explanation blue bibcodes open ADS page with paths to full text
Author name code: popescu
ADS astronomy entries on 2022-09-14
author:Popescu Braileanu, Beatrice
---------------------------------------------------------
Title: Three-Dimensional MHD Wave Propagation Near a Coronal Null
Point: a New Wave Mode Decomposition Approach
Authors: Yadav, Nitin; Keppens, Rony; Popescu Braileanu, Beatrice
2022cosp...44.2546Y Altcode:
Ubiquitous vortex flows at the solar surface excite magnetohydrodynamic
(MHD) waves that propagate to higher layers of the solar atmosphere. In
the solar corona, these waves frequently encounter magnetic null
points. The interaction of MHD waves with a coronal magnetic null
in realistic 3D setups requires an appropriate wave identification
method. We present a new MHD wave decomposition method that overcomes
the limitations of existing wave identification methods. Our method
allows to investigate the energy fluxes in different MHD modes at
different locations of the solar atmosphere as waves generated by
vortex flows travel through the solar atmosphere and pass near the
magnetic null. We use the open-source MPI-AMRVAC code to simulate wave
dynamics through a coronal null configuration. We apply a rotational
wave driver at our bottom photospheric boundary to mimic vortex flows
at the solar surface. To identify the wave energy fluxes associated
with different MHD wave modes, we employ a wave-decomposition method
that is able to uniquely distinguish different MHD modes. Our proposed
method utilizes the geometry of an individual magnetic field-line in 3D
space to separate out velocity perturbations associated with the three
fundamental MHD waves. We compare our method with an existing wave
decomposition method that uses magnetic flux surfaces instead. Over
selected flux surfaces, we calculate and analyze temporally averaged
wave energy fluxes, as well as acoustic and magnetic energy fluxes. Our
wave decomposition method allows us to estimate the relative strengths
of individual MHD wave energy fluxes. Our method for wave identification
is consistent with previous flux-surface-based methods and gives
expected results in terms of wave energy fluxes at various locations of
the null configuration. We show that ubiquitous vortex flows excite MHD
waves that contribute significantly to the Poynting flux in the solar
corona. Alfvén wave energy flux accumulates on the fan surface and fast
wave energy flux accumulates near the null point. There is a strong
current density buildup at the spine and fan surface. The proposed
method has advantages over previously utilized wave decomposition
methods, since it may be employed in realistic simulations or magnetic
extrapolations, as well as in real solar observations, whenever the 3D
field line shape is known. The essential characteristics of MHD wave
propagation near a null, such as wave energy flux accumulation and
current buildup at specific locations, translate to the more realistic
setup presented here. The enhancement in energy flux associated with
magneto-acoustic waves near nulls may have important implications in
the formation of jets and impulsive plasma flows.
---------------------------------------------------------
Title: Two-fluid implementation in MPI-AMRVAC, with applications in
the solar chromosphere
Authors: Popescu Braileanu, B.; Keppens, R.
2022arXiv220505049P Altcode:
The chromosphere is a partially ionized layer of the solar atmosphere,
the transition between the photosphere where the gas is almost neutral
and the fully ionized corona. As the collisional coupling between
neutral and charged particles decreases in the upper part of the
chromosphere, the hydrodynamical timescales may become comparable to
the collisional timescale, and a two-fluid model is needed. In this
paper we describe the implementation and validation of a two-fluid
model which simultaneously evolves charges and neutrals, coupled
by collisions. The two-fluid equations are implemented in the fully
open-source MPI-AMRVAC code. In the photosphere and the lower part of
the solar atmosphere, where collisions between charged and neutral
particles are very frequent, an explicit time-marching would be too
restrictive, since for stability the timestep needs to be proportional
to the inverse of the collision frequency. This is overcome by
evaluating the collisional terms implicitly using an explicit-implicit
(IMEX) scheme. The cases presented cover very different collisional
regimes and our results are fully consistent with related literature
findings. If collisional time and length scales are smaller than the
hydrodynamical scales usually considered in the solar chromosphere,
density structures seen in the neutral and charged fluids are similar,
with the effect of elastic collisions between charges and neutrals being
similar to diffusivity. Otherwise, density structures are different and
the decoupling in velocity between the two species increases. The use of
IMEX schemes efficiently avoids the small timestep constraints of fully
explicit implementations in strongly collisional regimes. Adaptive
Mesh Refinement (AMR) greatly decreases the computational cost,
compared to uniform grid runs at the same effective resolution.
