used numerical codes are Bifrost, CO5BOLD, MANCHA3D and MURaM. Here
we test and compare the wave propagation characteristics in model
runs from these four codes by measuring the dispersion relation of
acoustic-gravity waves at various heights. We find considerable
differences between the various models. The height dependence of
wave power, in particular of high-frequency waves, varies by up to
two orders of magnitude between the models, and the phase difference
spectra of several models show unexpected features, including ±180°
phase jumps. This article is part of the Theo Murphy meeting issue
`High-resolution wave dynamics in the lower solar atmosphere'.
Title: Effects of neutrals on magnetic Rayleigh Taylor instability
in solar prominences
Authors: Khomenko, Elena; Lukin, Vyacheslav; Popescu Braileanu,
Beatrice
Bibcode: 2021cosp...43E.976K
Altcode:
The Rayleigh Taylor instability has been frequently observed at
the interface between solar prominences and corona. Prominence
plasma contains a large fraction of neutrals, and their role on the
stability of these structures is not fully understood. Here we study
the behavior of plasma and neutral components during the Rayleigh
Taylor instability in a thread of prominence material, using two-fluid
numerical simulations. Our model takes into account elastic collisions,
ionization/recombination, thermal exchange, neutral viscosity and
conductivity. We study the effects of the magnetic field strength,
orientation, and the density contrast of the thread on the growth
rate of the instability, both in the linear and non-linear phases. We
observe that, while large-scale harmonics grow exponentially in the
linear phase of the instability, for intermediate and small scales,
affected by viscosity, thermal conduction, and the interaction
between neutrals and charges, the linear and the nonlinear phases are
superposed. This behavior affects the thermal and dynamic decoupling
of the components. We observe differences in the neutral and plasma
velocities of the order of several hundreds of m/s, and differences
in their temperature of the order of several tens of Kelvin. The
ionization-recombination imbalance results in creation of a layer of
increased density of charges. This layer follows the evolution of the
eddies, and it potentially observable. Elastic collisions influence
the growth rate similarly to the viscosity: an increase of collisions
results in a larger growth rate at small scales. Larger density contrast
increases the growth rate, contrary to the effect of increasing the
strength of the magnetic field.
Title: Joint action of Hall and ambipolar effects in 3D
magneto-convection simulations of the quiet Sun. I. Dissipation and
generation of waves
Authors: González-Morales, P. A.; Khomenko, E.; Vitas, N.; Collados,
M.
Bibcode: 2020A&A...642A.220G
Altcode: 2020arXiv200810429G
The partial ionization of the solar plasma causes several nonideal
effects such as the ambipolar diffusion, the Hall effect, and the
Biermann battery effect. Here we report on the first three-dimensional
realistic simulations of solar local dynamo where all three effects
were taken into account. The simulations started with a snapshot of
already saturated battery-seeded dynamo, where two new series were
developed: one with solely ambipolar diffusion and another one also
taking into account the Hall term in the generalized Ohm's law. The
simulations were then run for about 4 h of solar time to reach the
stationary regime and improve the statistics. In parallel, a purely
MHD dynamo simulation was also run for the same amount of time. The
simulations are compared in a statistical way. We consider the average
properties of simulation dynamics, the generation and dissipation
of compressible and incompressible waves, and the magnetic Poynting
flux. The results show that, with the inclusion of the ambipolar
diffusion, the amplitudes of the incompressible perturbations related
to Alfvén waves are reduced, and the Poynting flux is absorbed, with
a frequency dependence. The Hall effect causes the opposite action:
significant excess of incompressible perturbations is generated and an
excess of the Poynting flux is observed in the chromospheric layers. The
model with ambipolar diffusion shows, on average, sharper current
sheets and slightly more abundant fast magneto-acoustic shocks in the
chromosphere. The model with the Hall effect has higher temperatures at
the lower chromosphere and stronger and more vertical magnetic field
concentrations all over the chromosphere. The study of high-frequency
waves reveals that significant power of incompressible perturbations
is associated with areas with intense and more vertical magnetic
fields and larger temperatures. This behavior explains the large
Poynting fluxes in the simulations with the Hall effect and provides
confirmation as to the role of Alfvén waves in chromospheric heating
in internetwork regions, under the action of both Hall and ambipolar
effects. We find a positive correlation between the magnitude of the
ambipolar heating and the temperature increase at the same location
after a characteristic time of 102 s.
Title: Numerical simulations of large-amplitude oscillations in flux
rope solar prominences
Authors: Liakh, V.; Luna, M.; Khomenko, E.
Bibcode: 2020sea..confE.204L
Altcode:
This study is based on the 2.5D numerical simulations of the
large-amplitude oscillations (LAOs) in the flux rope solar
prominences. The prominence models with two different values of the
initial shear angle and the density contrast were considered. In
order to investigate the possible normal modes of the structure,
the prominence was perturbed with the horizontal and vertical
disturbances. To study the mechanism of the external LAOs triggering,
we used the disturbance placed out of the flux rope. The transverse
and longitudinal oscillation periods do not show a strong dependence
on the shear angle and the density contrast. The external perturbation
excites the oscillations of both polarizations, and their properties are
a mixture of those excited by purely horizontal and vertical driving.
Title: Local dynamo in stars beyond the Sun: Study for a K0V star
Authors: Perdomo García, A.; Vitas, N.; Khomenko, E.; Collados Vera,
M. A.
Bibcode: 2020sea..confE.206P
Altcode:
We present the first results of application of the MANCHA3D code (Felipe
2010; Khomenko et al. 2017, 2018) to a K0V cool star. Initially we run
the code solving purely hydrodynamic equations until the stationary
convection is reached. Then we produce the magnetic field generation
and amplification by Biermann's battery seed and local dynamo. We find
values around 100 Gauss for the amplified saturated magnetic field,
similar to those found in Khomenko et al. (2017) for the solar case.
Title: 2D simulations of the Biermann battery mechanism in partially
ionized plasmas
Authors: Martínez-Gómez, D.; Popescu Braileanu, B.; Khomenko, E.;
Hunana, P.
Bibcode: 2020sea..confE.205M
Altcode:
In the absence of an initial seed, the Biermann battery term of
a non-ideal induction equation acts a source that generates weak
magnetic fields. Here, we study this mechanism in the context of
partially ionized plasmas, using a model in which the charged and
neutral components of the plasma are treated as two different fluids
that interact by means of collisions. We investigate the effect that
the ionization degree and the charged-neutral interaction have on the
generation of magnetic field. We use the numerical code MANCHA-2F to
perform 2D simulations of the Kelvin-Helmholtz instability. We study how
the magnetic field generated by the Biermann battery process depends on
the ionization degree of the plasma and on the different collisional
terms included in the equations of the model. We find that when the
collisional coupling is taken into account, the generation of magnetic
field is increased as the ionization degree is decreased. We also
find that this effect depends on the total density of the plasma and
that as this parameter is increased, the numerical two-fluid results
converge to the analytical results from a single-fluid model.
Title: Numerical simulations of large-amplitude oscillations in flux
rope solar prominences
Authors: Liakh, V.; Luna, M.; Khomenko, E.
Bibcode: 2020A&A...637A..75L
Altcode: 2020arXiv200304343L
Context. Large-amplitude oscillations (LAOs) of solar prominences
are a very spectacular, but poorly understood, phenomena. These
motions have amplitudes larger than 10 km s-1 and can
be triggered by the external perturbations such as Moreton or EIT
waves.
Aims: Our aim is to analyze the properties of LAOs using
realistic prominence models and the triggering mechanism by external
disturbances.
Methods: We performed time-dependent numerical
simulations of LAOs using a magnetic flux rope model with the two
values of shear angle and density contrast. We studied the internal
modes of the prominence using horizontal and vertical triggering. In
addition, we used perturbation that arrives from outside to understand
how such external disturbance can produce LAOs.
Results: The
period of longitudinal oscillations and its behavior with height show
good agreement with the pendulum model. The period of the transverse
oscillations remains constant with height, suggesting a global normal
mode. The transverse oscillations typically have shorter periods than
longitudinal oscillations.
Conclusions: The periods of the
longitudinal and transverse oscillations show only weak dependence
on the shear angle of the magnetic structure and prominence density
contrast. The external disturbance perturbs the flux rope exciting
oscillations of both polarizations. Their properties are a mixture of
those excited by purely horizontal and vertical driving.
Title: Two-dimensional simulations of coronal rain dynamics. I. Model
consisting of a vertical magnetic field and an unbounded atmosphere
Authors: Martínez-Gómez, D.; Oliver, R.; Khomenko, E.; Collados, M.
Bibcode: 2020A&A...634A..36M
Altcode: 2019arXiv191106638M
Context. Coronal rain often comes about as the final product
of evaporation and condensation cycles that occur in active
regions. Observations show that the condensed plasma falls with an
acceleration that is less than that of free fall.
Aims: We aim
to improve the understanding of the physical mechanisms behind the
slower than free-fall motion and the two-stage evolution (an initial
phase of acceleration followed by an almost constant velocity phase)
detected in coronal rain events.
Methods: Using the MANCHA3D
code, we solve the 2D ideal magnetohydrodynamic equations. We
represent the solar corona as an isothermal vertically stratified
atmosphere with a uniform vertical magnetic field. We represent the
plasma condensation as a density enhancement described by a 2D Gaussian
profile. We analyse the temporal evolution of the descending plasma and
study its dependence on such parameters as density and magnetic field
strength.
Results: We confirm previous findings that indicate
that the pressure gradient is the main force that opposes the action
of gravity and slows down the blob descent, and that larger densities
require larger pressure gradients to reach the constant speed phase. We
find that the shape of a condensation with a horizontal variation of
density is distorted during its fall because the denser parts of the
blob fall faster than the lighter ones. This is explained by the fact
that the duration of the initial acceleration phase and, therefore,
the maximum falling speed attained by the plasma, increases with the
ratio of blob to coronal density. We also find that the magnetic
field plays a fundamental role in the evolution of the descending
condensations. A strong enough magnetic field (greater than 10 G
in our simulations) forces each plasma element to follow the path
given by a particular field line, which allows for the description
of the evolution of each vertical slice of the blob in terms of 1D
dynamics, without the influence of the adjacent slices. In addition,
under the typical conditions of the coronal rain events, the magnetic
field prevents the development of Kelvin-Helmholtz instability. Movies associated to Figs. 1, 8 and 10 are available at https://www.aanda.org
Title: An introductory guide to fluid models with anisotropic
temperatures. Part 1. CGL description and collisionless fluid
hierarchy
Authors: Hunana, P.; Tenerani, A.; Zank, G. P.; Khomenko, E.;
Goldstein, M. L.; Webb, G. M.; Cally, P. S.; Collados, M.; Velli,
M.; Adhikari, L.
Bibcode: 2019JPlPh..85f2002H
Altcode: 2019arXiv190109354H
We present a detailed guide to advanced collisionless fluid models
that incorporate kinetic effects into the fluid framework, and that are
much closer to the collisionless kinetic description than traditional
magnetohydrodynamics. Such fluid models are directly applicable to
modelling the turbulent evolution of a vast array of astrophysical
plasmas, such as the solar corona and the solar wind, the interstellar
medium, as well as accretion disks and galaxy clusters. The text
can be viewed as a detailed guide to Landau fluid models and it is
divided into two parts. Part 1 is dedicated to fluid models that
are obtained by closing the fluid hierarchy with simple (non-Landau
fluid) closures. Part 2 is dedicated to Landau fluid closures. Here
in Part 1, we discuss the fluid model of Chew-Goldberger-Low (CGL)
in great detail, together with fluid models that contain dispersive
effects introduced by the Hall term and by the finite Larmor radius
corrections to the pressure tensor. We consider dispersive effects
introduced by the non-gyrotropic heat flux vectors. We investigate
the parallel and oblique firehose instability, and show that the
non-gyrotropic heat flux strongly influences the maximum growth rate of
these instabilities. Furthermore, we discuss fluid models that contain
evolution equations for the gyrotropic heat flux fluctuations and that
are closed at the fourth-moment level by prescribing a specific form
for the distribution function. For the bi-Maxwellian distribution,
such a closure is known as the `normal' closure. We also discuss a
fluid closure for the bi-kappa distribution. Finally, by considering
one-dimensional Maxwellian fluid closures at higher-order moments,
we show that such fluid models are always unstable. The last possible
non Landau fluid closure is therefore the `normal' closure, and beyond
the fourth-order moment, Landau fluid closures are required.
Title: An introductory guide to fluid models with anisotropic
temperatures. Part 2. Kinetic theory, Padé approximants and Landau
fluid closures
Authors: Hunana, P.; Tenerani, A.; Zank, G. P.; Goldstein, M. L.;
Webb, G. M.; Khomenko, E.; Collados, M.; Cally, P. S.; Adhikari, L.;
Velli, M.
Bibcode: 2019JPlPh..85f2003H
Altcode: 2019arXiv190109360H
In Part 2 of our guide to collisionless fluid models, we concentrate
on Landau fluid closures. These closures were pioneered by Hammett
and Perkins and allow for the rigorous incorporation of collisionless
Landau damping into a fluid framework. It is Landau damping that sharply
separates traditional fluid models and collisionless kinetic theory,
and is the main reason why the usual fluid models do not converge to the
kinetic description, even in the long-wavelength low-frequency limit. We
start with a brief introduction to kinetic theory, where we discuss in
detail the plasma dispersion function Z(ζ), and the associated plasma
response function R(ζ)=1+ζZ(ζ)=-Z^' }(ζ)/2. We then consider a
one-dimensional (1-D) (electrostatic) geometry and make a significant
effort to map all possible Landau fluid closures that can be constructed
at the fourth-order moment level. These closures for parallel moments
have general validity from the largest astrophysical scales down to
the Debye length, and we verify their validity by considering examples
of the (proton and electron) Landau damping of the ion-acoustic mode,
and the electron Landau damping of the Langmuir mode. We proceed by
considering 1-D closures at higher-order moments than the fourth order,
and as was concluded in Part 1, this is not possible without Landau
fluid closures. We show that it is possible to reproduce linear
Landau damping in the fluid framework to any desired precision,
thus showing the convergence of the fluid and collisionless kinetic
descriptions. We then consider a 3-D (electromagnetic) geometry in the
gyrotropic (long-wavelength low-frequency) limit and map all closures
that are available at the fourth-order moment level. In appendix Ae
provide comprehensive tables with Padé approximants of R(ζ) up to
the eighth-pole order, with many given in an analytic form.
Title: Science Requirement Document (SRD) for the European Solar
Telescope (EST) (2nd edition, December 2019)
Authors: Schlichenmaier, R.; Bellot Rubio, L. R.; Collados, M.;
Erdelyi, R.; Feller, A.; Fletcher, L.; Jurcak, J.; Khomenko, E.;
Leenaarts, J.; Matthews, S.; Belluzzi, L.; Carlsson, M.; Dalmasse,
K.; Danilovic, S.; Gömöry, P.; Kuckein, C.; Manso Sainz, R.;
Martinez Gonzalez, M.; Mathioudakis, M.; Ortiz, A.; Riethmüller,
T. L.; Rouppe van der Voort, L.; Simoes, P. J. A.; Trujillo Bueno,
J.; Utz, D.; Zuccarello, F.
Bibcode: 2019arXiv191208650S
Altcode:
The European Solar Telescope (EST) is a research infrastructure
for solar physics. It is planned to be an on-axis solar telescope
with an aperture of 4 m and equipped with an innovative suite of
spectro-polarimetric and imaging post-focus instrumentation. The EST
project was initiated and is driven by EAST, the European Association
for Solar Telescopes. EAST was founded in 2006 as an association
of 14 European countries. Today, as of December 2019, EAST consists
of 26 European research institutes from 18 European countries. The
Preliminary Design Phase of EST was accomplished between 2008 and
2011. During this phase, in 2010, the first version of the EST Science
Requirement Document (SRD) was published. After EST became a project
on the ESFRI roadmap 2016, the preparatory phase started. The goal
of the preparatory phase is to accomplish a final design for the
telescope and the legal governance structure of EST. A major milestone
on this path is to revisit and update the Science Requirement Document
(SRD). The EST Science Advisory Group (SAG) has been constituted by
EAST and the Board of the PRE-EST EU project in November 2017 and has
been charged with the task of providing with a final statement on the
science requirements for EST. Based on the conceptual design, the SRD
update takes into account recent technical and scientific developments,
to ensure that EST provides significant advancement beyond the current
state-of-the-art. The present update of the EST SRD has been developed
and discussed during a series of EST SAG meetings. The SRD develops
the top-level science objectives of EST into individual science
cases. Identifying critical science requirements is one of its main
goals. Those requirements will define the capabilities of EST and the
post-focus instrument suite. The technical requirements for the final
design of EST will be derived from the SRD.
Title: Fast-to-Alfvén Mode Conversion and Ambipolar Heating in
Structured Media. I. Simplified Cold Plasma Model
Authors: Cally, Paul S.; Khomenko, Elena
Bibcode: 2019ApJ...885...58C
Altcode:
Linear fast magnetoacoustic waves are introduced into a cold
stratified plasma model made up of a doubly periodic ensemble of
straight diminished-Alfvén-speed tubes. Coupling between fast and
Alfvén waves is produced by stratification and the tube structures,
and scattering is strong for wavelengths comparable to the inter-tube
separation. Ambipolar diffusion is found to be enhanced by the
structuring and is potentially significant at high frequencies. The
production of kink waves is discussed and modeled. It is found that
the tube structure significantly alters the wave energy reaching the
corona and the form that it takes, even for moderate fast-slow tube
contrast, with Alfvén waves becoming prominent.
Title: Fast-to-Alfvén Mode Conversion and Ambipolar Heating in
Structured Media. II. Numerical Simulation
Authors: Khomenko, Elena; Cally, Paul S.
Bibcode: 2019ApJ...883..179K
Altcode:
This paper studies the effectiveness of production of Alfvén waves
in the solar atmosphere through the processes of mode conversion,
taking into account several new effects that have not been considered
before. We perform simulations of wave propagation and conversion from
the photosphere to the corona. Both magnetic field and plasma parameters
are structured in the form of small-scale flux tubes with characteristic
scale significantly below the wavelength of the waves. The waves are
allowed to dissipate through the ambipolar diffusion (AD) mechanism. We
use an analytical magneto-static equilibrium model, which provides
the AD coefficient values at the lower end of what is expected for
the quiet solar regions. This work extends the simplified study of
mode conversion by Cally and Cally & Khomenko to the case of warm,
partially ionized, and structured plasma. We conclude that interaction
of waves with the flux tube ensemble produces a discrete spectrum of
high-order harmonics. The scattering is a linear process: however,
the nonlinear effects have considerable influence upon the amplitudes
of the harmonics. The magnetic Poynting flux reaching the corona is
enhanced by about 35% and the reflection of waves at the transition
region is decreased by about 50% when the flux tubes structure is
present, relative to the horizontally homogeneous case. The energy
flux of Alfvén waves exceeds that of acoustic waves at coronal
heights. Ambipolar diffusion decreases the magnetic Poynting flux in
the corona because the fast waves entering the transformation region
at chromospheric heights are degraded and have lower amplitudes. The
effect of the enhancement of Alfvén wave production due to interaction
with flux tubes is independent of the numerical resolution, while the
effect of the AD is resolution-dependent and is not converged at the
10 km resolution of our best simulations.
Title: Two-fluid simulations of waves in the solar
chromosphere. II. Propagation and damping of fast magneto-acoustic
waves and shocks
Authors: Popescu Braileanu, B.; Lukin, V. S.; Khomenko, E.; de
Vicente, Á.
Bibcode: 2019A&A...630A..79P
Altcode: 2019arXiv190805262P
Waves and shocks traveling through the solar chromospheric plasma are
influenced by its partial ionization and weak collisional coupling, and
may become susceptible to multi-fluid effects, similar to interstellar
shock waves. In this study, we consider fast magneto-acoustic shock wave
formation and propagation in a stratified medium, that is permeated by
a horizontal magnetic field, with properties similar to that of the
solar chromosphere. The evolution of plasma and neutrals is modeled
using a two-fluid code that evolves a set of coupled equations for
two separate fluids. We observed that waves in neutrals and plasma,
initially coupled at the upper photosphere, become uncoupled at higher
heights in the chromosphere. This decoupling can be a consequence
of either the characteristic spatial scale at the shock front,
that becomes similar to the collisional scale, or the change in
the relation between the wave frequency, ion cyclotron frequency,
and the collisional frequency with height. The decoupling height is
a sensitive function of the wave frequency, wave amplitude, and the
magnetic field strength. We observed that decoupling causes damping
of waves and an increase in the background temperature due to the
frictional heating. The comparison between analytical and numerical
results allows us to separate the role of the nonlinear effects from
the linear ones on the decoupling and damping of waves.
