Dissipative instabilities in a partially ionised prominence plasma slab: II. The effect of compressibility

TL;DR

This study investigates how compressibility affects dissipative instabilities in partially ionised prominence plasma slabs, revealing that compressibility lowers the flow speed threshold for instability and highlighting the roles of neutrals and viscosity.

Contribution

It provides an analytical and numerical analysis of compressibility effects on dissipative instabilities in prominence plasmas, extending previous models to include these effects.

Findings

Compressibility reduces the flow speed threshold for instability.

Neutrals and viscosity influence the instability conditions in opposite ways.

Time-scales of instability range from 10^5 to 10^7 seconds.

Abstract

This present study deals with the dissipative instability that appears in a compressible partially ionised plasma slab embedded in a uniform magnetic field, modelling the state of the plasma in solar prominences. In the partially ionised plasma, the dominant dissipative effect is the Cowling resistivity. The regions outside the slab (modelling the solar corona) are fully ionised, and the dominant mechanism of dissipation is viscosity. Analytical solutions to the extended magnetohydrodynamic (MHD) equations are found inside and outside of the slab and solutions are matched at the boundaries of the slab. The dispersion relation is derived and solutions are found analytically in the slender slab limit, while the conditions necessary for the appearance of the instability is investigated numerically for the entire parameter space. Our study is focussed on the effect of the compressibility on the generation and evolution of instabilities. We find that compressibility reduces the threshold of the equilibrium flow, where waves can be unstable, to a level that is comparable to the internal cusp speed, which is of the same order of flow speeds that are currently observed in solar prominences. Our study addresses only the slow waves, as these are the most likely perturbations to become unstable, however the time-scales of the instability are found to be rather large ranging from $10^5$-$10^7$ seconds. It is determined that the instability threshold is further influenced by the concentration of neutrals and the strength of the viscosity of the corona. Interestingly, these two latter aspects have opposite effects. Our numerical analysis shows that the interplay between the equilibrium flow, neutrals and dispersion can change considerably the nature of waves.