---------------------------------------------------------
Title: 3D MHD wave propagation near a coronal null point: New wave
mode decomposition approach
Authors: Yadav, N.; Keppens, R.; Popescu Braileanu, B.
2022A&A...660A..21Y Altcode: 2022arXiv220109704Y
Context. Ubiquitous vortex flows at the solar surface excite
magnetohydrodynamic (MHD) waves that propagate to higher layers of
the solar atmosphere. In the solar corona, these waves frequently
encounter magnetic null points. The interaction of MHD waves with a
coronal magnetic null in realistic 3D setups requires an appropriate
wave identification method. <BR /> Aims: We present a new MHD wave
decomposition method that overcomes the limitations of existing wave
identification methods. Our method allows for an investigation of the
energy fluxes in different MHD modes at different locations of the
solar atmosphere as waves generated by vortex flows travel through
the solar atmosphere and pass near the magnetic null. <BR /> Methods:
We used the open-source MPI-AMRVAC code to simulate wave dynamics
through a coronal null configuration. We applied a rotational wave
driver at our bottom photospheric boundary to mimic vortex flows at
the solar surface. To identify the wave energy fluxes associated with
different MHD wave modes, we employed a wave decomposition method that
is able to uniquely distinguish different MHD modes. Our proposed
method utilizes the geometry of an individual magnetic field-line
in the 3D space to separate the velocity perturbations associated
with the three fundamental MHD waves. We compared our method with an
existing wave decomposition method that uses magnetic flux surfaces
instead. Over the selected flux surfaces, we calculated and analyzed
the temporally averaged wave energy fluxes, as well as the acoustic
and magnetic energy fluxes. Our wave decomposition method allowed
us to estimate the relative strengths of individual MHD wave energy
fluxes. <BR /> Results: Our method for wave identification is consistent
with previous flux-surface-based methods and provides the expected
results in terms of the wave energy fluxes at various locations of
the null configuration. We show that ubiquitous vortex flows excite
MHD waves that contribute significantly to the Poynting flux in the
solar corona. Alfvén wave energy flux accumulates on the fan surface
and fast wave energy flux accumulates near the null point. There is
a strong current density buildup at the spine and fan surface. <BR />
Conclusions: The proposed method has advantages over previously utilized
wave decomposition methods, since it may be employed in realistic
simulations or magnetic extrapolations, as well as in real solar
observations whenever the 3D fieldline shape is known. The essential
characteristics of MHD wave propagation near a null - such as wave
energy flux accumulation and current buildup at specific locations -
translate to the more realistic setup presented here. The enhancement
in energy flux associated with magneto-acoustic waves near nulls may
have important implications in the formation of jets and impulsive
plasma flows.
---------------------------------------------------------
Title: Magnetic field amplification and structure formation by the
Rayleigh-Taylor instability
Authors: Popescu Braileanu, B.; Lukin, V. S.; Khomenko, E.
2021arXiv211213043P Altcode:
We report on results of high resolution two fluid non-linear simulations
of the Rayleigh Taylor Instability (RTI) at the interface between
a solar prominence and the corona. These follow results reported
earlier by Popescu Braileanu et al. (2021a,b) on linear and early
non-linear RTI dynamics in this environment. The simulations use a
two fluid model that includes collisions between neutrals and charges,
including ionization/recombination, energy and momentum transfer, and
frictional heating. High resolution 2.5D magnetized RTI simulations
with the magnetic field dominantly normal to and slightly sheared
with respect to the prominence plane demonstrate that in a fully
developed state of RTI a large fraction of the gravitational energy
of a prominence thread can be converted into quasi-turbulent energy
of the magnetic field. RTI magnetic energy generation is further
accompanied by magnetic and plasma density structure formation,
including dynamic formation, break-up, and merging of current sheets
and plasmoid sub-structures. The simulations show the role of flow
decoupling and ionization/recombination reactions between the neutrals
and charges on the structure formation in magnetized RTI. We provide
a careful examination of sources and form of numerical dissipation of
the evolving magnetic field structures.
---------------------------------------------------------
Title: Effects of ambipolar diffusion on waves in the solar
chromosphere
Authors: Popescu Braileanu, B.; Keppens, R.