Title: Two-fluid simulations of waves in the solar
chromosphere. I. Numerical code verification
Authors: Popescu Braileanu, B.; Lukin, V. S.; Khomenko, E.; de
Vicente, Á.
Bibcode: 2019A&A...627A..25P
Altcode: 2019arXiv190503559P
Solar chromosphere consists of a partially ionized plasma, which
makes modeling the solar chromosphere a particularly challenging
numerical task. Here we numerically model chromospheric waves using
a two-fluid approach with a newly developed numerical code. The
code solves two-fluid equations of conservation of mass, momentum,
and energy, together with the induction equation for the case of the
purely hydrogen plasma with collisional coupling between the charged
and neutral fluid components. The implementation of a semi-implicit
algorithm allows us to overcome the numerical stability constraints due
to the stiff collisional terms. We test the code against analytical
solutions of acoustic and Alfvén wave propagation in uniform medium
in several regimes of collisional coupling. The results of our
simulations are consistent with the analytical estimates, and with
other results described in the literature. In the limit of a large
collisional frequency, the waves propagate with a common speed of a
single fluid. In the other limit of a vanishingly small collisional
frequency, the Alfvén waves propagate with an Alfvén speed of the
charged fluid only, while the perturbation in neutral fluid is very
small. The acoustic waves in these limits propagate with the sound
speed corresponding to either the charges or the neutrals, while the
perturbation in the other fluid component is negligible. Otherwise,
when the collision frequency is similar to the real part of the
wave frequency, the interaction between charges and neutrals through
momentum-transfer collisions cause alterations of the waves frequencies
and damping of the wave amplitudes.
Title: Spiral-shaped wavefronts in a sunspot umbra
Authors: Felipe, T.; Kuckein, C.; Khomenko, E.; Thaler, I.
Bibcode: 2019A&A...621A..43F
Altcode: 2018arXiv181011257F
Context. Solar active regions show a wide variety of oscillatory
phenomena. The presence of the magnetic field leads to the appearance
of several wave modes whose behavior is determined by the sunspot
thermal and magnetic structure.
Aims: We aim to study the
relation between the umbral and penumbral waves observed at the high
photosphere and the magnetic field topology of the sunspot.
Methods: Observations of the sunspot in active region NOAA 12662
obtained with the GREGOR telescope (Observatorio del Teide, Tenerife,
Spain) were acquired on 2017 June 17. The data set includes a temporal
series in the Fe I 5435 Å line obtained with the imaging spectrograph
GREGOR Fabry-Pérot Interferometer (GFPI) and a spectropolarimetric
raster map acquired with the GREGOR Infrared Spectrograph (GRIS)
in the 10 830 Å spectral region. The Doppler velocity deduced from
the restored Fe I 5435 Å line has been determined, and the magnetic
field vector of the sunspot has been inferred from spectropolarimetric
inversions of the Ca I 10 839 Å and the Si I 10 827 Å lines.
Results: A two-armed spiral wavefront has been identified in the
evolution of the two-dimensional velocity maps from the Fe I 5435 Å
line. The wavefronts initially move counterclockwise in the interior
of the umbra, and develop into radially outward propagating running
penumbral waves when they reach the umbra-penumbra boundary. The
horizontal propagation of the wavefronts approximately follows the
direction of the magnetic field, which shows changes in the magnetic
twist with height and horizontal position.
Conclusions:
The spiral wavefronts are interpreted as the visual pattern of slow
magnetoacoustic waves which propagate upward along magnetic field
lines. Their apparent horizontal propagation is due to their sequential
arrival to different horizontal positions at the formation height of the
Fe I 5435 Å line, as given by the inclination and orientation of the
magnetic field. The movie associated to Fig. 2 is available at https://www.aanda.org
Title: Fast-to-Alfvén Mode Conversion Mediated by Hall
Current. II. Application to the Solar Atmosphere
Authors: González-Morales, P. A.; Khomenko, E.; Cally, P. S.
Bibcode: 2019ApJ...870...94G
Altcode: 2018arXiv181106565G
Coupling between fast magnetoacoustic and Alfvén waves can be observed
in fully ionized plasmas mediated by stratification and 3D geometrical
effects. In Paper I, Cally & Khomenko have shown that in a weakly
ionized plasma, such as the solar photosphere and chromosphere, the
Hall current introduces a new coupling mechanism. The present study
extends the results from Paper I to the case of warm plasma. We report
on numerical experiments where mode transformation is studied using
quasi-realistic stratification in thermodynamic parameters resembling
the solar atmosphere. This redresses the limitation of the cold plasma
approximation assumed in Paper I, in particular allowing the complete
process of coupling between fast and slow magnetoacoustic modes and
subsequent coupling of the fast mode to the Alfvén mode through
the Hall current. Our results confirm the efficacy of the mechanism
proposed in Paper I for the solar case. We observe that the efficiency
of the transformation is a sensitive function of the angle between
the wave propagation direction and the magnetic field, and of the
wave frequency. The efficiency increases when the field direction and
the wave direction are aligned for increasing wave frequencies. After
scaling our results to typical solar values, the maximum amplitude of
the transformed Alfvén waves, for a frequency of 1 Hz, corresponds
to an energy flux (measured above the height of peak Hall coupling)
of ∼103 W m-2, based on an amplitude of 500
m s-1 at β = 1, which is sufficient to play a major role
in both quiet and active region coronal heating.
Title: Three-dimensional simulations of solar magneto-convection
including effects of partial ionization
Authors: Khomenko, E.; Vitas, N.; Collados, M.; de Vicente, A.
Bibcode: 2018A&A...618A..87K
Altcode: 2018arXiv180701061K
In recent decades, REALISTIC three-dimensional
radiative-magnetohydrodynamic simulations have become the dominant
theoretical tool for understanding the complex interactions between the
plasma and magnetic field on the Sun. Most of such simulations are based
on approximations of magnetohydrodynamics, without directly considering
the consequences of the very low degree of ionization of the solar
plasma in the photosphere and bottom chromosphere. The presence of a
large amount of neutrals leads to a partial decoupling of the plasma and
magnetic field. As a consequence, a series of non-ideal effects, i.e.,
the ambipolar diffusion, Hall effect, and battery effect, arise. The
ambipolar effect is the dominant in the solar chromosphere. We
report on the first three-dimensional realistic simulations
of magneto-convection including ambipolar diffusion and battery
effects. The simulations are carried out using the newly developed
MANCHA3Dcode. Our results reveal that ambipolar diffusion causes
measurable effects on the amplitudes of waves excited by convection
in the simulations, on the absorption of Poynting flux and heating,
and on the formation of chromospheric structures. We provide a low
limit on the chromospheric temperature increase owing to the ambipolar
effect using the simulations with battery-excited dynamo fields. The movies associated to Figs. 16 and 17 are available at https://www.aanda.org
Title: MHDSTS: a new explicit numerical scheme for simulations of
partially ionised solar plasma
Authors: González-Morales, P. A.; Khomenko, E.; Downes, T. P.;
de Vicente, A.
Bibcode: 2018A&A...615A..67G
Altcode: 2018arXiv180304891G
The interaction of plasma with magnetic field in the partially
ionised solar atmosphere is frequently modelled via a single-fluid
approximation, which is valid for the case of a strongly coupled
collisional media, such as solar photosphere and low chromosphere. Under
the single-fluid formalism the main non-ideal effects are described
by a series of extra terms in the generalised induction equation
and in the energy conservation equation. These effects are: Ohmic
diffusion, ambipolar diffusion, the Hall effect, and the Biermann
battery effect. From the point of view of the numerical solution of
the single-fluid equations, when ambipolar diffusion or Hall effects
dominate can introduce severe restrictions on the integration time step
and can compromise the stability of the numerical scheme. In this paper
we introduce two numerical schemes to overcome those limitations. The
first of them is known as super time-stepping (STS) and it is designed
to overcome the limitations imposed when the ambipolar diffusion term
is dominant. The second scheme is called the Hall diffusion scheme
(HDS) and it is used when the Hall term becomes dominant. These two
numerical techniques can be used together by applying Strang operator
splitting. This paper describes the implementation of the STS and HDS
schemes in the single-fluid code MANCHA3D. The validation for each of
these schemes is provided by comparing the analytical solution with
the numerical one for a suite of numerical tests.
Title: Partially Ionized Plasmas in Astrophysics
Authors: Ballester, José Luis; Alexeev, Igor; Collados, Manuel;
Downes, Turlough; Pfaff, Robert F.; Gilbert, Holly; Khodachenko,
Maxim; Khomenko, Elena; Shaikhislamov, Ildar F.; Soler, Roberto;
Vázquez-Semadeni, Enrique; Zaqarashvili, Teimuraz
Bibcode: 2018SSRv..214...58B
Altcode: 2017arXiv170707975B
Partially ionized plasmas are found across the Universe in many
different astrophysical environments. They constitute an essential
ingredient of the solar atmosphere, molecular clouds, planetary
ionospheres and protoplanetary disks, among other environments, and
display a richness of physical effects which are not present in fully
ionized plasmas. This review provides an overview of the physics of
partially ionized plasmas, including recent advances in different
astrophysical areas in which partial ionization plays a fundamental
role. We outline outstanding observational and theoretical questions
and discuss possible directions for future progress.
Title: Fast-to-Alfvén Mode Conversion in the Presence of Ambipolar
Diffusion
Authors: Cally, Paul S.; Khomenko, Elena
Bibcode: 2018ApJ...856...20C
Altcode:
It is known that fast magnetohydrodynamic waves partially convert to
upward and/or downward propagating Alfvén waves in a stratified
atmosphere where Alfvén speed increases with height. This
happens around the fast wave reflection height, where the fast
wave’s horizontal phase speed equals the Alfvén speed (in a
low-β plasma). Typically, this takes place in the mid to upper
solar chromosphere for low-frequency waves in the few-millihertz
band. However, this region is weakly ionized and thus susceptible to
nonideal MHD processes. In this article, we explore how ambipolar
diffusion in a zero-β plasma affects fast waves injected from
below. Classical ambipolar diffusion is far too weak to have any
significant influence at these low frequencies, but if enhanced
by turbulence (in the quiet-Sun chromosphere but not in sunspot
umbrae) or the production of sufficiently small-scale structure,
can substantially absorb waves for turbulent ambipolar Reynolds
numbers of around 20 or less. In that case, it is found that the mode
conversion process is not qualitatively altered from the ideal case,
though conversion to Alfvén waves is reduced because the fast wave
flux reaching the conversion region is degraded. It is also found
that any upward propagating Alfvén waves generated in this process
are almost immune to further ambipolar attenuation, thereby reducing
local ambipolar heating compared to cases without mode conversion. In
that sense, mode conversion provides a form of “Alfvén cooling.”
Title: Rayleigh-Taylor instabilities with sheared magnetic fields
in partially ionised plasmas
Authors: Ruderman, M. S.; Ballai, I.; Khomenko, E.; Collados, M.
Bibcode: 2018A&A...609A..23R
Altcode: 2017A&A...609A..23R
Aims: In the present study we investigate the nature of
the magnetic Rayleigh-Taylor instability appearing at a tangential
discontinuity in a partially ionised plasma when the effect of magnetic
shear is taken into account.
Methods: The partially ionised
character of the plasma is described by the ambipolar diffusion in
the induction equation. The dynamics of the plasma is investigated
in a single-fluid approximation. After matching the solutions on both
sides of the interface we derive a dispersion equation and calculate
the instability increment using analytical methods for particular
cases of parameters, and numerical investigation for a wide range
of parameters.
Results: We calculated the dependence of
the instability increment on the perturbation wavenumber. We also
calculated the dependence of the maximum instability increment on the
shear angle of the magnetic field for various values of the ionisation
degree.
Conclusions: Our results show that the Rayleigh-Taylor
instability becomes sensitive to the degree of plasma ionisation only
for plasmas with small values of plasma beta and in a very weakly
ionised state. Perturbations are unstable only for those wavenumbers
that are below a cut-off value.
Title: Signatures of the impact of flare-ejected plasma on the
photosphere of a sunspot light bridge
Authors: Felipe, T.; Collados, M.; Khomenko, E.; Rajaguru, S. P.;
Franz, M.; Kuckein, C.; Asensio Ramos, A.
Bibcode: 2017A&A...608A..97F
Altcode: 2017arXiv170806133F
Aims: We investigate the properties of a sunspot light bridge,
focusing on the changes produced by the impact of a plasma blob ejected
from a C-class flare.
Methods: We observed a sunspot in active
region NOAA 12544 using spectropolarimetric raster maps of the four
Fe I lines around 15 655 Å with the GREGOR Infrared Spectrograph,
narrow-band intensity images sampling the Fe I 6173 Å line with
the GREGOR Fabry-Pérot Interferometer, and intensity broad-band
images in G-band and Ca II H-band with the High-resolution Fast
Imager. All these instruments are located at the GREGOR telescope at
the Observatorio del Teide, Tenerife, Spain. The data cover the time
before, during, and after the flare event. The analysis is complemented
with Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager
data from the Solar Dynamics Observatory. The physical parameters of
the atmosphere at differents heights were inferred using spectral-line
inversion techniques.
Results: We identify photospheric and
chromospheric brightenings, heating events, and changes in the Stokes
profiles associated with the flare eruption and the subsequent arrival
of the plasma blob to the light bridge, after traveling along an
active region loop.
Conclusions: The measurements suggest that
these phenomena are the result of reconnection events driven by the
interaction of the plasma blob with the magnetic field topology of the
light bridge. Movies attached to Figs. 1 and 3 are available at http://www.aanda.org
Title: Flux Tube: Solar model
Authors: Khomenko, E.
Bibcode: 2017ascl.soft12010K
Altcode:
Flux Tube is a nonlinear, two-dimensional, numerical simulation of
magneto-acoustic wave propagation in the photosphere and chromosphere
of small-scale flux tubes with internal structure. Waves with realistic
periods of three to five minutes are studied, after horizontal and
vertical oscillatory perturbations are applied to the equilibrium
model. Spurious reflections of shock waves from the upper boundary
are minimized by a special boundary condition.
Title: Numerical simulations of quiet Sun magnetic fields seeded by
the Biermann battery
Authors: Khomenko, E.; Vitas, N.; Collados, M.; de Vicente, A.
Bibcode: 2017A&A...604A..66K
Altcode: 2017arXiv170606037K
The magnetic fields of the quiet Sun cover at any time more than
90% of its surface and their magnetic energy budget is crucial to
explain the thermal structure of the solar atmosphere. One of the
possible origins of these fields is the action of the local dynamo
in the upper convection zone of the Sun. Existing simulations of the
local solar dynamo require an initial seed field and sufficiently
high spatial resolution in order to achieve the amplification of the
seed field to the observed values in the quiet Sun. Here we report
an alternative model of seeding based on the action of the Bierman
battery effect. This effect generates a magnetic field due to the
local imbalances in electron pressure in the partially ionized solar
plasma. We show that the battery effect self-consistently creates from
zero an initial seed field of a strength of the order of micro G, and
together with dynamo amplification allows the generation of quiet Sun
magnetic fields of a similar strength to those from solar observations.
Title: High-frequency waves in the corona due to null points
Authors: Santamaria, I. C.; Khomenko, E.; Collados, M.; de Vicente, A.
Bibcode: 2017A&A...602A..43S
Altcode: 2017arXiv170406551S
This work aims to understand the behavior of non-linear waves in
the vicinity of a coronal null point. In previous works we have
shown that high-frequency waves are generated in such a magnetic
configuration. This paper studies those waves in detail in order to
provide a plausible explanation of their generation. We demonstrate
that slow magneto-acoustic shock waves generated in the chromosphere
propagate through the null point and produce a train of secondary shocks
that escape along the field lines. A particular combination of the
shock wave speeds generates waves at a frequency of 80 mHz. We speculate
that this frequency may be sensitive to the atmospheric parameters in
the corona and therefore can be used to probe the structure of this
solar layer. Movies attached to Figs 2 and 4 are available at http://www.aanda.org
Title: Dependence of sunspot photospheric waves on the depth of the
source of solar p-modes
Authors: Felipe, T.; Khomenko, E.
Bibcode: 2017A&A...599L...2F
Altcode: 2017arXiv170200997F
Photospheric waves in sunspots moving radially outward at speeds faster
than the characteristic wave velocities have been recently detected. It
has been suggested that they are the visual pattern of p-modes excited
around 5 Mm beneath the sunspot's surface. Using numerical simulations,
we performed a parametric study of the waves observed at the photosphere
and higher layers that were produced by sources located at different
depths beneath the sunspot's surface. The observational measurements
are consistent with waves driven between approximately 1 Mm and 5 Mm
below the sunspot's surface.
Title: On the effects of ion-neutral interactions in solar plasmas
Authors: Khomenko, Elena
Bibcode: 2017PPCF...59a4038K
Altcode: 2016arXiv161106063K
Solar photosphere and chromosphere are composed of weakly ionized plasma
for which collisional coupling decreases with height. This implies a
breakdown of some hypotheses underlying magnetohydrodynamics at low
altitudes and gives rise to non-ideal MHD effects such as ambipolar
diffusion, Hall effect, etc. Recently, there has been progress in
understanding the role of these effects for the dynamics and energetics
of the solar atmosphere. There are evidences that phenomena such as
wave propagation and damping, magnetic reconnection, formation of
stable magnetic field concentrations, magnetic flux emergence, etc
can be affected. This paper reviews the current state-of-the-art of
multi-fluid MHD modeling of the coupled solar atmosphere.
Title: The role of partial ionization in solar chromospheric heating
Authors: Shelyag, S.; Khomenko, E.; Przybylski, D.; Vitas, N.; de
Vicente, A.
Bibcode: 2016AGUFMSH21E2565S
Altcode:
The most energetic part of the Sun, its interior, due to its plasma
parameters is hidden below the solar surface and invisible to the
observer. Nevertheless, the solar interior generates the energy and
provokes atmospheric magnetic activity. Despite great progress in
both observational and simulational methods, the mechanism of energy
transport from the solar convection zone into the upper atmosphere,
and the upper-atmospheric heating mechanism remain the main unresolved
problems in solar and stellar structure. In this presentation, we
analyse the role of non-ideal plasma effects and partial ionization in
the solar atmospheric energy transport and chromospheric heating. Using
numerical magneto-hydrodynamic modelling we create detailed models
of magnetic flux tubes and realistic simulations of the coupled
solar interior and atmosphere with different levels of magnetic
activity, which take into account the effects of partial ionisation
and ion-neutral interaction in the solar atmospheric plasma. We show
that compressible and incompressible oscillations in solar magnetic
fields, indeed, are able to provide sufficient energy to compensate
chromospheric radiative losses. Detailed radiative diagnostics of the
simulated models is carried out to create a link between the simulations
and observational data. This gives an opportunity to directly compare
the simulation results with modern solar observations.
Title: Three-dimensional structure of a sunspot light bridge
Authors: Felipe, T.; Collados, M.; Khomenko, E.; Kuckein, C.; Asensio
Ramos, A.; Balthasar, H.; Berkefeld, T.; Denker, C.; Feller, A.;
Franz, M.; Hofmann, A.; Joshi, J.; Kiess, C.; Lagg, A.; Nicklas, H.;
Orozco Suárez, D.; Pastor Yabar, A.; Rezaei, R.; Schlichenmaier,
R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki,
S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.;
von der Lühe, O.; Waldmann, T.
Bibcode: 2016A&A...596A..59F
Altcode: 2016arXiv161104803F
Context. Active regions are the most prominent manifestations of solar
magnetic fields; their generation and dissipation are fundamental
problems in solar physics. Light bridges are commonly present during
sunspot decay, but a comprehensive picture of their role in the
removal of the photospheric magnetic field is still lacking.
Aims: We study the three-dimensional configuration of a sunspot,
and in particular, its light bridge, during one of the last stages of
its decay.
