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 Bibcode: 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. Bibcode: 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. Bibcode: 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.
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.
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.
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.
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. 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: Effects of ambipolar diffusion on waves in the solar chromosphere Authors: Popescu Braileanu, B.; Keppens, R. Bibcode: 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.
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.
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.
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.
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. 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: 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: 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: 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: 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.