2021A&A...653A.131P Altcode: 2021arXiv210510285P
Context. The chromosphere is a partially ionized layer of the solar
atmosphere that mediates the transition between the photosphere where
the gas motion is determined by the gas pressure and the corona
dominated by the magnetic field. <BR /> Aims: We study the effect
of partial ionization for 2D wave propagation in a gravitationally
stratified, magnetized atmosphere characterized by properties that are
similar to those of the solar chromosphere. <BR /> Methods: We adopted
an oblique uniform magnetic field in the plane of propagation with
a strength that is suitable for a quiet sun region. The theoretical
model we used is a single fluid magnetohydrodynamic approximation,
where ion-neutral interaction is modeled by the ambipolar diffusion
term. Magnetic energy can be converted into internal energy through the
dissipation of the electric current produced by the drift between ions
and neutrals. We used numerical simulations in which we continuously
drove fast waves at the bottom of the atmosphere. The collisional
coupling between ions and neutrals decreases with the decrease in
the density and the ambipolar effect thus becomes important. <BR />
Results: Fast waves excited at the base of the atmosphere reach
the equipartition layer and are reflected or transmitted as slow
waves. While the waves propagate through the atmosphere and the
density drops, the waves steepen into shocks. <BR /> Conclusions: The
main effect of ambipolar diffusion is damping of the waves. We find
that for the parameters chosen in this work, the ambipolar diffusion
affects the fast wave before it is reflected, with damping being more
pronounced for waves which are launched in a direction perpendicular
to the magnetic field. Slow waves are less affected by ambipolar
effects. The damping increases for shorter periods and greater magnetic
field strengths. Small scales produced by the nonlinear effects and the
superposition of different types of waves created at the equipartition
height are efficiently damped by ambipolar diffusion.
---------------------------------------------------------
Title: Simulations of the Biermann battery mechanism in two-fluid
partially ionised plasmas
Authors: Martínez-Gómez, D.; Popescu Braileanu, B.; Khomenko, E.;
Hunana, P.
2021A&A...650A.123M Altcode: 2021arXiv210406956M
Context. In the absence of an initial seed, the Biermann battery term
of a non-ideal induction equation acts as a source that generates
weak magnetic fields. These fields are then amplified via a dynamo
mechanism. The Kelvin-Helmholtz instability is a fluid phenomenon that
takes place in many astrophysical scenarios and can trigger the action
of the Biermann battery and dynamo processes. <BR /> Aims: We aim to
investigate the effect of the ionisation degree of the plasma and the
interaction between the charged and neutral species on the generation
and amplification of magnetic fields during the different stages of the
instability. <BR /> Methods: We use the two-fluid model implemented in
the numerical code MANCHA-2F. We perform 2D simulations starting from a
configuration with no initial magnetic field and which is unstable due
to a velocity shear. We vary the ionisation degree of the plasma and
we analyse the role that the different collisional terms included in
the equations of the model play on the evolution of the instability and
the generation of magnetic field. <BR /> Results: We find that when no
collisional coupling is considered between the two fluids, the effect
of the Biermann battery mechanism does not depend on the ionisation
degree. However, when elastic collisions are taken into account, the
generation of magnetic field is increased as the ionisation degree
is reduced. This behaviour is slightly enhanced if the process of
charge-exchange is also considered. We also find a dependence on the
total density of the plasma related to the dependence on the coupling
degree between the two fluids. As the total density is increased,
the results from the two-fluid model converge to the predictions of
single-fluid models. <BR /> Conclusions: The charged-neutral interaction
in a partially ionised plasmas has a non-negligible effect on the
Biermann battery mechanism and it effectively enhances the generation
of a magnetic field. In addition, single-fluid models, which assume
a very strong coupling between the two species, may overestimate
the contribution of this interaction in comparison with two-fluid
models. <P />Movies associated to Figs. 2 and A.2 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202039113/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: Two-fluid simulations of Rayleigh-Taylor instability in a
magnetized solar prominence thread. II. Effects of collisionality
Authors: Popescu Braileanu, B.; Lukin, V. S.; Khomenko, E.; de
Vicente, Á.