Methods: We present the magnetic and thermodynamical
stratification inferred from full Stokes inversions of the photospheric
Si I 10 827 Å and Ca I 10 839 Å lines obtained with the GREGOR
Infrared Spectrograph of the GREGOR telescope at the Observatorio del
Teide, Tenerife, Spain. The analysis is complemented by a study of
continuum images covering the disk passage of the active region, which
are provided by the Helioseismic and Magnetic Imager on board the Solar
Dynamics Observatory.
Results: The sunspot shows a light bridge
with penumbral continuum intensity that separates the central umbra from
a smaller umbra. We find that in this region the magnetic field lines
form a canopy with lower magnetic field strength in the inner part. The
photospheric light bridge is dominated by gas pressure (high-β),
as opposed to the surrounding umbra, where the magnetic pressure
is higher. A convective flow is observed in the light bridge. This
flow is able to bend the magnetic field lines and to produce field
reversals. The field lines merge above the light bridge and become
as vertical and strong as in the surrounding umbra. We conclude that
this occurs because two highly magnetized regions approach each other
during the sunspot evolution. Movies associated to Figs. 2 and 13
are available at http://www.aanda.org
Title: Tracing p-mode Waves from the Photosphere to the Corona in
Active Regions
Authors: Zhao, Junwei; Felipe, Tobías; Chen, Ruizhu; Khomenko, Elena
Bibcode: 2016ApJ...830L..17Z
Altcode:
Atmosphere above sunspots is abundant with different types
of waves. Among these waves are running penumbral waves in the
chromosphere, quasi-periodic oscillations in the lower coronal loops,
and recently reported running waves in sunspots’ photosphere, all of
which were interpreted as magnetoacoustic waves by some authors. Are
these waves in different atmospheric layers related to each other,
what is the nature of these waves, and where are the ultimate sources
of these waves? Applying a time-distance helioseismic analysis over a
suite of multi-wavelength observations above a sunspot, we demonstrate
that the helioseismic p-mode waves are able to channel up from the
photosphere through the chromosphere and transition region into the
corona, and that the magnetoacoustic waves observed in different
atmospheric layers are a same wave originating from the photosphere
but exhibiting differently under different physical conditions. We
also show waves of different frequencies travel along different
paths, which can be used to derive the physical properties of the
atmosphere above sunspots. Our numerical simulation of traveling
of waves from a subphotospheric source qualitatively resembles the
observed properties of the waves and offers an interpretation of the
shapes of the wavefronts above the photosphere.
Title: Numerical simulations of magnetic Kelvin-Helmholtz instability
at a twisted solar flux tube
Authors: Murawski, K.; Chmielewski, P.; Zaqarashvili, T. V.;
Khomenko, E.
Bibcode: 2016MNRAS.459.2566M
Altcode: 2016MNRAS.tmp..632M; 2016MNRAS.tmp..596M
The paper aims to study the response of a solar small-scale and weak
magnetic flux tube to photospheric twisting motions. We numerically
solve three-dimensional ideal magnetohydrodynamic equations to describe
the evolution of the perturbation within the initially static flux tube,
excited by twists in the azimuthal component of the velocity. These
twists produce rotation of the magnetic field lines. Perturbation
of magnetic field lines propagates upwardly, driving vertical and
azimuthal flow as well as plasma compressions and rarefactions in the
form of eddies. We conclude that these eddies result from the sheared
azimuthal flow which seeds Kelvin-Helmholtz instability (KHI) between
the flux tube and the ambient medium. Numerically obtained properties
of the KHI confirm the analytical predictions for the occurrence of
the instability.
Title: Observational Detection of Drift Velocity between Ionized
and Neutral Species in Solar Prominences
Authors: Khomenko, Elena; Collados, Manuel; Díaz, Antonio J.
Bibcode: 2016ApJ...823..132K
Altcode: 2016arXiv160401177K
We report the detection of differences in the ion and neutral velocities
in prominences using high-resolution spectral data obtained in 2012
September at the German Vacuum Tower Telescope (Observatorio del
Teide, Tenerife). A time series of scans of a small portion of a solar
prominence was obtained simultaneously with high cadence using the lines
of two elements with different ionization states, namely, Ca II 8542
Å and He I 10830 Å. The displacements, widths, and amplitudes of both
lines were carefully compared to extract dynamical information about the
plasma. Many dynamical features are detected, such as counterstreaming
flows, jets, and propagating waves. In all of the cases, we find a
very strong correlation between the parameters extracted from the
lines of both elements, confirming that both lines trace the same
plasma. Nevertheless, we also find short-lived transients where this
correlation is lost. These transients are associated with ion-neutral
drift velocities of the order of several hundred m s-1. The
patches of non-zero drift velocity show coherence in time-distance
diagrams.
Title: Vortex waves in sunspots
Authors: López Ariste, A.; Centeno, R.; Khomenko, E.
Bibcode: 2016A&A...591A..63L
Altcode:
Context. Waves in the magnetized solar atmosphere are one of the
favourite means of transferring and depositing energy into the solar
corona. The study of waves brings information not just on the dynamics
of the magnetized plasma, but also on the possible ways in which the
corona is heated.
Aims: The identification and analysis of the
phase singularities or dislocations provide us with a complementary
approach to the magnetoacoustic and Aflvén waves propagating in the
solar atmosphere. They allow us to identify individual wave modes,
shedding light on the probability of excitation or the nature of the
triggering mechanism.
Methods: We use a time series of Doppler
shifts measured in two spectral lines, filtered around the three-minute
period region. The data show a propagating magnetoacoustic slow
mode with several dislocations and, in particular, a vortex line. We
study under what conditions the different wave modes propagating in
the umbra can generate the observed dislocations.
Results:
The observed dislocations can be fully interpreted as a sequence
of sausage and kink modes excited sequentially on average during
15 min. Kink and sausage modes appear to be excited independently
and sequentially. The transition from one to the other lasts less
than three minutes. During the transition we observe and model the
appearance of superoscillations inducing large phase gradients and
phase mixing.
Conclusions: The analysis of the observed wave
dislocations leads us to the identification of the propagating wave
modes in umbrae. The identification in the data of superoscillatory
regions during the transition from one mode to the other may be an
important indicator of the location of wave dissipation.
Title: The possible origin of facular brightness in the solar
atmosphere
Authors: Kostik, R.; Khomenko, E.
Bibcode: 2016A&A...589A...6K
Altcode: 2016arXiv160203369K
This paper studies the dependence of the Ca II H line core brightness
on the strength and inclination of the photospheric magnetic field, and
on the parameters of convective and wave motions in a facular region
at the center of the solar disc. We use three simultaneous data sets
that were obtained at the German Vacuum Tower Telescope (Observatorio
del Teide, Tenerife): (1) spectra of Ba II 4554 Å line, registered
with the instrument TESOS to measure the variations of intensity
and velocity through the photosphere up to the temperature minimum;
(2) spectropolarimetric data in Fe I 1.56 μm lines (registered with
the instrument TIP II) to measure photospheric magnetic fields;
(3) filtergrams in Ca II H that give information about brightness
fluctuations in the chromosphere. The results show that the Ca II H
brightness in the facula strongly depends on the power of waves with
periods in the 5-min range, which propagate upwards, and also on the
phase shift between velocity oscillations at the bottom photosphere
and around the temperature minimum height that is measured from Ba
II line. The Ca II H brightness is maximum at locations where the
phase shift between temperature and velocity oscillations lies within
0°-100°. There is an indirect influence of convective motions on the
Ca II H brightness. The higher the amplitude of convective velocities
is and the greater the height is where they change their direction
of motion, the brighter the facula. In summary, our results lead
to conclusions that facular regions appear bright not only because
of the Wilson depression in magnetic structures, but also owing to
real heating.
Title: Tracing Helioseismic Waves from the Photosphere to the Corona
Authors: Zhao, Junwei; Felipe, Tobias; Chen, Ruizhu; Khomenko, Elena
Bibcode: 2016SPD....4730307Z
Altcode:
Can p-mode waves in sunspots propagate to the chromosphere and the
corona? And what are their counterparts in different atmospheric
heights? In order to study the connection between the photospheric
p-mode waves and the waves observed above the photosphere, we use a
helioseismic analysis technique, namely time-distance helioseismology,
and analyze multi-height observations from different instruments. We
find clear evidences that some p-mode waves in the photosphere, running
penumbral waves in the chromosphere, and the periodic disturbances in
the coronal fan structures are actually same magnetoacoustic waves that
exhibit differently at the different atmospheric heights. The 6-mHz
waves, with inclined wavefronts, propagate slantingly upward along
magnetic field lines. The 3-mHz waves, forming backward-'C'-shape
wavefronts, propagate mostly horizontally. Through numerical
simulations, we demonstrate that these p-mode waves that can travel
upward to the corona, possibly originate from sources located a few
megameters beneath sunspots' surface.
Title: Simulated interaction of magnetohydrodynamic shock waves with
a complex network-like region
Authors: Santamaria, I. C.; Khomenko, E.; Collados, M.; de Vicente, A.
Bibcode: 2016A&A...590L...3S
Altcode: 2016arXiv160408783S
We provide estimates of the wave energy reaching the solar chromosphere
and corona in a network-like magnetic field topology, including a
coronal null point. The waves are excited by an instantaneous strong
subphotospheric source and propagate through the subphotosphere,
photosphere, chromosphere, transition region, and corona with the
plasma beta and other atmospheric parameters varying by several
orders of magnitude. We compare two regimes of the wave propagation:
a linear and nonlinear regime. While the amount of energy reaching
the corona is similar in both regimes, this energy is transmitted at
different frequencies. In both cases the dominant periods of waves
at each height strongly depend on the local magnetic field topology,
but this distribution is only in accordance with observations in the
nonlinear case. Movies are available in electronic form at http://www.aanda.org
Title: Heating of the Partially Ionized Solar Chromosphere by Waves
in Magnetic Structures
Authors: Shelyag, S.; Khomenko, E.; de Vicente, A.; Przybylski, D.
Bibcode: 2016ApJ...819L..11S
Altcode: 2016arXiv160203373S
In this paper, we show a “proof of concept” of the heating
mechanism of the solar chromosphere due to wave dissipation caused by
the effects of partial ionization. Numerical modeling of non-linear wave
propagation in a magnetic flux tube, embedded in the solar atmosphere,
is performed by solving a system of single-fluid quasi-MHD equations,
which take into account the ambipolar term from the generalized Ohm’s
law. It is shown that perturbations caused by magnetic waves can be
effectively dissipated due to ambipolar diffusion. The energy input
by this mechanism is continuous and shown to be more efficient than
dissipation of static currents, ultimately leading to chromospheric
temperature increase in magnetic structures.
Title: On the Robustness of the Pendulum Model for Large-amplitude
Longitudinal Oscillations in Prominences
Authors: Luna, M.; Terradas, J.; Khomenko, E.; Collados, M.; de
Vicente, A.
Bibcode: 2016ApJ...817..157L
Altcode: 2015arXiv151205125L
Large-amplitude longitudinal oscillations (LALOs) in prominences are
spectacular manifestations of solar activity. In such events nearby
energetic disturbances induce periodic motions on filaments with
displacements comparable to the size of the filaments themselves and
with velocities larger than 20 {km} {{{s}}}-1. The pendulum
model, in which the gravity projected along a rigid magnetic field is
the restoring force, was proposed to explain these events. However,
it can be objected that in a realistic situation where the magnetic
field reacts to the mass motion of the heavy prominence, the simplified
pendulum model could be no longer valid. We have performed nonlinear
time-dependent numerical simulations of LALOs considering a dipped
magnetic field line structure. In this work we demonstrate that for
even relatively weak magnetic fields the pendulum model works very
well. We therefore validate the pendulum model and show its robustness,
with important implications for prominence seismology purposes. With
this model it is possible to infer the geometry of the dipped field
lines that support the prominence.
Title: Evershed flow observed in neutral and singly ionized iron lines
Authors: Khomenko, E.; Collados, M.; Shchukina, N.; Díaz, A.
Bibcode: 2015A&A...584A..66K
Altcode: 2015arXiv151000334K
The amplitudes of the Evershed flow are measured using pairs of
carefully selected Fe i and Fe ii spectral lines that are close in
wavelength and registered simultaneously. A sunspot belonging to the
NOAA 11582 group was scanned using the spectrograph of the German Vacuum
Tower Telescope (Observatorio del Teide, Tenerife). Velocities were
extracted from intensity profiles using the λ-meter technique. The
formation heights of the observed spectral lines were calculated using
semi-empirical models of a bright and dark penumbral filament taking
into account the sunspot location at the limb. Our objective is to
compare azimuthally averaged amplitudes of the Evershed flow extracted
from neutral and ion lines. We find measurable differences in the radial
component of the flow. All five pairs of lines show the same tendency;
the flow measured from the Fe i lines has an amplitude that is a few
hundred ms-1 larger than that of the Fe ii lines. This
tendency is preserved at all photospheric heights and radial distances
in the penumbra. We discuss the possible origin of this effect.
Title: Oscillations and Waves in Sunspots
Authors: Khomenko, Elena; Collados, Manuel
Bibcode: 2015LRSP...12....6K
Altcode:
A magnetic field modifies the properties of waves in a complex
way. Significant advances have been made recently in our understanding
of the physics of sunspot waves with the help of high-resolution
observations, analytical theories, as well as numerical simulations. We
review the current ideas in the field, providing the most coherent
picture of sunspot oscillations as by present understanding.
Title: Fast-to-Alfvén Mode Conversion Mediated by the Hall
Current. I. Cold Plasma Model
Authors: Cally, Paul S.; Khomenko, Elena
Bibcode: 2015ApJ...814..106C
Altcode: 2015arXiv151003927C
The photospheric temperature minimum in the Sun and solar-like
stars is very weakly ionized, with an ionization fraction f as
low as 10-4. In galactic star-forming regions, f can be
10-10 or lower. Under these circumstances, the Hall current
can couple low-frequency Alfvén and magnetoacoustic waves via the
dimensionless Hall parameter ɛ =ω /{{{Ω }}}{{i}}f,
where ω is the wave frequency and {{{Ω }}}{{i}} is the
mean ion gyrofrequency. This is analyzed in the context of a cold
(zero-β) plasma and in less detail for a warm plasma. It is found
that Hall coupling preferentially occurs where the wavevector is nearly
field-aligned. In these circumstances, Hall coupling in theory produces
a continual oscillation between fast and Alfvén modes as the wave
passes through the weakly ionized region. At low frequencies (mHz),
characteristic of solar and stellar normal modes, ɛ is probably too
small for more than a fraction of one oscillation to occur. On the
other hand, the effect may be significant at the far higher frequencies
(Hz) associated with magnetic reconnection events. In another context,
characteristic parameters for star-forming gas clouds suggest that
{O}(1) or more full oscillations may occur in one cloud crossing. This
mechanism is not expected to be effective in sunspots, due to their high
ion gyrofrequencies and Alfvén speeds, since the net effect depends
inversely on both and therefore inverse quadratically on field strength.
Title: On the Source of Propagating Slow Magnetoacoustic Waves
in Sunspots
Authors: Krishna Prasad, S.; Jess, D. B.; Khomenko, Elena
Bibcode: 2015ApJ...812L..15K
Altcode: 2015arXiv151003275K; 2015ApJ...812L..15P
Recent high-resolution observations of sunspot oscillations using
simultaneously operated ground- and space-based telescopes reveal
the intrinsic connection between different layers of the solar
atmosphere. However, it is not clear whether these oscillations are
externally driven or generated in situ. We address this question by
using observations of propagating slow magnetoacoustic waves along a
coronal fan loop system. In addition to the generally observed decreases
in oscillation amplitudes with distance, the observed wave amplitudes
are also found to be modulated with time, with similar variations
observed throughout the propagation path of the wave train. Employing
multi-wavelength and multi-instrument data, we study the amplitude
variations with time as the waves propagate through different layers of
the solar atmosphere. By comparing the amplitude modulation period in
different layers, we find that slow magnetoacoustic waves observed in
sunspots are externally driven by photospheric p-modes, which propagate
upward into the corona before becoming dissipated.
Title: On the nature of transverse coronal waves revealed by wavefront
dislocations
Authors: López Ariste, A.; Luna, M.; Arregui, I.; Khomenko, E.;
Collados, M.
Bibcode: 2015A&A...579A.127L
Altcode: 2015arXiv150503348L
Context. Coronal waves are an important aspect of the dynamics of the
plasma in the corona. Wavefront dislocations are topological features
of most waves in nature and also of magnetohydrodynamic waves. Are there
dislocations in coronal waves?
Aims: The finding and explanation
of dislocations may shed light on the nature and characteristics of the
propagating waves, their interaction in the corona, and in general on
the plasma dynamics.
Methods: We positively identify dislocations
in coronal waves observed by the Coronal Multi-channel Polarimeter
(CoMP) as singularities in the Doppler shifts of emission coronal
lines. We study the possible singularities that can be expected in
coronal waves and try to reproduce the observed dislocations in terms of
localization and frequency of appearance.
Results: The observed
dislocations can only be explained by the interference of a kink and
sausage wave modes propagating with different frequencies along the
coronal magnetic field. In the plane transverse to the propagation,
the cross-section of the oscillating plasma must be smaller than the
spatial resolution, and the two waves result in net longitudinal and
transverse velocity components that are mixed through projection onto
the line of sight. Alfvén waves can be responsible for the kink mode,
but a magnetoacoustic sausage mode is necessary in all cases. Higher
(flute) modes are excluded. The kink mode has a pressure amplitude
that is less than the pressure amplitude of the sausage mode, though
its observed velocity is higher. This concentrates dislocations on
the top of the loop.
Conclusions: To explain dislocations,
any model of coronal waves must include the simultaneous propagation
and interference of kink and sausage wave modes of comparable but
different frequencies with a sausage wave amplitude much smaller than
the kink one. Appendix A is available in electronic form at http://www.aanda.org
Title: Magnetohydrodynamic wave propagation from the subphotosphere
to the corona in an arcade-shaped magnetic field with a null point
Authors: Santamaria, I. C.; Khomenko, E.; Collados, M.
Bibcode: 2015A&A...577A..70S
Altcode: 2015arXiv150303094S
Aims: The aim of this work is to study the energy transport
by means of Magnetohydrodynamic (MHD) waves propagating in quiet-Sun
magnetic topology from layers below the surface to the corona. Upwardly
propagating waves find obstacles, such as the equipartition layer
with plasma β = 1, the transition region, and null points, and they
get transmitted, converted, reflected, and refracted. Understanding
the mechanisms by which MHD waves can reach the corona can give us
information about the solar atmosphere and the magnetic structures.
Methods: We carried out two-dimensional numerical simulations of
wave propagation in a magnetic field structure that consists of two
vertical flux tubes with the same polarity separated by an arcade-shaped
magnetic field. This configuration contains a null point in the corona,
which significantly modifies the behavior of the waves as they pass
near it.
Results: We describe in detail the wave propagation
through the atmosphere under different driving conditions. We also
present the spatial distribution of the mean acoustic and magnetic
energy fluxes for the cases where these calculations are possible,
as well as the spatial distribution of the dominant frequencies in
the whole domain.
Conclusions: We conclude that the energy
reaches the corona preferably along almost vertical magnetic fields,
that is, inside the vertical flux tubes. This energy is acoustic
in nature. Most of the magnetic energy stays concentrated below the
transition region owing to the refraction of the magnetic waves and
the continuous conversion of acoustic-like waves into fast magnetic
waves in the equipartition layer located in the photosphere where
plasma β = 1. However, part of the magnetic energy reaches the low
corona when propagating in the region where the arcades are located,
but waves are sent back downward into the lower atmosphere at the
null-point surroundings. This phenomenon, together with the reflection
and refraction of waves in the TR and the lower turning point, act as
a re-feeding of the atmosphere, which keeps oscillating during all the
simulation time even if a driver with a single pulse was used as initial
perturbation. In the frequency distribution, we find that high frequency
waves can reach the corona outside the vertical flux tubes. Movies
related to Figs. 3, 7, and 11 are available in electronic form at http://www.aanda.org
Title: Beyond MHD: modeling and observation of partially ionized
solar plasma processes
Authors: Khomenko, E.
Bibcode: 2015hsa8.conf..677K
Altcode: 2015arXiv150401578K
The temperature and density conditions in the magnetized photosphere
and chromosphere of the Sun lead to a very small degree of atomic
ionization. At particular heights, the magnetic field may be
strong enough to give rise to a cyclotron frequency larger than the
collisional frequency for some species. These circumstances influence
the collective behavior of the particles and some of the hypotheses
of magnetohydrodynamics may be relaxed, giving rise to non-ideal MHD
effects. In this paper we discuss our recent developments in modeling
non-ideal plasma effects, as well as their observational consequences.
