Simulations of the Biermann battery mechanism in two-fluid partially ionised plasmas

Abstract

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.

Large Ion-neutral Drift Velocities and Plasma Heating in Partially Ionized Coronal Rain Blobs

“…After analyzing the evolution of the drift velocities and the frictional heating, it is also interesting to investigate the variations of temperature related to the charged-neutral interaction. As shown by Martínez-Gómez et al (2021), the frictional heating produces an increase of temperature of each fluid given by the following expressions:…”

“…The details of the full set of equations included in the twofluid model can be found in Popescu Braileanu et al (2019b) and Martínez-Gómez et al (2021). Here we only show the most relevant terms for the present investigation, which are the momentum transfer and heating due to collisions, given by…”

“…From Martínez-Gómez et al (2021), the equations that describe the temporal evolution of the internal energy of the charged and the neutral fluids, e c and e n , respectively, are given (in the absence of Joule heating) by…”

Ambipolar Diffusion in the Lower Solar Atmosphere: Magnetohydrodynamic Simulations of a Sunspot

“…It is becoming clear that the degree of plasma ionization in the solar atmosphere must be accounted for in a wide variety of situations. Plasma with a particularly large number of neutrals often has significantly modified properties, resulting in changes to the physical processes that drive important aspects of solar dynamics (Arber et al 2007;Pandey & Wardle 2008;Ni et al 2015;Khomenko et al 2018Khomenko et al , 2021Martínez-Gómez et al 2021;Popescu Braileanu & Keppens 2021); see Ballester et al (2018) for a review.…”

“…Partial ionization effects primarily arise due to the substantial quantity of neutral species in the plasma, and their insufficient collisional coupling to the charged species, leading to additional physical processes that need to be taken into consideration. For example, accounting for the presence of neutrals, and the degree of coupling between the neutral and charged plasma species, significantly affects the Biermann battery effect (Biermann 1950) and the strength of the magnetic field it generates within the plasma (Martínez-Gómez et al 2021).…”