2021A&A...650A.181P Altcode: 2021arXiv210112731P
Solar prominences are formed by partially ionized plasma
with inter-particle collision frequencies generally warranting
magnetohydrodynamic treatment. In this work we explore the dynamical
impacts and observable signatures of two-fluid effects in the parameter
regimes when ion-neutral collisions do not fully couple the neutral
and charged fluids. We perform 2.5D two-fluid (charges-neutrals)
simulations of the Rayleigh-Taylor instability (RTI) at a smoothly
changing interface between a solar prominence thread and the corona. The
purpose of this study is to deepen our understanding of the RTI and
the effects of the partial ionization on the development of RTI using
nonlinear two-fluid numerical simulations. Our two-fluid model takes
into account neutral viscosity, thermal conductivity, and collisional
interaction between neutrals and charges: ionization-recombination,
energy and momentum transfer, and frictional heating. In this paper, the
sensitivity of the RTI dynamics to collisional effects for different
magnetic field configurations supporting the prominence thread
is explored. This is done by artificially varying, or eliminating,
effects of both elastic and inelastic collisions by modifying the model
equations. We find that ionization and recombination reactions between
ionized and neutral fluids do not substantially impact the development
of the primary RTI. However, such reactions can impact the development
of secondary structures during the mixing of the cold prominence and
hotter surrounding coronal material. We find that collisionality
within and between ionized and neutral particle populations plays
an important role in both linear and nonlinear development of RTI;
ion-neutral collision frequency is the primary determining factor in
development or damping of small-scale structures. We also observe that
the degree and signatures of flow decoupling between ion and neutral
fluids can depend on the inter-particle collisionality and on the
magnetic field configuration of the prominence thread.
---------------------------------------------------------
Title: Two-fluid simulations of Rayleigh-Taylor instability in
a magnetized solar prominence thread. I. Effects of prominence
magnetization and mass loading
Authors: Popescu Braileanu, B.; Lukin, V. S.; Khomenko, E.; de
Vicente, Á.
2021A&A...646A..93P Altcode: 2020arXiv200715984P
Solar prominences are formed by partially ionized plasma with
inter-particle collision frequencies, which generally warrant
magnetohydrodynamic treatment. In this work, we explore the dynamical
impacts and observable signatures of two-fluid effects in the
parameter regimes when ion-neutral collisions do not fully couple
the neutral and charged fluids. We performed 2.5D two-fluid (charge -
neutrals) simulations of the Rayleigh-Taylor instability (RTI) at a
smoothly changing interface between a solar prominence thread and the
corona. The purpose of this study is to deepen our understanding of the
RTI and the effects of partial ionization on the development of the RTI
using nonlinear two-fluid numerical simulations. Our two-fluid model
takes into account viscosity, thermal conductivity, and collisional
interaction between neutrals and charge: ionization or recombination,
energy and momentum transfer, and frictional heating. In this paper,
we explore the sensitivity of the RTI dynamics to the prominence
equilibrium configuration, including the impact of the magnetic field
strength and shear supporting the prominence thread, and the amount of
prominence mass-loading. We show that at small scales, a realistically
smooth prominence-corona interface leads to qualitatively different
linear RTI evolution than that which is expected for a discontinuous
interface, while magnetic field shear has the stabilizing effect
of reducing the growth rate or eliminating the instability. In the
nonlinear phase, we observe that in the presence of field shear the
development of the instability leads to formation of coherent and
interacting 2.5D magnetic structures, which, in turn, can lead to
substantial plasma flow across magnetic field lines and associated
decoupling of the fluid velocities of charged particles and neutrals.
---------------------------------------------------------
Title: Effects of neutrals on magnetic Rayleigh Taylor instability
in solar prominences
Authors: Khomenko, Elena; Lukin, Vyacheslav; Popescu Braileanu,
Beatrice
2021cosp...43E.976K Altcode:
The Rayleigh Taylor instability has been frequently observed at
the interface between solar prominences and corona. Prominence
plasma contains a large fraction of neutrals, and their role on the
stability of these structures is not fully understood. Here we study
the behavior of plasma and neutral components during the Rayleigh
Taylor instability in a thread of prominence material, using two-fluid
numerical simulations. Our model takes into account elastic collisions,
ionization/recombination, thermal exchange, neutral viscosity and
conductivity. We study the effects of the magnetic field strength,
orientation, and the density contrast of the thread on the growth
rate of the instability, both in the linear and non-linear phases. We
observe that, while large-scale harmonics grow exponentially in the
linear phase of the instability, for intermediate and small scales,
affected by viscosity, thermal conduction, and the interaction
between neutrals and charges, the linear and the nonlinear phases are
superposed. This behavior affects the thermal and dynamic decoupling
of the components. We observe differences in the neutral and plasma
velocities of the order of several hundreds of m/s, and differences
in their temperature of the order of several tens of Kelvin. The
ionization-recombination imbalance results in creation of a layer of
increased density of charges. This layer follows the evolution of the
eddies, and it potentially observable. Elastic collisions influence
the growth rate similarly to the viscosity: an increase of collisions
results in a larger growth rate at small scales. Larger density contrast
increases the growth rate, contrary to the effect of increasing the
strength of the magnetic field.