Title: Synthetic Observations of Wave Propagation in a Sunspot Umbra
Authors: Felipe, T.; Socas-Navarro, H.; Khomenko, E.
Bibcode: 2014ApJ...795....9F
Altcode: 2014arXiv1408.6565F
Spectropolarimetric temporal series from Fe I λ6301.5 Å and Ca II
infrared triplet lines are obtained by applying the Stokes synthesis
code NICOLE to a numerical simulation of wave propagation in a sunspot
umbra from MANCHA code. The analysis of the phase difference between
Doppler velocity and intensity core oscillations of the Fe I λ6301.5
Å line reveals that variations in the intensity are produced by
opacity fluctuations rather than intrinsic temperature oscillations,
except for frequencies between 5 and 6.5 mHz. On the other hand, the
photospheric magnetic field retrieved from the weak field approximation
provides the intrinsic magnetic field oscillations associated to wave
propagation. Our results suggest that this is due to the low magnetic
field gradient of our sunspot model. The Stokes parameters of the
chromospheric Ca II infrared triplet lines show striking variations as
shock waves travel through the formation height of the lines, including
emission self-reversals in the line core and highly abnormal Stokes V
profiles. Magnetic field oscillations inferred from the Ca II infrared
lines using the weak field approximation appear to be related with
the magnetic field strength variation between the photosphere and
the chromosphere.
Title: Fluid description of multi-component solar partially ionized
plasma
Authors: Khomenko, E.; Collados, M.; Díaz, A.; Vitas, N.
Bibcode: 2014PhPl...21i2901K
Altcode: 2014arXiv1408.1871K
We derive self-consistent formalism for the description of
multi-component partially ionized solar plasma, by means of the coupled
equations for the charged and neutral components for an arbitrary
number of chemical species, and the radiation field. All approximations
and assumptions are carefully considered. Generalized Ohm's law is
derived for the single-fluid and two-fluid formalism. Our approach is
analytical with some order-of-magnitude support calculations. After
general equations are developed, we particularize to some frequently
considered cases as for the interaction of matter and radiation.
Title: Rayleigh-Taylor instability in prominences from numerical
simulations including partial ionization effects
Authors: Khomenko, E.; Díaz, A.; de Vicente, A.; Collados, M.;
Luna, M.
Bibcode: 2014A&A...565A..45K
Altcode: 2014arXiv1403.4530K
We study the Rayleigh-Taylor instability (RTI) at a prominence-corona
transition region in a non-linear regime. Our aim is to understand how
the presence of neutral atoms in the prominence plasma influences the
instability growth rate, as well as the evolution of velocity, magnetic
field vector, and thermodynamic parameters of turbulent drops. We
perform 2.5D numerical simulations of the instability initiated by
a multi-mode perturbation at the corona-prominence interface using a
single-fluid magnetohydrodynamic (MHD) approach including a generalized
Ohm's law. The initial equilibrium configuration is purely hydrostatic
and contains a homogeneous horizontal magnetic field forming an
angle with the direction in which the plasma is perturbed. We analyze
simulations with two different orientations of the magnetic field. For
each field orientation we compare two simulations, one for the pure
MHD case, and one including the ambipolar diffusion in Ohm's law (AD
case). Other than that, both simulations for each field orientation are
identical. The numerical results in the initial stage of the instability
are compared with the analytical linear calculations. We find that the
configuration is always unstable in the AD case. The growth rate of
the small-scale modes in the non-linear regime is up to 50% larger in
the AD case than in the purely MHD case and the average velocities of
flows are a few percentage points higher. Significant drift momenta
are found at the interface between the coronal and the prominence
material at all stages of the instability, produced by the faster
downward motion of the neutral component with respect to the ionized
component. The differences in temperature of the bubbles between the
ideal and non-ideal case are also significant, reaching 30%. There is
an asymmetry between large rising bubbles and small-scale down flowing
fingers, favoring the detection of upward velocities in observations.
Title: Rayleigh-Taylor instability in partially ionized compressible
plasmas: One fluid approach
Authors: Díaz, A. J.; Khomenko, E.; Collados, M.
Bibcode: 2014A&A...564A..97D
Altcode: 2014arXiv1401.5388D
Aims: We study the modification of the classical criterion for
the linear onset and growth rate of the Rayleigh-Taylor instability
(RTI) in a partially ionized (PI) plasma in the one-fluid description
by considering a generalized induction equation.
Methods:
The governing linear equations and appropriate boundary conditions,
including gravitational terms, are derived and applied to the case of
the RTI in a single interface between two partially ionized plasmas. The
boundary conditions lead to an equation for the frequencies in which
some have positive complex parts, marking the appearance of the
RTI. We study the ambipolar term alone first, extending the result to
the full induction equation later.
Results: The configuration
is always unstable because of the presence of a neutral species. In
the classical stability regime, the growth rate is small, since the
collisions prevent the neutral fluid to fully develop the RTI. For
parameters in the classical instability regime, the growth rate is
lowered, but the differences with the compressible MHD case are small
for the considered theoretical values of the collision frequencies
and diffusion coefficients for solar prominences.
Conclusions:
The PI modifies some aspects of the linear RTI instability, since it
takes into account that neutrals do not feel the stabilizing effect
of the magnetic field. For the set of parameters representative for
solar prominences, our model gives the resulting timescale comparable
to observed lifetimes of RTI plumes.
Title: Rayleigh-Taylor instability in partially ionized prominence
plasma
Authors: Khomenko, E.; Díaz, A.; de Vicente, A.; Collados, M.;
Luna, M.
Bibcode: 2014IAUS..300...90K
Altcode: 2013arXiv1310.7016K
We study Rayleigh-Taylor instability (RTI) at the coronal-prominence
boundary by means of 2.5D numerical simulations in a single-fluid MHD
approach including a generalized Ohm's law. The initial configuration
includes a homogeneous magnetic field forming an angle with the
direction in which the plasma is perturbed. For each field inclination
we compare two simulations, one for the pure MHD case, and one including
the ambipolar diffusion in the Ohm's law, otherwise identical. We find
that the configuration containing neutral atoms is always unstable. The
growth rate of the small-scale modes in the non-linear regime is larger
than in the purely MHD case.
Title: 3D simulations of Rayleigh-Taylor instability in prominences
including partial ionization effects
Authors: Khomenko, Elena; Collados, Manuel; De Vicente, Angel; Luna,
Manuel; Diaz, Antonio
Bibcode: 2014cosp...40E1476K
Altcode:
We study the Rayleigh-Taylor instability (RTI) at a prominence-corona
transition region in a non-linear regime. Our aim is to understand how
the presence of neutral atoms in the prominence plasma influences the
instability growth rate, and the evolution of velocity, magnetic field
vector and thermodynamic parameters of turbulent drops. We perform
3D numerical simulations of the instability initiated by a multi-mode
perturbation at the corona-prominence interface using a single-fluid
MHD approach including a generalized Ohm's law. Pairs of simulations
are compared, one of them done under ideal MHD conditions, and others
include ambipolar diffusion (AD) in the Ohm's law. Other than that,
the simulations of each pair are identical in their magnetic field
orientation and thermal parameters. The numerical results in the
initial stage of the instability are compared with the analytical
linear calculations. We find that the configuration is always
unstable in the AD case. The growth rate of the small-scale modes in
the non-linear regime is up to 50% larger in the AD case than in the
purely MHD case and the average velocities of flows are a few percent
larger. Significant drift momenta are found at the interface between the
coronal and the prominence material at all stages of the instability,
produced by the faster downward motion of the neutral component with
respect to the ionized component. The differences in temperature of
the bubbles between the ideal and non-ideal case are also significant.
Title: Properties of oscillatory motions in a facular region
Authors: Kostik, R.; Khomenko, E.
Bibcode: 2013A&A...559A.107K
Altcode: 2013arXiv1310.0184K
Aims: We study the properties of waves in a facular region of
moderate strength in the photosphere and chromosphere. Our aim is to
statistically analyse the wave periods, power, and phase relations
as a function of the magnetic field strength and inclination.
Methods: Our work is based on observations obtained at the German Vacuum
Tower Telescope (Observatorio del Teide, Tenerife) using two different
instruments: the Triple Etalon SOlar Spectrometer (TESOS) in the Ba
ii 4554 Å line to measure velocity and intensity variations through
the photosphere and, simultaneously, the Tenerife Infrared Polarimeter
(TIP-II), in the Fe i 1.56 μm lines to measure the Stokes parameters
and magnetic field strength in the lower photosphere. Additionally,
we use the simultaneous broad-band filtergrams in the Ca ii H line to
obtain information about intensity oscillations in the chromosphere.
Results: We find several clear trends in the oscillation behaviour:
(i) the period of oscillation increases by 15-20% with the magnetic
field increasing from 500 to 1500 G. (ii) The temperature-velocity
phase shifts show a strikingly different distribution in the facular
region compared to the quiet region, a significant number of cases
in the range from - 180° to 180° is detected in the facula. (iii)
The most powerful chromospheric Ca ii H intensity oscillations are
observed at locations with strong magnetic fields (1.3-1.5 kG) inclined
by 10-12 degrees, as a result of upward propagating waves with rather
low phase speeds, and temperature-velocity phase shifts between 0°
and 90°. (iv) The power of the photospheric velocity oscillations
from the Ba ii line increases linearly with decreasing magnetic field
inclination, reaching its maximum at strong field locations.
Title: Dislocations in Magnetohydrodynamic Waves in a Stellar
Atmosphere
Authors: López Ariste, A.; Collados, M.; Khomenko, E.
Bibcode: 2013PhRvL.111h1103L
Altcode: 2013arXiv1308.0145L
We describe the presence of wave front dislocations in
magnetohydrodynamic waves in stratified stellar atmospheres. Scalar
dislocations such as edges and vortices can appear in Alfvén waves, as
well as in general magnetoacoustic waves. We detect those dislocations
in observations of magnetohydrodynamic waves in sunspots in the solar
chromosphere. Through the measured charge of all the dislocations
observed, we can give for the first time estimates of the modal
contribution in the waves propagating along magnetic fields in solar
sunspots.
Title: Atmosphere Dynamics of the Active Region NOAA 11024
Authors: Kondrashova, N. N.; Pasechnik, M. N.; Chornogor, S. N.;
Khomenko, E. V.
Bibcode: 2013SoPh..284..499K
Altcode: 2012arXiv1212.1307K
We present results of the study of chromospheric and photospheric
line-of-sight velocity fields in the young active region
NOAA 11024. Multi-layer, multi-wavelength observational data
were used for the analysis of the emerging flux in this active
region. Spectropolarimetric observations were carried out with the
telescope THEMIS on Tenerife (Canary Islands) on 4 July 2009. In
addition, space-borne data from SOHO/MDI, STEREO and GOES were also
considered. The combination of data from ground- and space-based
telescopes allowed us to study the dynamics of the lower atmosphere
of the active region with high spatial, spectral, and temporal
resolutions. THEMIS spectra show strong temporal variations of the
velocity in the chromosphere and photosphere for various activity
features: two pores, active and quiet plage regions, and two
surges. The range of variations of the chromospheric line-of-sight
velocity at the heights of the formation of the Hα core was extremely
large. Both upward and downward motions were observed in these
layers. In particular, a surge with upward velocities up to −73 km
s−1 was detected. In the photosphere, predominantly upward
motions were found, varying from −3.1 km s−1 upflows
to 1.4 km s−1 downflows in different structures. The
velocity variations at different levels in the lower atmosphere are
compatible with the emergence of magnetic flux.
Title: Magnetohydrodynamic waves driven by p-modes
Authors: Khomenko, Elena; Calvo Santamaria, Irantzu
Bibcode: 2013JPhCS.440a2048K
Altcode: 2013arXiv1302.4351K
Waves are observed at all layers of the solar atmosphere and the
magnetic field plays a key role in their propagation. While deep down
in the atmosphere the p-modes are almost entirely of acoustic nature,
in the upper layers magnetic forces are dominating, leading to a large
variety of new wave modes. Significant advances have been made recently
in our understanding of the physics of waves interaction with magnetic
structures, with the help of analytical theories, numerical simulations,
as well as high-resolution observations. In this contribution, we review
recent observational findings and current theoretical ideas in the
field, with an emphasis on the following questions: (i) Peculiarities
of the observed wave propagation in network, plage and facular regions;
(ii) Role of the mode transformation and observational evidences of
this process: (iii) Coupling of the photosphere, chromosphere, and
above by means of waves propagating in magnetic structures.
Title: MHD wave propagation in the solar network
Authors: Calvo Santamaria, I.; Khomenko, E.; Cally, P. S.; Collados, M.
Bibcode: 2013hsa7.conf..806C
Altcode:
Magneto-acoustic and Alfvénic waves are ubiquitous in solar coronal
loops, possibly being excited by photospheric motions. It is not
clear, though, how these waves get so high, having obstacles such as
the acoustic cut-off frequency, reflection and refraction of fast
MHD waves and also the strongly reflecting transition region. In
this contribution we report on 2D numerical modelling of waves in
magnetic arcade structures extending from photospheric layers through
the transition region to the corona. Waves in the arcade are excited
by sub-photospheric p-modes. We discuss the behaviour of waves, their
conversion and propagation properties and possible mechanisms allowing
their escape through the transition region.
Title: Simulations of Chromospheric Heating by Ambipolar Diffusion
Authors: Khomenko, E.; Collados Vera, M.
Bibcode: 2012ASPC..463..281K
Altcode: 2012arXiv1202.2252K
We propose a mechanism for efficient heating of the solar chromosphere
based on non-ideal plasma effects. Three ingredients are needed for the
work of this mechanism: (1) presence of neutral atoms; (2) presence
of a non-potential magnetic field; (3) decrease of the collisional
coupling of the plasma. Due to the decrease of collisional coupling, a
net relative motion appears between the neutral and ionized components,
usually referred to as “ambipolar diffusion.” This results in
a significant enhancement of current dissipation as compared to the
classical MHD case. We propose that the current dissipation in this
situation is able to provide enough energy to heat the chromosphere
by several kK on the time scale of minutes, or even seconds. In this
paper, we show that this energy supply might be sufficient to balance
the radiative energy losses of the chromosphere.
Title: Solar Fe abundance and magnetic fields. Towards a consistent
reference metallicity
Authors: Fabbian, D.; Moreno-Insertis, F.; Khomenko, E.; Nordlund, Å.
Bibcode: 2012A&A...548A..35F
Altcode: 2012arXiv1209.2771F
Aims: We investigate the impact on Fe abundance determination of
including magnetic flux in series of 3D radiation-magnetohydrodynamics
(MHD) simulations of solar convection, which we used to synthesize
spectral intensity profiles corresponding to disc centre.
Methods: A differential approach is used to quantify the changes
in theoretical equivalent width of a set of 28 iron spectral lines
spanning a wide range in wavelength, excitation potential, oscillator
strength, Landé factor, and formation height. The lines were computed
in local thermodynamic equilibrium (LTE) using the spectral synthesis
code LILIA. We used input magnetoconvection snapshots covering 50 min
of solar evolution and belonging to series having an average vertical
magnetic flux density of ⟨ Bvert ⟩ = 0,50,100, and
200 G. For the relevant calculations we used the Copenhagen Stagger
code.
Results: The presence of magnetic fields causes both a
direct (Zeeman-broadening) effect on spectral lines with non-zero
Landé factor and an indirect effect on temperature-sensitive
lines via a change in the photospheric T - τ stratification. The
corresponding correction in the estimated atomic abundance ranges
from a few hundredths of a dex up to |Δlog ɛ(Fe)⊙|
~ 0.15 dex, depending on the spectral line and on the amount of
average magnetic flux within the range of values we considered. The
Zeeman-broadening effect gains relatively more importance in the
IR. The largest modification to previous solar abundance determinations
based on visible spectral lines is instead due to the indirect effect,
i.e., the line-weakening caused by a warmer stratification as seen on
an optical depth scale. Our results indicate that the average solar
iron abundance obtained when using magnetoconvection models can be ~
0.03-0.11 dex higher than when using the simpler hydrodynamics (HD)
convection approach.
Conclusions: We demonstrate that accounting
for magnetic flux is important in state-of-the-art solar photospheric
abundance determinations based on 3D convection simulations.
Title: Properties of convective motions in facular regions
Authors: Kostik, R.; Khomenko, E. V.
Bibcode: 2012A&A...545A..22K
Altcode: 2012arXiv1207.4340K
Aims: We study the properties of solar granulation in a facular
region from the photosphere up to the lower chromosphere. Our aim is
to investigate the dependence of granular structure on magnetic field
strength.
Methods: We used observations obtained at the German
Vacuum Tower Telescope (Observatorio del Teide, Tenerife) using two
different instruments: the Triple Etalon SOlar Spectrometer (TESOS)
to measure velocity and intensity variations along the photosphere in
the Ba ii 4554 Å line; and, simultaneously, the Tenerife Infrared
Polarimeter (TIP-II) to the measure Stokes parameters and the
magnetic field strength at the lower photosphere in the Fe i 1.56
μm lines.
Results: We find that the convective velocities of
granules in the facular area decrease with magnetic field while the
convective velocities of intergranular lanes increase with the field
strength. Similar to the quiet areas, there is a contrast and velocity
sign reversal taking place in the middle photosphere. The reversal
heights depend on the magnetic field strength and are, on average,
about 100 km higher than in the quiet regions. The correlation between
convective velocity and intensity decreases with magnetic field at
the bottom photosphere, but increases in the upper photosphere. The
contrast of intergranular lanes observed close to the disk center is
almost independent of the magnetic field strength.
Conclusions:
The strong magnetic field of the facular area seems to stabilize the
convection and to promote more effective energy transfer in the upper
layers of the solar atmosphere, since the convective elements reach
greater heights.
Title: First Results from the SUNRISE Mission
Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.;
Gandorfer, A.; Hirzberger, J.; Jafarzadeh, S.; Lagg, A.; Riethmüller,
T. L.; Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; González,
M. J. M.; Pillet, V. M.; Khomenko, E.; Yelles Chaouche, L.; Iniesta,
J. C. d. T.; Domingo, V.; Palacios, J.; Knölker, M.; González,
N. B.; Borrero, J. M.; Berkefeld, T.; Franz, M.; Roth, M.; Schmidt,
W.; Steiner, O.; Title, A. M.
Bibcode: 2012ASPC..455..143S
Altcode:
The SUNRISE balloon-borne solar observatory consists of a 1m aperture
Gregory telescope, a UV filter imager, an imaging vector polarimeter,
an image stabilization system, and further infrastructure. The first
science flight of SUNRISE yielded high-quality data that reveal the
structure, dynamics, and evolution of solar convection, oscillations,
and magnetic fields at a resolution of around 100 km in the quiet
Sun. Here we describe very briefly the mission and the first results
obtained from the SUNRISE data, which include a number of discoveries.
Title: Heating of the Magnetized Solar Chromosphere by Partial
Ionization Effects
Authors: Khomenko, E.; Collados, M.
Bibcode: 2012ApJ...747...87K
Altcode: 2011arXiv1112.3374K
In this paper, we study the heating of the magnetized solar
chromosphere induced by the large fraction of neutral atoms present
in this layer. The presence of neutrals, together with the decrease
with height of the collisional coupling, leads to deviations from the
classical magnetohydrodynamic behavior of the chromospheric plasma. A
relative net motion appears between the neutral and ionized components,
usually referred to as ambipolar diffusion. The dissipation of currents
in the chromosphere is enhanced by orders of magnitude due to the
action of ambipolar diffusion, as compared with the standard ohmic
diffusion. We propose that a significant amount of magnetic energy
can be released to the chromosphere just by existing force-free
10-40 G magnetic fields there. As a consequence, we conclude that
ambipolar diffusion is an important process that should be included
in chromospheric heating models, as it has the potential to rapidly
heat the chromosphere. We perform analytical estimations and numerical
simulations to prove this idea.
Title: Influence of phase-diversity image reconstruction techniques
on circular polarization asymmetries
Authors: Asensio Ramos, A.; Martínez González, M. J.; Khomenko,
E.; Martínez Pillet, V.
