Partially Ionized Plasmas in Astrophysics

Abstract

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,

MHDSTS: a new explicit numerical scheme for simulations of partially ionised solar plasma

“…Since typically the scales of interest are significantly larger than collisional scales (see Khomenko et al 2014), the most efficient approach for the mathematical treatment of such plasma is a single-fluid quasi-MHD formalism. The derivation of conservation equations and the generalised induction equation for such a case can be consulted elsewhere (see e.g., Ballester et al 2017). Following the derivation of the equations from Bittencourt (2004), and using a specific definition for the macroscopic variables, see Appendix D of Khomenko et al (2014), the non-ideal effects are encoded into several terms in the induction and total energy conservation equations.…”

“…It can be expected that in such conditions the neutral and charged particles behave in a different way, with direct repercussions on the plasma dynamics and energy exchange. A recent review of the behaviour of partially ionised plasmas in different astrophysical conditions is provided by Ballester et al (2017).…”

“…Mathematically, the treatment of partially ionised plasmas depends on the degree of collisional coupling. The most widely used formalisms are either a two-fluid or single-fluid approach, see for example the recent discussion in Khomenko et al (2014) and a review by Ballester et al (2017). The two-fluid formalism, including the treatment of ionisation-recombination terms was recently provided by Meier & Shumlak (2012).…”

“…For the Sun, it has been demonstrated that the ambipolar diffusion is orders of magnitude larger than Ohmic diffusion in the chromospheric layers, therefore greatly contributing to chromospheric heating via Joule dissipation (Khomenko & Collados Vera 2012;Priest 2014). In the two-fluid treatment, the energy equation for the charged fluid also has a Joule heating term, similar to the single-fluid case, and the expression for the electric field comes from the generalised Ohm's law (see, e.g., Khomenko et al 2014;Ballester et al 2017). It is interesting to note, however, that ambipolar diffusion term is not explicitly present in the two-fluid Ohm's law, and in the energy conservation equation in the two-fluid treatment, and the corresponding effect is expressed via explicit ion-neutral collisional terms.…”

“…The Hall term is not dissipative, so it does not contribute to the heating of the environment, however it does redistribute magnetic energy in the system. Therefore, the Hall effect has an important role at many scales, from laboratory to astrophysical plasmas, while ambipolar diffusion is typically only important at low ionisation fractions (Parker 2007;Pandey & Wardle 2008;Priest 2014;Ballester et al 2017).…”