---------------------------------------------------------
Title: 2D simulations of the Biermann battery mechanism in partially
ionized plasmas
Authors: Martínez-Gómez, D.; Popescu Braileanu, B.; Khomenko, E.;
Hunana, P.
2020sea..confE.205M Altcode:
In the absence of an initial seed, the Biermann battery term of
a non-ideal induction equation acts a source that generates weak
magnetic fields. Here, we study this mechanism in the context of
partially ionized plasmas, using a model in which the charged and
neutral components of the plasma are treated as two different fluids
that interact by means of collisions. We investigate the effect that
the ionization degree and the charged-neutral interaction have on the
generation of magnetic field. We use the numerical code MANCHA-2F to
perform 2D simulations of the Kelvin-Helmholtz instability. We study how
the magnetic field generated by the Biermann battery process depends on
the ionization degree of the plasma and on the different collisional
terms included in the equations of the model. We find that when the
collisional coupling is taken into account, the generation of magnetic
field is increased as the ionization degree is decreased. We also
find that this effect depends on the total density of the plasma and
that as this parameter is increased, the numerical two-fluid results
converge to the analytical results from a single-fluid model.
---------------------------------------------------------
Title: Two-fluid simulations of waves in the solar
chromosphere. II. Propagation and damping of fast magneto-acoustic
waves and shocks
Authors: Popescu Braileanu, B.; Lukin, V. S.; Khomenko, E.; de
Vicente, Á.
2019A&A...630A..79P Altcode: 2019arXiv190805262P
Waves and shocks traveling through the solar chromospheric plasma are
influenced by its partial ionization and weak collisional coupling, and
may become susceptible to multi-fluid effects, similar to interstellar
shock waves. In this study, we consider fast magneto-acoustic shock wave
formation and propagation in a stratified medium, that is permeated by
a horizontal magnetic field, with properties similar to that of the
solar chromosphere. The evolution of plasma and neutrals is modeled
using a two-fluid code that evolves a set of coupled equations for
two separate fluids. We observed that waves in neutrals and plasma,
initially coupled at the upper photosphere, become uncoupled at higher
heights in the chromosphere. This decoupling can be a consequence
of either the characteristic spatial scale at the shock front,
that becomes similar to the collisional scale, or the change in
the relation between the wave frequency, ion cyclotron frequency,
and the collisional frequency with height. The decoupling height is
a sensitive function of the wave frequency, wave amplitude, and the
magnetic field strength. We observed that decoupling causes damping
of waves and an increase in the background temperature due to the
frictional heating. The comparison between analytical and numerical
results allows us to separate the role of the nonlinear effects from
the linear ones on the decoupling and damping of waves.
---------------------------------------------------------
Title: Two-fluid simulations of waves in the solar
chromosphere. I. Numerical code verification
Authors: Popescu Braileanu, B.; Lukin, V. S.; Khomenko, E.; de
Vicente, Á.
2019A&A...627A..25P Altcode: 2019arXiv190503559P
Solar chromosphere consists of a partially ionized plasma, which
makes modeling the solar chromosphere a particularly challenging
numerical task. Here we numerically model chromospheric waves using
a two-fluid approach with a newly developed numerical code. The
code solves two-fluid equations of conservation of mass, momentum,
and energy, together with the induction equation for the case of the
purely hydrogen plasma with collisional coupling between the charged
and neutral fluid components. The implementation of a semi-implicit
algorithm allows us to overcome the numerical stability constraints due
to the stiff collisional terms. We test the code against analytical
solutions of acoustic and Alfvén wave propagation in uniform medium
in several regimes of collisional coupling. The results of our
simulations are consistent with the analytical estimates, and with
other results described in the literature. In the limit of a large
collisional frequency, the waves propagate with a common speed of a
single fluid. In the other limit of a vanishingly small collisional
frequency, the Alfvén waves propagate with an Alfvén speed of the
charged fluid only, while the perturbation in neutral fluid is very
small. The acoustic waves in these limits propagate with the sound
speed corresponding to either the charges or the neutrals, while the
perturbation in the other fluid component is negligible. Otherwise,
when the collision frequency is similar to the real part of the
wave frequency, the interaction between charges and neutrals through
momentum-transfer collisions cause alterations of the waves frequencies
and damping of the wave amplitudes.