Bibcode: 2012A&A...539A..42A
Altcode: 2011arXiv1111.2496A
Context. Full Stokes filter-polarimeters are key instruments for
investigating the rapid evolution of magnetic structures on the solar
surface. To this end, the image quality is routinely improved using
a-posteriori image reconstruction methods.
Aims: We analyze
the robustness of circular polarization asymmetries to phase-diversity
image reconstruction techniques.
Methods: We used snapshots of
magneto-hydrodynamical simulations carried out with different initial
conditions to synthesize spectra of the magnetically sensitive Fe
i line at 5250.2 Å. We degraded the synthetic profiles spatially
and spectrally to simulate observations with the IMaX full Stokes
filter-polarimeter. We also simulated the focused/defocused pairs of
images used by the phase-diversity algorithm for reconstruction and the
polarimetric modulation scheme. We assume that standard optimization
methods are able to infer the projection of the wavefront on the Zernike
polynomials with 10% precision. We also consider the less favorable case
of 25% precision. We obtain reconstructed monochromatic modulated images
that are later demodulated and compared with the original maps.
Results: Although asymmetries are often difficult to define in the
quiet Sun due to the complexity of the Stokes V profiles, we show
how asymmetries are degraded with spatial and spectral smearing. The
results indicate that, although image reconstruction techniques reduce
the spatial smearing, they can modify the asymmetries of the profiles,
which are mainly caused by the appearance of spatially-correlated noise.
Title: Beyond single fluid MHD: multi-fluid modeling of the coupled
solar atmosphere
Authors: Khomenko, Elena
Bibcode: 2012decs.confE..26K
Altcode:
The particular temperature and density conditions in the magnetized
photosphere and chromosphere of the Sun usually lead to a very small
degree of atomic ionization. In addition, at particular heights,
the magnetic field may be strong enough to give rise to a cyclotron
frequency larger than the collisional frequency for some species,
while for others the opposite may happen. These circumstances can
influence the collective behaviour of the particles and some of the
hypotheses of magnetohydrodynamics may be relaxed, giving rise to
non-ideal MHD effects. These effects are potentially important for the
dynamics and energy exchange in the solar photosphere and, especially,
chromosphere. In particular, there are evidences that such phenomena
as wave propagation and damping, magnetic reconnection, formation
of stable magnetic field concentrations, magnetic flux emergence,
etc. can be affected. In this contribution, I will discuss the current
state-of-the-art of multi-fluid MHD modelling of the coupled solar
atmosphere. I will revise the major issues, physical mechanisms and
assumptions of the MHD approach, and discuss future simulations that
would be required to address some unresolved topics. I will present
the first results of numerical simulations using the modified MHD
equations, showing that the chromosphere can be effectively heated
due to non-ideal MHD effects.
Title: Numerical Simulations of Conversion to Alfvén Waves in
Sunspots
Authors: Khomenko, E.; Cally, P. S.
Bibcode: 2012ApJ...746...68K
Altcode: 2011arXiv1111.2851K
We study the conversion of fast magnetoacoustic waves to Alfvén waves
by means of 2.5D numerical simulations in a sunspot-like magnetic
configuration. A fast, essentially acoustic, wave of a given frequency
and wave number is generated below the surface and propagates upward
through the Alfvén/acoustic equipartition layer where it splits
into upgoing slow (acoustic) and fast (magnetic) waves. The fast wave
quickly reflects off the steep Alfvén speed gradient, but around and
above this reflection height it partially converts to Alfvén waves,
depending on the local relative inclinations of the background magnetic
field and the wavevector. To measure the efficiency of this conversion
to Alfvén waves we calculate acoustic and magnetic energy fluxes. The
particular amplitude and phase relations between the magnetic field
and velocity oscillations help us to demonstrate that the waves
produced are indeed Alfvén waves. We find that the conversion to
Alfvén waves is particularly important for strongly inclined fields
like those existing in sunspot penumbrae. Equally important is the
magnetic field orientation with respect to the vertical plane of
wave propagation, which we refer to as "field azimuth." For a field
azimuth less than 90° the generated Alfvén waves continue upward, but
above 90° downgoing Alfvén waves are preferentially produced. This
yields negative Alfvén energy flux for azimuths between 90° and
180°. Alfvén energy fluxes may be comparable to or exceed acoustic
fluxes, depending upon geometry, though computational exigencies limit
their magnitude in our simulations.
Title: Magneto-acoustic wave energy in sunspots: observations and
numerical simulations
Authors: Felipe, T.; Khomenko, E.; Collados, M.; Beck, C.
Bibcode: 2011hsa6.conf..630F
Altcode:
We have reproduced some sunspot wave signatures obtained
from spectropolarimetric observations through 3D MHD
numericalsimulations. The results of the simulations arecompared with
the oscillations observed simultaneously at different heights from the
SiI lambda10827Å line, HeI lambda10830Å line, the CaII H core and
the FeI blends at the wings of the CaII H line. The simulations show
a remarkable agreement with the observations, and we have used them
to quantify the energy contribution of the magneto-acoustic waves to
the chromospheric heating in sunspots. Our findings indicate that the
energy supplied by these waves is 5-10 times lower than the amount
needed to balance the chromospheric radiative losses.
Title: The Sun at high resolution: first results from the Sunrise
mission
Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller,
A.; Gandorfer, A.; Hirzberger, J.; Lagg, A.; Riethmüller, T. L.;
Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; Pillet, V. Martínez;
Khomenko, E.; del Toro Iniesta, J. C.; Domingo, V.; Palacios, J.;
Knölker, M.; González, N. Bello; Borrero, J. M.; Berkefeld, T.;
Franz, M.; Roth, M.; Schmidt, W.; Steiner, O.; Title, A. M.
Bibcode: 2011IAUS..273..226S
Altcode:
The Sunrise balloon-borne solar observatory consists of a 1m aperture
Gregory telescope, a UV filter imager, an imaging vector polarimeter,
an image stabilization system and further infrastructure. The first
science flight of Sunrise yielded high-quality data that reveal the
structure, dynamics and evolution of solar convection, oscillations
and magnetic fields at a resolution of around 100 km in the quiet
Sun. Here we describe very briefly the mission and the first results
obtained from the Sunrise data, which include a number of discoveries.
Title: Magnetoacoustic Wave Energy from Numerical Simulations of an
Observed Sunspot Umbra
Authors: Felipe, T.; Khomenko, E.; Collados, M.
Bibcode: 2011ApJ...735...65F
Altcode: 2011arXiv1104.4138F
We aim at reproducing the height dependence of sunspot wave signatures
obtained from spectropolarimetric observations through three-dimensional
MHD numerical simulations. A magnetostatic sunspot model based on
the properties of the observed sunspot is constructed and perturbed
at the photosphere, introducing the fluctuations measured with the Si
I λ10827 line. The results of the simulations are compared with the
oscillations observed simultaneously at different heights from the He
I λ10830 line, the Ca II H core, and the Fe I blends in the wings of
the Ca II H line. The simulations show a remarkable agreement with the
observations. They reproduce the velocity maps and power spectra at
the formation heights of the observed lines, as well as the phase and
amplification spectra between several pairs of lines. We find that the
stronger shocks at the chromosphere are accompanied with a delay between
the observed signal and the simulated one at the corresponding height,
indicating that shocks shift the formation height of the chromospheric
lines to higher layers. Since the simulated wave propagation matches
very well the properties of the observed one, we are able to use the
numerical calculations to quantify the energy contribution of the
magnetoacoustic waves to the chromospheric heating in sunspots. Our
findings indicate that the energy supplied by these waves is too low to
balance the chromospheric radiative losses. The energy contained at the
formation height of the lowermost Si I λ10827 line in the form of slow
magnetoacoustic waves is already insufficient to heat the higher layers,
and the acoustic energy which reaches the chromosphere is around 3-9
times lower than the required amount of energy. The contribution of
the magnetic energy is even lower.
Title: Sunspot Models
Authors: Khomenko, E.
Bibcode: 2011ascl.soft05007K
Altcode:
These IDL codes create a thick magneto-static structure with parameters
of a typical sunspot in a solar like photosphere - chromosphere. The
variable parameters are field strength on the axis, radius, and Wilson
depression (displacement of the atmosphere on the axis with respect to
the field-free atmosphere). Output are magnetic field vector, pressure
and density distributions with radius and height. The structure has
azimuthal symmetry. The codes are relatively self explanatory and the
download packages contain README files.
Title: Unnoticed Magnetic Field Oscillations in the Very Quiet Sun
Revealed by SUNRISE/IMaX
Authors: Martínez González, M. J.; Asensio Ramos, A.; Manso Sainz,
R.; Khomenko, E.; Martínez Pillet, V.; Solanki, S. K.; López Ariste,
A.; Schmidt, W.; Barthol, P.; Gandorfer, A.
Bibcode: 2011ApJ...730L..37M
Altcode: 2011arXiv1103.0145M
We present observational evidence for oscillations of magnetic flux
density in the quiet areas of the Sun. The majority of magnetic
fields on the solar surface have strengths of the order of or lower
than the equipartition field (300-500 G). This results in a myriad of
magnetic fields whose evolution is largely determined by the turbulent
plasma motions. When granules evolve they squash the magnetic field
lines together or pull them apart. Here, we report on the periodic
deformation of the shapes of features in circular polarization observed
at high resolution with SUNRISE. In particular, we note that the
area of patches with a constant magnetic flux oscillates with time,
which implies that the apparent magnetic field intensity oscillates
in antiphase. The periods associated with this oscillatory pattern
are compatible with the granular lifetime and change abruptly, which
suggests that these oscillations might not correspond to characteristic
oscillatory modes of magnetic structures, but to the forcing by granular
motions. In one particular case, we find three patches around the same
granule oscillating in phase, which means that the spatial coherence
of these oscillations can reach 1600 km. Interestingly, the same kind
of oscillatory phenomenon is also found in the upper photosphere.
Title: Numerical simulations of conversion to Alfvén waves in solar
active regions
Authors: Khomenko, E.; Cally, P. S.
Bibcode: 2011JPhCS.271a2042K
Altcode: 2010arXiv1009.4575K
We study the coupling of magneto-acoustic waves to Alvén waves using
2.5D numerical simulations. In our experiment, a fast magnetoacoustic
wave of a given frequency and wavenumber is generated below the
surface. The magnetic field in the domain is assumed homogeneous and
inclined. The efficiency of the conversion to Alfvén waves near the
layer of equal acoustic and Alfven speeds is measured calculating
their energy flux. The particular amplitude and phase relations
between the oscillations of magnetic field and velocity help us to
demonstrate that the waves produced after the transformation and
reaching upper atmosphere are indeed Alfvén waves. We find that
the conversion from fast magneto-acoustic waves to Alfvén waves is
particularly important for the inclination θ and azimuth phi angles of
the magnetic field between 55 and 65 degrees, with the maximum shifted
to larger inclinations for lower frequency waves. The maximum Alfvén
flux transmitted to the upper atmosphere is about 2-3 times lower than
the corresponding acoustic flux.
Title: Magneto-acoustic waves in sunspots from observations and
numerical simulations
Authors: Felipe, T.; Khomenko, E.; Collados, M.; Beck, C.
Bibcode: 2011JPhCS.271a2040F
Altcode: 2010arXiv1009.5512F
We study the propagation of waves from the photosphere to the
chromosphere of sunspots. From time series of cospatial Ca II H
(including its line blends) intensity spectra and polarimetric spectra
of Si I λ 1082.7 nm and He I λ 1083.0 nm we retrieve the line-of-sight
velocity at several heights. The analysis of the phase difference and
amplification spectra shows standing waves for frequencies below 4 mHz
and propagating waves for higher frequencies, and allows us to infer
the temperature and height where the lines are formed. Using these
observational data, we have constructed a model of sunspot, and we
have introduced the velocity measured with the photospheric Si I λ
1082.7 nm line as a driver. The numerically propagated wave pattern
fits reasonably well with the observed using the lines formed at higher
layers, and the simulations reproduce many of the observed features. The
observed waves are slow MHD waves propagating longitudinally along
field lines.
Title: Solar Abundance Corrections Derived Through Three-dimensional
Magnetoconvection Simulations
Authors: Fabbian, D.; Khomenko, E.; Moreno-Insertis, F.; Nordlund, Å.
Bibcode: 2010ApJ...724.1536F
Altcode: 2010arXiv1006.0231F
We explore the effect of the magnetic field when using realistic
three-dimensional convection experiments to determine solar element
abundances. By carrying out magnetoconvection simulations with a
radiation-hydro code (the Copenhagen stagger code) and through a
posteriori spectral synthesis of three Fe I lines, we obtain evidence
that moderate amounts of mean magnetic flux cause a noticeable
change in the derived equivalent widths compared with those for a
non-magnetic case. The corresponding Fe abundance correction for a
mean flux density of 200 G reaches up to ~0.1 dex in magnitude. These
results are based on space- and time-averaged line profiles over a time
span of 2.5 solar hours in the statistically stationary regime of the
convection. The main factors causing the change in equivalent widths,
namely the Zeeman broadening and the modification of the temperature
stratification, act in different amounts and, for the iron lines
considered here, in opposite directions; yet, the resulting |Δlog
epsilonsun(Fe)| coincides within a factor of 2 in all
of them, even though the sign of the total abundance correction
is different for the visible and infrared lines. We conclude that
magnetic effects should be taken into account when discussing precise
values of the solar and stellar abundances and that an extended study
is warranted.
Title: Where the Granular Flows Bend
Authors: Khomenko, E.; Martínez Pillet, V.; Solanki, S. K.; del Toro
Iniesta, J. C.; Gandorfer, A.; Bonet, J. A.; Domingo, V.; Schmidt,
W.; Barthol, P.; Knölker, M.
Bibcode: 2010ApJ...723L.159K
Altcode: 2010arXiv1008.0517K
Based on IMaX/SUNRISE data, we report on a previously undetected
phenomenon in solar granulation. We show that in a very narrow region
separating granules and intergranular lanes, the spectral line width
of the Fe I 5250.2 Å line becomes extremely small. We offer an
explanation of this observation with the help of magneto-convection
simulations. These regions with extremely small line widths correspond
to the places where the granular flows bend from upflow in granules
to downflow in intergranular lanes. We show that the resolution and
image stability achieved by IMaX/SUNRISE are important requisites to
detect this interesting phenomenon.
Title: Numerical simulation of propagation of the MHD waves in
sunspots
Authors: Parchevsky, K.; Kosovichev, A.; Khomenko, E.; Olshevsky,
V.; Collados, M.
Bibcode: 2010HiA....15..354P
Altcode:
We present results of numerical 3D simulation of propagation of MHD
waves in sunspots. We used two self consistent magnetohydrostatic
background models of sunspots. There are two main differences
between these models: (i) the topology of the magnetic field and
(ii) dependence of the horizontal profile of the sound speed on
depth. The model with convex shape of the magnetic field lines near
the photosphere has non-zero horizorntal perturbations of the sound
speed up to the depth of 7.5 Mm (deep model). In the model with concave
shape of the magnetic field lines near the photosphere Δ c/c is close
to zero everywhere below 2 Mm (shallow model). Strong Alfven wave is
generated at the wave source location in the deep model. This wave is
almost unnoticeable in the shallow model. Using filtering technique
we separated magnetoacoustic and magnetogravity waves. It is shown,
that inside the sunspot magnetoacoustic and magnetogravity waves
are not spatially separated unlike the case of the horizontally
uniform background model. The sunspot causes anisotropy of the
amplitude distribution along the wavefront and changes the shape
of the wavefront. The amplitude of the waves is reduced inside the
sunspot. This effect is stronger for the magnetogravity waves than for
magnetoacoustic waves. The shape of the wavefront of the magnetogravity
waves is distorted stronger as well. The deep model causes bigger
anisotropy for both mgnetoacoustic and magneto gravity waves than the
shallow model.
Title: Modeling the Subsurface Structure of Sunspots
Authors: Moradi, H.; Baldner, C.; Birch, A. C.; Braun, D. C.; Cameron,
R. H.; Duvall, T. L.; Gizon, L.; Haber, D.; Hanasoge, S. M.; Hindman,
B. W.; Jackiewicz, J.; Khomenko, E.; Komm, R.; Rajaguru, P.; Rempel,
M.; Roth, M.; Schlichenmaier, R.; Schunker, H.; Spruit, H. C.;
Strassmeier, K. G.; Thompson, M. J.; Zharkov, S.
Bibcode: 2010SoPh..267....1M
Altcode: 2009arXiv0912.4982M; 2010SoPh..tmp..171M
While sunspots are easily observed at the solar surface, determining
their subsurface structure is not trivial. There are two main
hypotheses for the subsurface structure of sunspots: the monolithic
model and the cluster model. Local helioseismology is the only means
by which we can investigate subphotospheric structure. However, as
current linear inversion techniques do not yet allow helioseismology to
probe the internal structure with sufficient confidence to distinguish
between the monolith and cluster models, the development of physically
realistic sunspot models are a priority for helioseismologists. This
is because they are not only important indicators of the variety of
physical effects that may influence helioseismic inferences in active
regions, but they also enable detailed assessments of the validity of
helioseismic interpretations through numerical forward modeling. In
this article, we provide a critical review of the existing sunspot
models and an overview of numerical methods employed to model wave
propagation through model sunspots. We then carry out a helioseismic
analysis of the sunspot in Active Region 9787 and address the serious
inconsistencies uncovered by Gizon et al. (2009a, 2009b). We find that
this sunspot is most probably associated with a shallow, positive
wave-speed perturbation (unlike the traditional two-layer model)
and that travel-time measurements are consistent with a horizontal
outflow in the surrounding moat.
Title: Multi-layer Study of Wave Propagation in Sunspots
Authors: Felipe, T.; Khomenko, E.; Collados, M.; Beck, C.
Bibcode: 2010ApJ...722..131F
Altcode: 2010arXiv1008.4004F
We analyze the propagation of waves in sunspots from the photosphere
to the chromosphere using time series of co-spatial Ca II H intensity
spectra (including its line blends) and polarimetric spectra of Si
I λ10,827 and the He I λ10,830 multiplet. From the Doppler shifts
of these lines we retrieve the variation of the velocity along the
line of sight at several heights. Phase spectra are used to obtain
the relation between the oscillatory signals. Our analysis reveals
standing waves at frequencies lower than 4 mHz and a continuous
propagation of waves at higher frequencies, which steepen into shocks
in the chromosphere when approaching the formation height of the Ca
II H core. The observed nonlinearities are weaker in Ca II H than in
He I lines. Our analysis suggests that the Ca II H core forms at a
lower height than the He I λ10,830 line: a time delay of about 20 s is
measured between the Doppler signal detected at both wavelengths. We fit
a model of linear slow magnetoacoustic wave propagation in a stratified
atmosphere with radiative losses according to Newton's cooling law to
the phase spectra and derive the difference in the formation height
of the spectral lines. We show that the linear model describes well
the wave propagation up to the formation height of Ca II H, where
nonlinearities start to become very important.
Title: Magneto-acoustic Waves in Sunspots: First Results From a New
Three-dimensional Nonlinear Magnetohydrodynamic Code
Authors: Felipe, T.; Khomenko, E.; Collados, M.
Bibcode: 2010ApJ...719..357F
Altcode: 2010arXiv1006.2998F
Waves observed in the photosphere and chromosphere of sunspots
show complex dynamics and spatial patterns. The interpretation
of high-resolution sunspot wave observations requires modeling
of three-dimensional (3D) nonlinear wave propagation and mode
transformation in the sunspot upper layers in realistic spot model
atmospheres. Here, we present the first results of such modeling. We
have developed a 3D nonlinear numerical code specially designed to
calculate the response of magnetic structures in equilibrium to an
arbitrary perturbation. The code solves the 3D nonlinear MHD equations
for perturbations; it is stabilized by hyper-diffusivity terms and is
fully parallelized. The robustness of the code is demonstrated by a
number of standard tests. We analyze several simulations of a sunspot
perturbed by pulses of different periods at a subphotospheric level,
from short periods, introduced for academic purposes, to longer and
realistic periods of 3 and 5 minutes. We present a detailed description
of the 3D mode transformation in a non-trivial sunspot-like magnetic
field configuration, including the conversion between fast and slow
magneto-acoustic waves and the Alfvén wave, by calculation of the wave
energy fluxes. Our main findings are as follows: (1) the conversion from
acoustic to the Alfvén mode is only observed if the driving pulse is
located out of the sunspot axis, but this conversion is energetically
inefficient; (2) as a consequence of the cutoff effects and refraction
of the fast magneto-acoustic mode, the energy of the evanescent waves
with periods around 5 minutes remains almost completely below the level
β = 1; (3) waves with frequencies above the cutoff propagate field
aligned to the chromosphere and their power becomes dominating over that
of evanescent 5 minute oscillations, in agreement with observations.
