Magnetothermal instabilities in magnetized anisotropic plasmas

TL;DR

This paper investigates how pressure anisotropy affects magnetothermal and heat-flux-driven buoyancy instabilities in collisionless plasmas, revealing conditions under which anisotropy significantly alters instability growth rates.

Contribution

It provides a detailed analysis of the influence of pressure anisotropy on MTI and HBI instabilities using the 16-moment method, extending understanding beyond ideal MHD assumptions.

Findings

Pressure anisotropy can reduce instability growth times by nearly an order of magnitude.

In high-frequency conduction regimes, anisotropy has negligible effects on instabilities.

In low magnetic field environments, results align with ideal MHD predictions.

Abstract

Using the transport equations for an ideal anisotropic collisionless plasma derived from the Vlasov equation by the 16-moment method, we analyse the influence of pressure anisotropy exhibited by collisionless magnetized plasmas on the magnetothermal (MTI) and heat-flux-driven buoyancy (HBI) instabilities. We calculate the dispersion relation and the growth rates for these instabilities in the presence of a background heat flux and for configurations with static pressure anisotropy, finding that when the frequency at which heat conduction acts is much larger than any other frequency in the system (i.e. weak magnetic field) the pressure anisotropy has no effect on the MTI/HBI, provided the degree of anisotropy is small. In contrast, when this ordering of timescales does not apply the instability criteria depend on pressure anisotropy. Specifically, the growth time of the instabilities in the anisotropic case can be almost one order of magnitude smaller than its isotropic counterpart. We conclude that in plasmas where pressure anisotropy is present the MTI/HBI are modified. However, in environments with low magnetic fields and small anisotropy such as the ICM the results obtained from the 16-moment equations under the approximations considered are similar to those obtained from ideal MHD.