Overstability of acoustic waves in strongly magnetized anisotropic MHD shear flows

TL;DR

This paper analyzes the stability of acoustic waves in strongly magnetized, anisotropic MHD shear flows, revealing conditions under which thermo-acoustic waves become overstable or thermally unstable, with implications for astrophysical plasma environments.

Contribution

It introduces a detailed linear stability analysis incorporating heat flux effects in anisotropic MHD flows, highlighting the overstability of acoustic waves due to shear and heat flux parameters.

Findings

Thermo-acoustic waves become unstable when heat flux exceeds a critical value.

Velocity shear can cause thermo-acoustic waves to overstable even at subcritical heat flux.

Flow exhibits thermal instability at supercritical heat fluxes.

Abstract

We present a linear stability analysis of the perturbation modes in anisotropic MHD flows with velocity shear and strong magnetic field. Collisionless or weakly collisional plasma is described within the 16-momentum MHD fluid closure model, that takes into account not only the effect of pressure anisotropy, but also the effect of anisotropic heat fluxes. In this model the low frequency acoustic wave is revealed into a standard acoustic mode and higher frequency fast thermo-acoustic and lower frequency slow thermo-acoustic waves. It is shown that thermo-acoustic waves become unstable and grow exponentially when the heat flux parameter exceeds some critical value. It seems that velocity shear makes thermo-acoustic waves overstable even at subcritical heat flux parameters. Thus, when the effect of heat fluxes is not profound acoustic waves will grow due to the velocity shear, while at supercritical heat fluxes the flow reveals compressible thermal instability. Anisotropic thermal instability should be also important in astrophysical environments, where it will limit the maximal value of magnetic field that a low density ionized anisotropic flow can sustain.