Title: Mode transformation and frequency change with height in 3D
numerical simulations of magneto-acoustic wave propagation in sunspots
Authors: Felipe, T.; Khomenko, E.; Collados, M.
Bibcode: 2010arXiv1005.3684F
Altcode:
Three-dimensional numerical simulations of magnetoacoustic wave
propagation are performed in a sunspot atmosphere with a computational
domain covering from the photosphere to the chromosphere. The
wave source, with properties resembling the solar spectrum, is
located at different distances from the axis of the sunspot for
each simulation. These results are compared with the theory of mode
transformation and also with observational features. Simulations show
that the dominant oscillation frequency in the chromosphere decreases
with the radial distance from the sunspot axis. The energy flux of the
different wave modes involved, including de Alfvén mode, is evaluated
and discussed.
Title: Towards pulsation mode identification in 3-D: theoretical
simulations of line profile variations in roAp stars
Authors: Kochukhov, O.; Khomenko, E.
Bibcode: 2010arXiv1004.0139K
Altcode:
Time-resolved spectroscopic observations of rapidly oscillating Ap
(roAp) stars show a complex picture of propagating magneto-acoustic
pulsation waves, with amplitude and phase strongly changing as a
function of atmospheric height. We have recently conducted numerical,
non-linear MHD simulations to get an insight into the complex
atmospheric dynamics of magnetic pulsators. Here we use the resulting
time-dependent atmospheric structure and velocity field to predict line
profile variations for roAp stars. These calculations use realistic
atmospheric structure, account for vertical chemical stratification
and treat the line formation in pulsating stellar atmosphere without
relying on the simplistic single-layer approximation universally
adopted for non-radial pulsators. The new theoretical calculations
provide an essential tool for interpreting the puzzling complexity of
the spectroscopic pulsations in roAp stars.
Title: Numerical Simulation of Excitation and Propagation of
Helioseismic MHD Waves in Magnetostatic Models of Sunspots
Authors: Parchevsky, K.; Kosovichev, A.; Khomenko, E.; Olshevsky,
V.; Collados, M.
Bibcode: 2010arXiv1002.1117P
Altcode:
We present comparison of numerical simulations of propagation of
MHD waves,excited by subphotospheric perturbations, in two different
("deep" and "shallow") magnetostatic models of the sunspots. The "deep"
sunspot model distorts both the shape of the wavefront and its amplitude
stronger than the "shallow" model. For both sunspot models, the surface
gravity waves (f-mode) are affected by the sunspots stronger than
the acoustic p-modes. The wave amplitude inside the sunspot depends
on the photospheric strength of the magnetic field and the distance
of the source from the sunspot axis. For the source located at 9 Mm
from the center of the sunspot, the wave amplitude increases when
the wavefront passes through the central part of the sunspot. For
the source distance of 12 Mm, the wave amplitude inside the sunspot
is always smaller than outside. For the same source distance from
the sunspot center but for the models with different strength of the
magnetic field, the wave amplitude inside the sunspot increases with
the strength of the magnetic field. The simulations show that unlike
the case of the uniform inclined background magnetic field, the p-
and f-mode waves are not spatially separated inside the sunspot where
the magnetic field is strongly non-uniform. These properties have to
be taken into account for interpretation of observations of MHD waves
traveling through sunspot regions.
Title: Magnetic Fingerprints of Solar and Stellar Oscillations
Authors: Khomenko, Elena
Bibcode: 2010ASSP...14...51K
Altcode: 2010hsa5.conf...51K; 2008arXiv0812.0042K
Waves connect all the layers of the Sun, from its interior to the upper
atmosphere. It is becoming clear now the important role of magnetic
field on the wave propagation. Magnetic field modifies propagation
speed of waves, thus affecting the conclusions of helioseismological
studies. It can change the direction of the wave propagation, help
channeling them straight up to the corona, extending the dynamic
and magnetic couplings between all the layers. Modern instruments
provide measurements of complex patterns of oscillations in solar
active regions and of tiny effects such as temporal oscillations
of the magnetic field. The physics of oscillations in a variety of
magnetic structures of the Sun is similar to that of pulsations of
stars that posses strong magnetic fields, such as roAp stars. All these
arguments point toward a need of systematic self-consistent modeling
of waves in magnetic structures that is able to take into account
the complexity of the magnetic field configurations. In this paper,
we describe simulations of this kind, summarize our recent findings
and bring together results from the theory and observations.
Title: Simulations of Waves in Sunspots
Authors: Khomenko, E.
Bibcode: 2009ASPC..416...31K
Altcode: 2008arXiv0812.0040K
A magnetic field modifies the properties of waves in a complex
way. Significant advances have been made recently in our understanding
of the physics of waves in solar active regions with the help of
analytical theories, numerical simulations, and high-resolution
observations. In this contribution we review the current ideas in the
field, with the emphasis on theoretical models of waves in sunspots.
Title: NUMERICAL SIMULATION OF PROPAGATION AND SCATTERING OF THE
MHD WAVES IN SUNSPOTS
Authors: Parchevsky, K.; Kosovichev, A. G.; Khomenko, E.; Collados, M.
Bibcode: 2009AGUFMSH23B1535P
Altcode:
We present comparison of numerical simulation results of MHD wave
propagation in two different magnitostatic models of sunspots
refferred to as "deep" and "shallow" models. The "deep" model has
convex shape of magnetic field lines near the photosphere and non-zero
horizorntal perturbations of the sound speed up to the bottom of the
model (7.5 Mm). The "shallow" model has concave shape of the magnetic
field lines near the photosphere and horizontally uniform sound speed
below 2 Mm. Common feature of MHD waves behaviour in these two models
is that for weak magnetic field (less than 1kG at the photosphere)
waves reduce their amplitude when they reach the center of the sunspot
and restore the amplitude when pass the center. For the "deep" model
this effect is bigger than for the "shallow" model. The wave amplitude
inside sunspots depends on the strength of the magnetic field. For the
"shallow" model with photospheric magnetic field of 2.2 kG the wave
amplitude inside the sunspot becomes bigger than outside (opposite to
the weak magnetic field). The wave amplitude depends on the distance
of the source from the sunspot center. For the "shallow" model and
source distance of 9 Mm from the sunspot center the wave amplitude at
some moment (when the wavefront passes the sunspot center) becomes
bigger inside the sunspot than outside. For the source distance
of 12 Mm the wave amplitude remains smaller inside the sunspot
than outside for all moments of time. Using filtering technique we
separated magnetoacoustic and magnetogravity waves. Simulations show
that the sunspot changes the shape of the wave front and amplitude
of the f-modes significantly stronger than the p-modes. It is shown,
that inside the sunspot magnetoacoustic and magnetogravity waves are
not spatially separated unlike the case of the horizontally uniform
background model. Strong Alfven wave is generated at the wave source
location in the "deep" model. This wave exists in the "shallow" model
as well, but with much smaller amplitude.
Title: The solar Ba{II} 4554 Å line as a Doppler diagnostic: NLTE
analysis in 3D hydrodynamical model
Authors: Shchukina, N. G.; Olshevsky, V. L.; Khomenko, E. V.
Bibcode: 2009A&A...506.1393S
Altcode: 2009arXiv0905.0985S
Aims: The aim of this paper is to analyse the validity of the
Dopplergram and λ-meter techniques for the Doppler diagnostics
of solar photospheric velocities using the Ba II 4554 Å line.
Methods: Both techniques are evaluated by means of NLTE radiative
transfer calculations of the Ba II 4554 Å line in a three-dimensional
hydrodynamical model of solar convection. We consider the cases of
spatially unsmeared profiles and the profiles smeared to the resolution
of ground-based observations.
Results: We find that: (i)
speckle-reconstructed Dopplergram velocities reproduce the “true”
velocities well at heights around 300 km, except for intergranular lanes
with strong downflows where the velocity can be overestimated; (ii)
the λ-meter velocities give a good representation of the “true”
velocities through the whole photosphere, both under the original and
reduced spatial resolutions. The velocities derived from the inner wing
of smeared Ba II 4554 Å line profiles are more reliable than those for
the outer wing. Only under high spatial resolution does the inner wing
velocities calculated in intergranular regions give an underestimate
(or even a sign reversal) compared with the model velocities; (iii)
NLTE effects should be taken into account in modelling the Ba II 4554
Å line profiles. Such effects are more pronounced in intergranular
regions.
Conclusions: Our analysis supports the opinion that the
Dopplergram technique applied to the Ba II 4554 Å line is a valuable
tool for the Doppler diagnostics of the middle photosphere around
300 km. The λ-meter technique applied to this line gives us a good
opportunity to “trace” the non-thermal motions along the whole
photosphere up to the temperature minimum and lower chromosphere. Appendix is only available in electronic form at http://www.aanda.org
Title: Sunspot seismic halos generated by fast MHD wave refraction
Authors: Khomenko, E.; Collados, M.
Bibcode: 2009A&A...506L...5K
Altcode: 2009arXiv0905.3060K
Aims: We suggest an explanation for the high-frequency power excess
surrounding active regions known as seismic halos.
Methods:
We use numerical simulations of magneto-acoustic wave propagation in
a magnetostatic sunspot model.
Results: We propose that seismic
halos can be caused by the additional energy injected by high-frequency
fast mode waves refracted in the higher atmosphere due to the rapid
increase of the Alfvén speed. Our model qualitatively explains the
magnitude of the halo and allows us to make predictions of its behavior
that can be checked in future observations.
Title: Solar granulation from photosphere to low chromosphere observed
in Ba II 4554 Å line
Authors: Kostik, R.; Khomenko, E.; Shchukina, N.
Bibcode: 2009A&A...506.1405K
Altcode: 2009arXiv0909.1210K
Aims: The purpose of this paper is to characterize the statistical
properties of solar granulation in the photosphere and low chromosphere
up to 650 km.
Methods: We use velocity and intensity variations
obtained at different atmospheric heights from observations in Ba II
4554 Å. The observations were done during good seeing conditions at
the VTT at the Observatorio del Teide on Tenerife. The line core forms
rather high in the atmosphere and allows granulation properties to
be studied at heights that have been not accessed before in similar
studies. In addition, we analyze the synthetic profiles of the Ba II
4554 Å line by the same method computed taking NLTE effects into
account in the 3D hydrodynamical model atmosphere.
Results:
We suggest a 16-column model of solar granulation depending on the
direction of motion and on the intensity contrast measured in the
continuum and in the uppermost layer. We calculate the heights of
intensity contrast sign reversal and velocity sign reversal. We show
that both parameters depend strongly on the granulation velocity and
intensity at the bottom photosphere. The larger the two parameters,
the higher the reversal takes place in the atmosphere. On average, this
happens at about 200-300 km. We suggest that this number also depends
on the line depth of the spectral line used in observations. Despite
the intensity and velocity reversal, about 40% of the column structure
of granulation is preserved up to heights around 650 km.
Title: The energy of waves in the photosphere and lower
chromosphere. I. Velocity statistics
Authors: Beck, C.; Khomenko, E.; Rezaei, R.; Collados, M.
Bibcode: 2009A&A...507..453B
Altcode: 2009arXiv0905.1011B
Context: Acoustic waves are one of the primary suspects besides magnetic
fields for the chromospheric heating process to temperatures above
radiative equilibrium (RE).
Aims: We derived the mechanical
wave energy as seen in line-core velocities on disc centre to obtain
a measure of mechanical energy flux with height for a comparison
with the energy requirements in a semi-empirical atmosphere model,
the Harvard-Smithsonian reference atmosphere (HSRA).
Methods: We
analyzed a 1-hour time series and a large-area map of Ca II H spectra
on the traces of propagating waves. We analyzed the velocity statistics
of several spectral lines in the wing of Ca II H, and the line-core
velocity of Ca II H. We converted the velocity amplitudes into volume
(∝ ρ v^2) and mass energy densities (∝ v^2). For comparison, we
used the increase of internal energy (∝ R ρ Δ T) necessary to lift
a RE atmosphere to the HSRA temperature stratification.
Results:
We find that the velocity amplitude grows in agreement with linear
wave theory and thus slower with height than predicted from energy
conservation. The mechanical energy of the waves above around z ~ 500 km
is insufficient to maintain on a long-term average the chromospheric
temperature rise in the semi-empirical HSRA model. The intensity
variations of the Ca line core (z ~ 1000 km) can, however, be traced
back to the velocity variations of the lowermost forming spectral line
considered (z ~ 250 km).
Conclusions: The chromospheric intensity,
and hence, (radiation) temperature variations are seen to be induced by
passing waves originating in the photosphere. The wave energy is found
to be insufficient to maintain the temperature stratification of the
semi-empirical HSRA model above 500 km. We will in a following paper of
this series investigate the energy contained in the intensity variations
to see if the semi-empirical model is appropriate for the spectra.
Title: Simulations of Magnetoacoustic Pulsations in Atmospheres of
Rapidly Oscillating Ap Stars
Authors: Khomenko, E.; Kochukhov, O.
Bibcode: 2009ApJ...704.1218K
Altcode: 2009arXiv0909.1214K
Rapidly oscillating Ap (roAp) stars exhibit an astrophysically
interesting combination of strong, dipolar-like magnetic fields and
high-overtone p-mode pulsations similar to the Sun. Recent time-resolved
spectroscopy of these stars unravelled a complex picture of propagating
magnetoacoustic pulsation waves, with amplitude and phase strongly
changing as a function of atmospheric height. To interpret these
observations and gain a new insight into the atmospheric dynamics
of roAp stars we have carried out two-dimensional time-dependent,
non-linear magnetohydrodynamical simulations of waves for a realistic
atmospheric stratification of a cool Ap star. We explore a grid
of simulations in a wide parameter space, treating oscillations
of the velocity, magnetic field, and thermodynamic quantities in a
self-consistent manner. Our simulations foster a new understanding
of the influence of the atmosphere and the magnetic field on the
propagation and reflection properties of magnetoacoustic waves,
formation of node surfaces, and relative variation of different
quantities. Our simulations reproduce all main features of the observed
pulsational behavior of roAp stars. We show, for the first time, that
the overall depth dependence of the pulsations in roAp atmospheres is
strongly influenced by the density inversion at the photospheric base.
Title: Observational Signatures of Numerically Simulated MHD Waves
in Small-scale Flux Sheets
Authors: Khomenko, E.; Collados, M.; Felipe, T.
Bibcode: 2009ASPC..405..183K
Altcode: 2008arXiv0801.3966K
We present some results obtained from the synthesis of Stokes profiles
in small-scale flux sheets with propagating MHD waves. To that aim,
2D flux sheets showing internal structure have been excited with 5
min period drivers, allowing non-linear waves to propagate inside the
magnetic structure. The observational signatures of these waves in
Stokes profiles of several spectral lines that are commonly used in
spectropolarimetric measurements are discussed.
Title: Simulations of magneto-hydrodynamic waves in atmospheres of
roAp stars
Authors: Khomenko, Elena; Kochukhov, Oleg
Bibcode: 2009IAUS..259..409K
Altcode: 2009arXiv0901.1204K
We report 2D time-dependent non-linear magneto-hydrodynamical
simulations of waves in the atmospheres of roAp stars. We explore a grid
of simulations in a wide parameter space. The aim of our study is to
understand the influence of the atmosphere and the magnetic field on
the propagation and reflection properties of magneto-acoustic waves,
formation of shocks and node layers.
Title: Theoretical Modeling of Propagation of Magnetoacoustic Waves
in Magnetic Regions Below Sunspots
Authors: Khomenko, E.; Kosovichev, A.; Collados, M.; Parchevsky, K.;
Olshevsky, V.
Bibcode: 2009ApJ...694..411K
Altcode: 2008arXiv0809.0278K
We use two-dimensional numerical simulations and eikonal approximation
to study properties of magnetohydrodynamic (MHD) waves traveling below
the solar surface through the magnetic structure of sunspots. We
consider a series of magnetostatic models of sunspots of different
magnetic field strengths, from 10 Mm below the photosphere to the
low chromosphere. The purpose of these studies is to quantify the
effect of the magnetic field on local helioseismology measurements
by modeling waves excited by subphotospheric sources. Time-distance
propagation diagrams and wave travel times are calculated for models
of various field strengths and compared to the nonmagnetic case. The
results clearly indicate that the observed time-distance helioseismology
signals in sunspot regions correspond to fast MHD waves. The slow MHD
waves form a distinctly different pattern in the time-distance diagram,
which has not been detected in observations. The numerical results are
in good agreement with the solution in the short-wavelength (eikonal)
approximation, providing its validation. The frequency dependence of
the travel times is in good qualitative agreement with observations.
Title: Magnetohydrostatic Sunspot Models from Deep Subphotospheric
to Chromospheric Layers
Authors: Khomenko, E.; Collados, M.
Bibcode: 2008ApJ...689.1379K
Altcode: 2008arXiv0808.3571K
In order to understand the influence of magnetic fields on the
propagation properties of waves, as derived from different local
helioseismology techniques, forward modeling of waves is required. Such
calculations need a model in magnetohydrostatic equilibrium as an
initial atmosphere through which to propagate oscillations. We provide
a method to construct such a model in equilibrium for a wide range
of parameters, for use in simulations of artificial helioseismologic
data. The method combines the advantages of self-similar solutions and
current-distributed models. A set of models is developed by numerical
integration of magnetohydrostatic equations from the subphotospheric
to chromospheric layers.
Title: On the Possible Sources of Chromospheric Heating
Authors: Beck, C.; Collados, M. Vera; Khomenko, E.; Rezaei, R.
Bibcode: 2008ESPM...12.2.14B
Altcode:
The chromospheric temperature rise to values above the photospheric
temperature cannot be due to radiative energy transport alone. We will
outline different possibilities for the additional energy transport in
the solar atmosphere by processes that require (or exclude) the presence
of magnetic fields. We will discuss which of them could be identified
and studied in detail using current data. To find the signature of
the different heating processes and derive quantitative estimates
of their efficiency, we analyzed simultaneous spectropolarimetric
observations of photospheric magnetic fields (@630 nm) and intensity
spectra of the chromospheric Ca II H line (396 nm). The mechanical
energy flux at several height layers was derived from the velocity
amplitudes of propagating acoustic waves seen in different spectral
lines. The enhancement of chromospheric (radiation) temperature above
the radiative equilibrium values was taken from an inversion of the
Ca II H spectra with the SIR code assuming local thermal equilibrium
(LTE) and complete redistribution (CRD). We compare the obtained energy
values with each other and with the energy requirements demanded by
theoretical/semi-empirical atmospheric models. We find that
the most important agent of chromospheric heating are propagating
(magneto-)acoustic waves, which suffice to explain the brightenings in
Ca II H spectra and their corresponding temperature enhancements. The
energy contained in these intensity variations of the Ca II H line,
however, is found to be insufficient to maintain a full-time and
full-volume "hot" chromosphere. Additional energy transport mechanisms
without a signature in the Ca II H spectra are thus necessary. Finally,
we will outline which improvements are to be expected with future
observations of higher quality (spatial resolution, enhanced
polarimetric sensitivity, temporal cadence, other spectral lines)
to be achieved with new ground-based telescopes like GREGOR or EST.
Title: Multi-layer Study of Wave Propagation in Sunspots
Authors: Felipe, T.; Khomenko, E.; Collados, M.; Beck, C.
Bibcode: 2008ESPM...12.2.12F
Altcode:
Observations in different spectral lines give us information about
the different layers of the solar atmosphere. Here we analyze
the propagation of waves in sunspots from the photosphere to the
chromosphere using time series of cospatial Ca II H intensity
spectra and polarimetric spectra of Si I 10827 A and He I 10830
A multiplet. From the Doppler shifts of these lines we retrieve
the temporal variations of the velocity along the line-of-sight
at several heights. Phase spectra are used to get the relation
between oscillatory signals measured at each spectral signature. Our
analysis reveals standing waves for frequencies lower than 3.5 mHz and
propagating waves for higher frequencies, which steepen into shocks
in the chromosphere. Oscillations are detectable in Ca II H wings and
they are propagated along line wing layers to the line core. Ca II H
core forms at a lower height than the He I 10830 A line. A time delay
of about 30 s is measured between the Doppler signals detected at both
wavelengths. We also find that in "cold" sunspots the Si I 10827 A forms
deeper than in the quiet sun. This type of measurements demonstrate
the importance of simultaneous co-spatial observations at different
wavelengths. Future infrastructures, such as GREGOR and EST, should
include multi-wavelength capabilities to make possible the study of
the photosphere-chromosphere connection with the highest spatial and
temporal resolution.
Title: Nonlinear Numerical Simulations of Magneto-Acoustic Wave
Propagation in Small-Scale Flux Tubes
Authors: Khomenko, E.; Collados, M.; Felipe, T.
Bibcode: 2008SoPh..251..589K
Altcode: 2008SoPh..tmp...32K; 2007arXiv0710.3335K
We present results of nonlinear, two-dimensional, numerical simulations
of magneto-acoustic wave propagation in the photosphere and chromosphere
of small-scale flux tubes with internal structure. Waves with realistic
periods of three to five minutes are studied, after horizontal and
vertical oscillatory perturbations are applied to the equilibrium
model. Spurious reflections of shock waves from the upper boundary
are minimized by a special boundary condition. This has allowed us to
increase the duration of the simulations and to make it long enough to
perform a statistical analysis of oscillations. The simulations show
that deep horizontal motions of the flux tube generate a slow (magnetic)
mode and a surface mode. These modes are efficiently transformed
into a slow (acoustic) mode in the vA<cS
atmosphere. The slow (acoustic) mode propagates vertically along
the field lines, forms shocks, and remains always within the flux
tube. It might effectively deposit the energy of the driver into the
chromosphere. When the driver oscillates with a high frequency, above
the cutoff, nonlinear wave propagation occurs with the same dominant
driver period at all heights. At low frequencies, below the cutoff,
the dominant period of oscillations changes with height from that
of the driver in the photosphere to its first harmonic (half period)
in the chromosphere. Depending on the period and on the type of the
driver, different shock patterns are observed.
Title: Seismology of Sunspots: An Interplay between Temperature and
Magnetic Field Structures
Authors: Olshevsky, V.; Khomenko, E.; Collados, M.
Bibcode: 2008ESPM...12..3.2O
Altcode:
Using a numerical three-dimensional MHD modelling of magneto-acoustic
wave propagation in a realistic magnetostatic sunspot model we
investigate the influence of the magnetic field on the parameters
measured by local helioseismology. We find that the variations of
temperature as well as the presence of the magnetic field cause
important changes to the wave travel times. Magnetic field speeds up
the waves to considerable amount, while the temperature depression
within a sunspot causes the opposite action. The calculated travel time
differences between the unmagnetized and magnetized atmospheres lie in
the range typically obtained from local helioseismology correlation
analysis. Our numerical results are also in agreement with the
analytical calculations of the travel times applying WKB technique.
Title: Channeling 5 Minute Photospheric Oscillations into the Solar
Outer Atmosphere through Small-Scale Vertical Magnetic Flux Tubes
Authors: Khomenko, E.; Centeno, R.; Collados, M.; Trujillo Bueno, J.
Bibcode: 2008ApJ...676L..85K
Altcode: 2008arXiv0802.0938K
We report two-dimensional MHD simulations which demonstrate that
photospheric 5 minute oscillations can leak into the chromosphere
inside small-scale vertical magnetic flux tubes. The results of
our numerical experiments are compatible with those inferred from
simultaneous spectropolarimetric observations of the photosphere and
chromosphere obtained with the Tenerife Infrared Polarimeter (TIP)
at 10830 Å. We conclude that the efficiency of energy exchange by
radiation in the solar photosphere can lead to a significant reduction
of the cutoff frequency and may allow for the propagation of the 5
minute waves vertically into the chromosphere.
Title: Multiline Spectropolarimetry of the Quiet Sun at 5250 and
6302 Å
Authors: Socas-Navarro, H.; Borrero, J. M.; Asensio Ramos, A.;
Collados, M.; Domínguez Cerdeña, I.; Khomenko, E. V.; Martínez
González, M. J.; Martínez Pillet, V.; Ruiz Cobo, B.; Sánchez
Almeida, J.
Bibcode: 2008ApJ...674..596S
Altcode:
The reliability of quiet-Sun magnetic field diagnostics based on the
Fe I lines at 6302 Å has been questioned by recent work. Here we
present the results of a thorough study of high-resolution multiline
observations taken with the new spectropolarimeter SPINOR, comprising
the 5250 and 6302 Å spectral domains. The observations were analyzed
using several inversion algorithms, including Milne-Eddington,
LTE with 1 and 2 components, and MISMA codes. We find that the
line-ratio technique applied to the 5250 Å lines is not sufficiently
reliable to provide a direct magnetic diagnostic in the presence
of thermal fluctuations and variable line broadening. In general,
one needs to resort to inversion algorithms, ideally with realistic
magnetohydrodynamic constrains. When this is done, the 5250 Å lines
do not seem to provide any significant advantage over those at 6302
Å. In fact, our results point toward a better performance with the
latter (in the presence of turbulent line broadening). In any case,
for very weak flux concentrations, neither spectral region alone
provides sufficient constraints to fully disentangle the intrinsic
field strengths. Instead, we advocate for a combined analysis of both
spectral ranges, which yields a better determination of the quiet-Sun
magnetic properties. Finally, we propose the use of two other Fe I
lines (at 4122 and 9000 Å) with identical line opacities that seem
to work much better than the others.
Title: Observations of a bright plume in solar granulation
Authors: Kostik, R. I.; Khomenko, E. V.
Bibcode: 2007A&A...476..341K
Altcode:
Aims:The aim of this paper is to analyze the thermal properties,
oscillatory, and flow motions of a bright, long-lasting feature
observed in solar granulation, which we call the plume.
Methods:
We used the spectral observations of quiet granulation at solar disc
center, including the two Fe II 5234 and Fe I 6393 Å lines recorded
simultaneously at the German Vacuum Tower telescope in Tenerife. The
recorded data revealed a stable, bright structure of 3-4 arcsec size
present during the whole 2.5 h of observations. We compare the velocity
fields extracted by means of a λ-meter method and temperature and
pressure stratification obtained after inversion of the profiles related
to granules, intergranular lanes, and the plume.
Results: The
following results were obtained: (i) the correlation between variations
in convective velocity and intensity in the plume is close to zero
at all observed heights; (ii) the velocity flow in the plume changes
from a downflow in the deep layers to an upflow in the upper layers;
(iii) the brightness of the plume increases with height; (iv) the
amplitudes of the five-minute oscillations of intensity and velocity
are twice lower in the plume than outside, and vertically propagating
waves are observed; (v) the plume is hotter and denser than the quiet
Sun in the upper photosphere.
Conclusions: We conclude that the
observed phenomenon has a non-convective origin. The decrease in the
amplitudes of oscillations in the plume cannot be attributed to the
higher density in comparison to the surrounding atmosphere. Along with
other findings, this indicates the possible presence of magnetic field.
Title: Multi-Line Quiet Sun Spectro-Polarimetry at 5250 and 6302 Å
Authors: Socas-Navarro, H.; Borrero, J.; Asensio Ramos, A.; Collados,
M.; Domínguez Cerdeña, I.; Khomenko, E. V.; Martínez González,
M. J.; Martínez Pillet, V.; Ruiz Cobo, B.; Sánchez Almeida, J.
Bibcode: 2007arXiv0710.1099S
Altcode:
The reliability of quiet Sun magnetic field diagnostics based on the
\ion{Fe}{1} lines at 6302 Åhas been questioned by recent work. We
present here the results of a thorough study of high-resolution
multi-line observations taken with the new spectro-polarimeter SPINOR,
comprising the 5250 and 6302 Åspectral domains. The observations were
analyzed using several inversion algorithms, including Milne-Eddington,
LTE with 1 and 2 components, and MISMA codes. We find that the
line-ratio technique applied to the 5250 Ålines is not sufficiently
reliable to provide a direct magnetic diagnostic in the presence
of thermal fluctuations and variable line broadening. In general,
one needs to resort to inversion algorithms, ideally with realistic
magneto-hydrodynamical constrains. When this is done, the 5250 Ålines
do not seem to provide any significant advantage over those at 6302
Å. In fact, our results point towards a better performance with the
latter (in the presence of turbulent line broadening). In any case,
for very weak flux concentrations, neither spectral region alone
provides sufficient constraints to fully disentangle the intrinsic field
strengths. Instead, we advocate for a combined analysis of both spectral
ranges, which yields a better determination of the quiet Sun magnetic
properties. Finally, we propose the use of two other \ion{Fe}{1} lines
(at 4122 and 9000 Å) with identical line opacities that seem to work
much better than the others.
Title: On the Stokes V Amplitude Ratio as an Indicator of the Field
Strength in the Solar Internetwork
Authors: Khomenko, E.; Collados, M.
Bibcode: 2007ApJ...659.1726K
Altcode:
The results of the determination of magnetic field strength from weak
polarimetric signals in solar internetwork regions are contradictory. We
investigate the origin of this contradiction with the help of MHD
simulations. It is shown that the Stokes V amplitude ratio of the
Fe I λλ15652-15648 lines is a good indicator of kG magnetic field
concentrations, even for magnetic fields with a complex internal
structure like those in MHD simulations. The Stokes V amplitude ratio
of the Fe I λλ5247-5250 lines also shows a good correlation with
magnetic field strength. However, in simulations with a flux level
appropriate for the internetwork, it gives values corresponding to
sub-kG fields. The reason is the rapid decrease of the field strength
with height in kG magnetic field concentrations. These lines sample
high regions of the atmosphere, where the field is already below
kG levels. We also find that the Stokes V amplitude ratio of the Fe
I λλ6301-6302 lines shows no correlation with the magnetic field
strength. The reason lies in the large difference in the heights of
formation of these two lines. The value of the magnetic field strength
obtained from the Fe I λλ6301 and 6302 lines depends crucially on the
treatment of gradients of the magnetic field, line-of-sight velocity,
and temperature, even at a numerical spatial resolution of 20 km.
Title: Line ratio method applied to inter-network magnetic fields
Authors: Khomenko, E.; Collados, M.
Bibcode: 2007msfa.conf..303K
Altcode:
We investigate the validity of the Stokes V amplitude ratio as an
indicator of the magnetic field strength in solar inter-network
regions with the help ofMHD simulations. We show that the Stokes V
amplitude ratio of the Fe I 15652-15648 Å lines and Fe I 5247-5250 Å
lines show a good correlation with the magnetic field strength even
for magnetic fields with a complex internal structure like those in
MHD simulations. However, in the latter case, the amplitude ratio
sub-estimates the magnetic field strength, always revealing sub-kG
values. The Stokes V amplitude ratio of the Fe I 6301-6302 Å lines
shows no correlation with the magnetic field strength. The reasons of
this behaviour are explained.
Title: Numerical modeling of MHD wave propagation in sunspots:
a 3D case
Authors: Olshevsky, V.; Khomenko, E.; Collados, M.
Bibcode: 2007msfa.conf..347O
Altcode:
We present the first results of a 3D numerical modeling of linear MHD
wave propagation in a realistic sunspot model. In our simulations,
a piston located at the base of the photosphere generates waves with
a certain period. The ratio between the acoustic and the Alfven speed,
cS /vA, decreases from much larger than one at the photosphere to much
lower than one in the chromosphere in our simulation domain. Waves
propagate through the region where cS << vA, where mode
transformation is observed. At a somewhat higher region, where cS =
vA, the fast (magnetic) mode reflects back to the photosphere due
to the vertical and horizontal gradients of vA. The slow (acoustic)
mode propagates to the upper layers and increases its velocity
amplitude. Unlike the 2D simulations, the Alfven mode is also generated
by the piston and experiences transformations at the cS = vA layer. The
behaviour of this mode requires further study.
Title: Magnetic field inversions from Stokes profiles generated by
MHD simulations
Authors: Khomenko, E.; Collados, M.
Bibcode: 2007MmSAI..78..166K
Altcode:
We report tests of inversion methods applied to complex Stokes spectra
generated by realistic MHD simulations. The average magnetic field
strength of the simulations used is of 30 and 140 G, which we believe
is representative of quiet solar regions. The behaviour of the Fe I at
1.56 mu m and 630 nm lines is analyzed. The tests have been done with
the original resolution of simulations (20 km) and also with resolution
of 0.6'' and 1.4'' (after having conveniently degraded the images).
Title: On the Determination of Magnetic Field Strength and Flux
in Inter-Network
Authors: Khomenko, E.; Collados, M.,
Bibcode: 2006ASPC..358...42K
Altcode:
The results of the determination of magnetic field strength and
flux from weak polarimetric signals in solar inter-network regions
are contradictory. We investigate the origin of this contradiction
with the help of MHD simulations. It is shown that the Stokes-V line
ratio of the Fe I 5247/5250 Å and 15652/15648 Å line pairs is a good
indicator of kG magnetic field concentrations, even for magnetic fields
with a complex internal structure like those in MHD simulations. On the
contrary, the Stokes-V line ratio of the Fe I 6301/6302 Å lines shows
no correlation with magnetic field strength. The reason lies in the
large difference in the heights of formation of these two lines. The
value of the magnetic field strength obtained from the inversion of
the Fe I 6301 Å and 6302 Å lines depends crucially on the treatment
of gradients of magnetic field, LOS velocity, and temperature even at
numerical spatial resolution of 20 km.
Title: Numerical Modeling of Magnetohydrodynamic Wave Propagation
and Refraction in Sunspots
Authors: Khomenko, E.; Collados, M.
Bibcode: 2006ApJ...653..739K
Altcode:
We present numerical simulations of magnetoacoustic wave propagation
from the photosphere to the low chromosphere in a magnetic sunspot-like
structure. A thick flux tube, with dimensions typical of a small
sunspot, is perturbed by a vertical or horizontal velocity pulse
at the photospheric level. The type of mode generated by the pulse
depends on the ratio between the sound speed cS and the
Alfvén speed vA, on the magnetic field inclination at the
location of the driver, and on the shape of the pulse in the horizontal
direction. Mode conversion is observed to occur in the region in which
both characteristic speeds have similar values. The fast (magnetic)
mode in the region cS<vA does not reach the
chromosphere and reflects back to the photosphere at a somewhat higher
layer than the cS=vA line. This behavior is due
to wave refraction, caused primarily by the vertical and horizontal
gradients of the Alfvén speed. The slow (acoustic) mode continues up
to the chromosphere along the magnetic field lines with increasing
amplitude. We show that this behavior is characteristic for waves
in a wide range of periods generated at different distances from the
sunspot axis. Since an important part of the energy of the pulse is
returned back to the photosphere by the fast mode, the mechanism of
energy transport from the photosphere to the chromosphere by waves in
sunspots is rather ineffective.
Title: Diagnostics of Quiet-Sun Magnetism
Authors: Khomenko, E.
Bibcode: 2006ASPC..354...63K
Altcode:
Most of our knowledge of solar surface magnetism comes from the analysis
of polarization spectra. The Stokes spectra contain detailed information
on the structure and dynamics of the magnetized photospheric plasma
and its interaction with convection, i.e., magnetoconvection. The
interpretation of high-resolution observations requires sophisticated
techniques such as radiative transfer of polarized light in 3D model
atmospheres. On the other hand, 3D magnetoconvection simulations include
elaborate physics and are becoming sufficiently realistic to make
predictions about the complex processes that take place in the Sun's
magnetized atmosphere. This paper concentrates on the diagnostics of
the magnetic fields in quiet solar photospheric regions outside sunspots
and active regions. Until recently the influence of the magnetic field
on the dynamics of these regions was considered unimportant. However,
it turns out that a considerable amount of magnetic energy is probably
stored in the ``quiet'' Sun. The issue of quiet solar magnetism remains
open and is much debated in the literature.
Title: Fine structure of wave motions in the solar photosphere:
Observations and theory
Authors: Kostyk, R. I.; Shchukina, N. G.; Khomenko, E. V.
Bibcode: 2006ARep...50..588K
Altcode:
Spectral observations of the 639.361-nm FeI line at the center of
the quiet solar disk with high spatial (0.4″) and temporal (10
s) resolution are used to investigate the behavior of local 5-min
oscillations over granules and intergranular lanes. The power of the
5-min oscillations in the upper photosphere (at heights of H ≈ 490 km)
is higher the faster the convective motions in the lower photosphere
(H ≈ 10 km). This suggests that turbulent convection is responsible
for the excitation of local solar oscillations. A statistical analysis
of the oscillations shows that, on average, both the intensity and
velocity of the oscillation amplitudes are greater over intergranular
lanes. This difference in amplitudes is present throughout the studied
heights in the photosphere (H = 0-490 km). The period at which the
power spectrum of velocity oscillations reaches its maximum is longer
over intergranules than over granules. Simulations of the propagation
of acoustic-gravity waves in an atmosphere taking into account the
convection pattern give a satisfactory explanation for the above
observed effects. It is concluded that the atmospheric modulation
of the 5-min oscillations is an additional or alternative mechanism
responsible for differences between these oscillations over granules
and intergranules.
Title: Simulations of - Acoustic Waves in Sunspots
Authors: Khomenko, E. V.; Collados, M.
Bibcode: 2005ESASP.596E..40K
Altcode: 2005ccmf.confE..40K
No abstract at ADS
Title: Stokes diagnostics of simulations of magnetoconvection of
mixed-polarity quiet-Sun regions
Authors: Khomenko, E. V.; Shelyag, S.; Solanki, S. K.; Vögler, A.
Bibcode: 2005A&A...442.1059K
Altcode:
Realistic solar magneto-convection simulations including the
photospheric layers are used to study the polarization of the Fe i
Zeeman-sensitive spectral lines at 6301.5, 6302.5, 15 648 and 15 652
Å. The Stokes spectra are synthesized in a series of snapshots with
a mixed-polarity magnetic field whose average unsigned strength varies
from < B > = 10 to 140 G. The effects of spatial resolution and
of the amount of magnetic flux in the simulation box on the profiles
shapes, amplitudes and shifts are discussed. The synthetic spectra show
many properties in common with those observed in quiet solar regions. In
particular, the simulations reproduce the width and depth of spatially
averaged Stokes I profiles, the basic classes of the Stokes V profiles
and their amplitude and area asymmetries, as well as the abundance of
the irregular-shaped Stokes V profiles. It is demonstrated that the
amplitudes of the 1.56 μm lines observed in the inter-network are
consistent with a "true" average unsigned magnetic field strength of
20 G. We show that observations using these and visible lines, carried
out under different seeing conditions (e.g., simultaneous observations
at different telescopes), may result in different asymmetries and
even opposite polarities of the profiles in the two spectral regions
observed at the same spatial point.
Title: Magnetic flux in the internetwork quiet Sun
Authors: Khomenko, E. V.; Martínez González, M. J.; Collados, M.;
Vögler, A.; Solanki, S. K.; Ruiz Cobo, B.; Beck, C.
Bibcode: 2005A&A...436L..27K
Altcode:
We report a direct comparison of the amplitudes of Stokes spectra of the
Fe i 630 nm and 1.56 μm lines produced by realistic MHD simulations
with simultaneous observations in the same spectral regions. The
Stokes spectra were synthesized in snapshots with a mixed polarity
magnetic field having a spatially averaged strength, < B >,
between 10 and 30 G. The distribution of Stokes V amplitudes depends
sensitively on < B >. A quiet inter-network region was observed
at the German VTT simultaneously with TIP (1.56 μm) and POLIS (630
nm). We find that the Stokes V amplitudes of both infrared and visible
observations are best reproduced by the simulation snapshot with <
B > = 20 G. In observations with 1 resolution, up to 2/3 of the
magnetic flux can remain undetected.
Title: Bright features in the solar photosphere
Authors: Kostik, R. I.; Khomenko, E. V.
Bibcode: 2005KFNTS...5..141K
Altcode:
We report thermodynamical properties of so-called ``thermal plume''
observed near the solar disc centre in 2001. The spectral observations
of two iron lines analysed were obtained with the use of the Vacuum
Tower Telescope (Tenerife).
Title: Helioseismology space and ground-based studies
Authors: Kostik, R. I.; Osipov, S. N.; Khomenko, E. V.; Lebedev, N. I.
Bibcode: 2005KFNTS...5..138K
Altcode:
We give a preliminary report on the observations of solar irradiance
fluctuations with the DIFOS photometer aboard the Russian-Ukrainian
satellite CORONAS-F launched in 2001. In addition, the parallel
ground-based spectral observations (VTT, Tenerife) carried out with
20-day observing space campaign are described.
Title: Convective and wave motions in a thermal plume
Authors: Kostik, Roman I.; Khomenko, Elena V.
Bibcode: 2004IAUS..223..271K
Altcode: 2005IAUS..223..271K
Here we report thermodynamical properties of a a so-called "thermal
plume" observed near the solar disc center in 2001. The spectral
observations of two iron lines analysed have been obtained using the
Vacuum Tower Telescope (Tenerife).
Title: Stokes diagnostics of magneto-convection. Profile shapes
and asymmetries
Authors: Khomenko, E. V.; Shelyag, S.; Solanki, S. K.; Vögler, A.;
Schüssler, M.
Bibcode: 2004IAUS..223..635K
Altcode: 2005IAUS..223..635K
We discuss the polarization signals produced in recent realistic 3D
simulations of solar magnetoconvection. The Stokes profiles of the
Fe I 6301.5, 6302.5, 15648 and 15652 mathrm{Å} Zeeman-sensitive
spectral lines are synthesised and smeared to simulate the image
degradation caused by the Earth's atmosphere and finite telescope
resolution. A Principal Component Analysis approach is applied to
classify the profiles. We find that the classes of Stokes V profiles
as well as their amplitude and area asymmetries are very close to the
observations in the network and inter-network regions.
Title: Helioseismology space and ground based studies
Authors: Kostik, R. I.; Osipov, S. N.; Khomenko, E. V.; Lebedev, N. I.
Bibcode: 2004IAUS..223..273K
Altcode: 2005IAUS..223..273K
This is a preliminary report on the observations of solar irradiance
fluctuations with the DIFOS photometer aboard the Russian-Ukrainian
satellite CORONAS-F launched in 2001. In addition the parallel
ground-based spectral observations (VTT, Tenerife) carried out with
the first 20-days observing space campaign are described.
Title: Stokes Diagnostics of Magnetoconvection. Profile shapes
and asymmetries.
Authors: Khomenko, E. V.; Shelyag, S.; Solanki, S. K.; Vogler, A.;
Schussler, M.
Bibcode: 2004cosp...35.2131K
Altcode: 2004cosp.meet.2131K
Stokes profiles observed in the quiet Sun have a broad range of
asymmetries and show a variety of shapes. These asymmetries are the
result of the velocity and magnetic field gradients both in horizontal
and vertical directions. We use the most recent realistic 3D simulations
of magnetoconvection at the solar surface to synthesize Stokes profiles
of some photospheric lines and to compare them with observations. Such
comparison provides an important constrains on the MHD models allowing
to conclude about their realism and, thus, to understand the nature of
solar magnetoconvection. The following Zeeman-sensitive spectral lines
are considered: Fe I 6301.5, 6302.5, 15648 and 15652 Å. These lines are
extensively used in observations. The computed Stokes profiles of these
lines were spatially smeared to simulate the effects of a telescope and
atmospheric seeing. A Principal Component Analysis approach is applied
to classify the profiles. The effects of spatial resolution and the
amount of the magnetic flux in the MHD model on the profile shapes are
discussed. The profiles of different classes are clustered together and
form patches on the surface. The size of these patches decreases with
increasing spatial resolution. The distributions of the amplitude and
area asymmetries of Stokes V profiles are very close to the observations
in network and inter-network regions. Some 15% of the profiles smeared
with a 0.''5 seeing have irregular shape with 3 or more lobes. Finally,
we show that simultaneous observations of the same area of the solar
disc using infrared Fe I 15648, 15652 Å and the visible Fe I 6301.5,
6302.5 Å lines done under different seeing conditions (for example
in the case of simultaneous observations at different telescopes)
may result in different asymmetries and even different polarities
of the profiles in two spectral regions observed at the same spatial
point. This work was partially supported by INTAS grant 00-00084.
Title: Quiet-Sun inter-network magnetic fields observed in the
infrared
Authors: Khomenko, E. V.; Collados, M.; Solanki, S. K.; Lagg, A.;
Trujillo Bueno, J.
Bibcode: 2003A&A...408.1115K
Altcode:
This paper presents the results of an investigation of the quiet Sun's
magnetic field based on high-resolution infrared spectropolarimetric
observations obtained with the Tenerife Infrared Polarimeter (TIP)
at the German VTT of the Observatorio del Teide. We observed two very
quiet regions at disc centre. The seeing was exceptionally good during
both observing runs, being excellent during one of them. In both cases
the network was intentionally avoided to the extent possible, to focus
the analysis on the characteristics of the weak polarization signals
of the inter-network regions. We find that the Stokes V profile of
Fe I 15648 Å line in almost 50% of the pixels and Stokes Q and/or
U in 20% of the pixels have a signal above 10-3 (in units
of continuum intensity Ic), which is significantly above
the noise level of 2-3 x 10-4. This implies that we detect
fluxes as low as 2 x 1015 Mx/px. We find evidence that we
have detected most of the net flux that is in principle detectable at
1'' resolution with the Zeeman effect. The observed linear
polarization resulting from the transverse Zeeman effect indicates that
the magnetic fields have a broad range of inclinations, although most
of the pixels show polarization signatures which imply an inclination
of about 20o. Nearly 30% of the selected V-profiles have
irregular shapes with 3 or more lobes, suggesting mixed polarities with
different LOS velocity within the resolution element. The profiles are
classified using a single value decomposition approach. The spatial
distribution of the magnetic signal shows that profiles of different
classes (having different velocities, splitting, asymmetries) are
clustered together and form patches, close to the spatial resolution
in size. Most of the field is found to be located in intergranular
lanes. The statistical properties of the mainly inter-network field
sampled by these observations are presented, showing that most of
the observed fields are weak with relatively few kG features. The
field strength distribution peaks at 350 G and has a FWHM of 300
G. Other parameters, such as profile asymmetries, filling factors and
line-of-sight velocities are also determined and discussed. Based
on observations with the German Vacuum Tower Telescope (VTT) operated by
the Kiepenheuer-Institut für Sonnenphysik at the Spanish Observatorio
del Teide of the Instituto de Astrofísica de Canarias (IAC).
Title: Magnetoacoustic Waves in Sunspots
Authors: Khomenko, E. V.; Collados, M.; Bellot Rubio, L. R.
Bibcode: 2003ApJ...588..606K
Altcode:
Observed variations of the magnetic field strength in sunspot umbrae
consist of intrinsic oscillations and ``false'' oscillations due to
time-dependent opacity effects. Here we present an approach intended for
the separation of these components. We develop a mathematical formalism
based on the analytical solution of the MHD equations including gravity,
inclination of the magnetic field, and effects of nonadiabaticity. The
theoretical results are compared with observations in the near-infrared
at 1.56 μm by Bellot Rubio and coworkers using the Tenerife Infrared
Polarimeter. It is shown that part of the detected field strength
variations can be intrinsic magnetic field oscillations caused by
magnetoacoustic waves.
Title: Local oscillations and their modification in inhomogeneous
solar atmosphere
Authors: Khomenko, E. V.
Bibcode: 2002KFNT...18..559K
Altcode:
Properties of the local 5-minute oscillations caused by their
interaction with convective motions are studied. We use a
vast statistical material of SOHO/MDI observations. We confirm
that oscillations above granules and intergranular lanes are
different. Powerful oscillations occur not only above intergranular
lanes but above granules as well. We show that the model for
oscillations in the inhomogeneous atmosphere which takes into account
the velocities of granular motions and reflection of waves allows
the observed dependencies to be explained qualitatively without any
assumption about different rate of excitation of oscillations in
granules and lanes.
Title: The Effect of Acoustic Waves on Spectral-Line Profiles in
the Solar Atmosphere: Observations and Theory
Authors: Kostyk, R. I.; Khomenko, E. V.
Bibcode: 2002ARep...46..925K
Altcode:
The fine structure of the FeI λ 532.4185-nm line of neutral iron is
studied with high spatial (0.5″) and temporal (9.3 s) resolution
using observations of a quiet region at the center of the solar
disk. The character of the line asymmetry depends strongly on the
nature of the velocity field, i.e., on whether it is due to convective
or wave motions. The magnitude of the asymmetry due to acoustic waves
is comparable to that due to convective motions. The propagation of
acoustic waves in moving granules and intergranular lanes is studied
by solving a system of hydrodynamical equations in a three-dimensional
model for the solar atmosphere. The temporal variations in the bisector
of the line synthesized in a non-LTE approximation agree well with
the observational data.
Title: Statistical properties of magnetic fields in intranetwork
Authors: Khomenko, E. V.; Collados, M.; Lagg, A.; Solanki, S. K.;
Trujillo Bueno, J.
Bibcode: 2002ESASP.505..445K
Altcode: 2002IAUCo.188..445K; 2002solm.conf..445K
We report a study of the quiet sun's magnetic field based
on high-resolution infrared spectropolarimetric observations
(TIP/VTT). We find that in almost 50% of the pixels Stokes V and in 15%
the Stokes Q and/or U profiles have a signal above 10-3. The
statistical properties of the mainly intranetwork field sampled by these
observations are presented, showing that most of the observed fields
are weak (the field strength distribution peaks at 350 G and has a FWHM
of 300 G) with very few kG features. The magnetized regions occupy a
very small fill fractions (about 2%). The field changes properties on
granular spatial scales and the size of the patches formed by similar
profiles is close to 1". Most of the parameters of the observed
polarization profiles show correlations with granulation parameters.
Title: Observation of Convective Collapse and Upward-moving Shocks
in the Quiet Sun
Authors: Bellot Rubio, Luis R.; Rodríguez Hidalgo, Inés; Collados,
Manuel; Khomenko, Elena; Ruiz Cobo, Basilio
Bibcode: 2001ApJ...560.1010B
Altcode:
We present spectropolarimetric evidence of convective collapse
and destruction of magnetic flux by upward-moving fronts in the
quiet Sun. The observational material consists of time series of
the full Stokes vector of two infrared spectral lines emerging from
regions associated with Ca II K network points. The amplitude of the
circular polarization profiles of a particular spatial point is seen to
increase while the profiles are redshifted. It then decreases during
a much shorter phase characterized by large blueshifts. Inspection
of the data indicates that the blueshift occurs because of the sudden
appearance of a new, strongly displaced Stokes V profile of the same
polarity. The amplification of the magnetic signal takes place in a
time interval of about 13 minutes, while blueshifts and the concomitant
decreasing Stokes V amplitudes last for only 2 minutes. An inversion
code based on the thin flux-tube scenario has been applied to the data
in order to derive the thermal, magnetic, and dynamic structures of
the atmosphere. According to our results, the field strength undergoes
a moderate increase from 400 to 600 G at z=0 km during the phase in
which redshifts are present. The observed redshifts are produced by
internal downflows of up to 6 km s-1 at z=0 km. After ~13
minutes, the material falling down inside the tube appears to bounce
off in the deeper layers, originating an upward-propagating front whose
manifestation on the Stokes V profiles is a large blueshift. The front
moves with a speed of 2.3 km s-1 and has a downflow-to-upflow
velocity difference of about 7 km s-1 initially and some
4 km s-1 after 2 minutes. It strongly weakens the magnetic
field strength and may be responsible for the complete destruction of
the magnetic feature. The observed behavior is in general agreement
with theoretical predictions of flux expulsion, convective collapse,
and development of shocks within magnetic flux tubes.
Title: Phases of the 5-min Photospheric Oscillations above Granules
and Intergranular Lines
Authors: Khomenko, E. V.
Bibcode: 2001ASSL..259..275K
Altcode: 2001dysu.conf..275K
In this work we re-examine the links between the 5-minute oscillations
and granulation using observations of the Fe I 5324 Å line obtained
with high spatial and temporal resolution. In contradiction to
the previous studies we show that oscillations above the brightest
granules as well as above the darkest intergranular lanes occur with
the smaller amount of radiative energy losses. This causes the amplitude
amplification with the contrast of granulation.
Title: Five-minute oscillations above granules and intergranular lanes
Authors: Khomenko, E. V.; Kostik, R. I.; Shchukina, N. G.
Bibcode: 2001A&A...369..660K
Altcode:
We discuss the links between the photospheric 5-min oscillations and
the granulation pattern using a 30-min time series of CCD spectrograms
of solar granulation recorded with high spatial (0{''}5) and
temporal (9.3 s) resolution. The observed images contain the Fe \sc{i
5324 Å spectral line with good height coverage from the low photosphere
up to the temperature minimum region. Amplitudes, phases and periods
of the 5-min oscillations are found to be different above granules
and intergranular lanes. Strong oscillations occur well separated
temporally and spatially. Many features of this different behaviour
can be described in the frame of a relatively simple model of wave
propagation in the solar atmosphere. To that aim, we have introduced
oscillations into a 3D snapshot of a theoretical time dependent solar
model atmosphere. NLTE synthesis of the time series of the Fe \sc{i
5324 Å line profiles was performed taking into account granular and
oscillatory components of the velocity field. Both, observations and
theoretical modeling, lead to similar results: (i) oscillations above
granules and intergranular lanes occur with different periods; (ii)
the most energetic intensity oscillations occur above intergranular
lanes; the most energetic velocity oscillations occur above granules
and lanes with maximum contrast, {i.e.} above the regions with maximum
convective velocities; (iii) velocity oscillations at the lower layers
of the atmosphere lead oscillations at the upper layers in intergranular
lanes. In granules the phase shift is nearly zero. We conclude that
differences in oscillations above granules and lanes are caused mainly
by variations of the physical conditions in these structures.
Title: Comparision of Observed and Theoretical Amplitudes of
Oscillations above granules and intergranular lanes
Authors: Khomenko, E. V.
Bibcode: 2001IAUS..203..192K
Altcode:
Differences in the amplitudes, phases and periods of the five-minute
oscillations above granules and intergranular lanes are found to be
well-described in a frame of a relatively simple model. We utilized
a 3D snapshot of the theoretically computed time depended solar model
atmosphere. We considered a vertical monochromatic wave propagation in
a moving, isothermal medium. NLTE synthesis of a time series of the FeI
5324 Å line profiles in the model atmospheres from a horizontal cut of
the snapshot was performed taking into account granular and oscillatory
components of the velocity field. Observations of the FeI 5324 Å line
in a quite solar disk center and our theoretical modeling lead to
the similar results: Periods of oscillations in intergranular lanes
are lower than in granules; Amplitudes of the velocity oscillations
grow with the contrast of granulation. Amplitudes of the intensity
oscillations are larger in intergranular lanes than in granules;
Velocity oscillations at the lower levels of the atmosphere lead
oscillations at the outer layers in intergranular lanes. In granules
this phase shift is nearly zero. The fact that our simple model
describes the basics features of the oscillations above granules and
intergranular lanes made us come to the conclusion that differences
in those oscillations are caused mainly by variations of the physical
conditions in these structures.
Title: Simulation of the wave propagation in the 3D solar atmosphere
Authors: Khomenko, E. V.
Bibcode: 2001ESASP.464..589K
Altcode: 2001soho...10..589K
A theory of wave propagation in the solar atmosphere has been
applied to study peculiarities in the wave behaviour above granules
and intergranular lanes. We obtained analytical solution of the
hydrodynamical equations considering a vertical monochromatic
wave propagation in a moving isothermal medium. Effects of the wave
reflection were taken into account. We introduced oscillations into a 3D
snapshot of the theoretical time dependent solar model atmosphere. NLTE
synthesis of the two solar lines was performed: the relatively strong
Fe I 5324 Å which is formed over all the photosphere and a weaker
Ni I 6768 Å line. The latter is widely used in helioseismology. Both
granular and oscillatory components of the velocity field were taken
into account. We compared our modeling with the high spatial resolution
observations obtained from the ground using VTT on Tenerife and from
space using SOHO/MDI. In general, amplitudes and phases of oscillations
above granules and intergranular lanes are reproduced by the applied
model. We conclude that many differences in oscillations above granules
and lanes can be induced by variations of the physical conditions in
these structures.
Title: Simulation of Temporal Variations of the Solar Line Fe
I 532. 4185 nm by the 5-min Oscillations (CD-ROM Directory:
contribs/khomenko)
Authors: Khomenko, E. V.; Shchukina, N. G.
Bibcode: 2001ASPC..223..680K
Altcode: 2001csss...11..680K
No abstract at ADS
Title: Wave propagation in the solar atmosphere
Authors: Khomenko, E. V.
Bibcode: 2000KFNTS...3..456K
Altcode:
The theory of a wave propagation in the solar atmosphere has been
applied to study peculiarities in the wave behaviour above granules and
intergranular lanes. We introduced oscillations into a 3D snapshot of
the theoretical time dependent solar model atmosphere. NLTE synthesis
of the two solar lines was performed: the relatively strong Fe I 5324
Å which is formed over all the photosphere and a weaker Ni I 6768 Å
line. The latter is widely used in helioseismology. We compared our
modeling with the high spatial resolution observations obtained from the
ground using VTT on Tenerife and from space using SOHO/MDI. We conclude
that many differences in oscillations above granules and lanes can be
induced by variations of the physical conditions in these structures.
Title: Granulation and five-minute oscillations
Authors: Khomenko, E. V.; Kostik, R. I.; Shchukina, N. G.
Bibcode: 2000KFNTS...3..431K
Altcode:
We discuss the links between the photospheric 5-min oscillations and
granulation patterns using a 30-min time series of CCD spectral images
of solar granulation recorded with high spatial (0.5'') and temporal
(9.3 s) resolution. The observed images contain Fe I 5324 Å line. Our
observations and theoretical modeling lead to the similar results:
(i) period of oscillations varies above granules and lanes; (ii)
amplitudes of the velocity oscillations grow with the contrast of
granulation. Amplitudes of the intensity oscillations are larger in
intergranular lanes than in granules. Our simple model describes the
basic features of the oscillations above granules and intergranular
lanes. We conclude that differences in these oscillations are caused
mainly by variations of the physical conditions in these structures.
Title: Interaction of Granulation with the 5-min Photospheric
Oscillations
Authors: Kostik, R. I.; Shchukina, N. G.; Khomenko, E. V.
Bibcode: 1999ESASP.448..319K
Altcode: 1999ESPM....9..319K; 1999mfsp.conf..319K
No abstract at ADS
Title: Formation and Destruction of a Weak Magnetic Feature in the
Solar Photosphere
Authors: Khomenko, E.; Collados, M.; Bellot Rubio, L. R.; Rodríguez
Hidalgo, I.; Ruiz Cobo, B.
Bibcode: 1999ESASP.448..307K
Altcode: 1999mfsp.conf..307K; 1999ESPM....9..307K
No abstract at ADS
Title: Phase characteristics of the local five-minute oscillations.
Authors: Khomenko, E. V.
Bibcode: 1999KFNT...15..145K
Altcode: 1999KNFT...15..145K
Local five-minute oscillations were studied on the basis of
observations with very high spatial and temporal resolution. Phase
shifts between oscillations in the Doppler velocity, central residual
intensity, and equivalent width of the line Fe I λ 532.4185
nm were analyzed. Phase shifts between velocity and temperature
oscillations above different granulation structures in the middle and
upper photosphere were found. The phase shift magnitude is shown to
change depending on granulation brightness at the height of continuum
formation. Oscillations above the brightest granules and the darkest
intergranular lanes occur with smaller radiative energy losses as
compared to the oscillations above the granular structures of lower
contrast. Relaxation time of temperature inhomogeneities above granules
and intergranular lanes was estimated with the phase shift obtained.
Title: Phase characteristics of local five-minute oscillations of
the Sun.
Authors: Khomenko, E. V.
Bibcode: 1999KPCB...15..109K
Altcode:
Local five-minute oscillations were studied using observations with
high spatial and temporal resolution. Phase shifts between oscillations
in the radial velocity, central residual intensity, and equivalent
width of the Fe I λ532.4185 nm line were analyzed. Phase shifts
between the velocity and temperature oscillations were found to
exist in the middle and upper photosphere over various granulation
structures. Their magnitude depends on granulation brightness at
the height of the continuum formation. The oscillations over the
brightest granules and the darkest intergranular lanes occur with
smaller radiative energy losses as compared to the oscillations over
the granular structures of lower contrast. Estimates were made for the
relaxation time of temperature inhomogeneities over granules and dark
lanes of different contrast.