Buoyancy Instabilities in Weakly Magnetized Low Collisionality Plasmas

TL;DR

This paper analyzes the linear stability of stratified, weakly magnetized, low-collisionality plasmas, revealing a heat flux-driven buoyancy instability that occurs regardless of the temperature gradient direction, with implications for galaxy clusters.

Contribution

It demonstrates a new buoyancy instability driven by background heat flux in low-collisionality plasmas with weak magnetic fields, extending previous understanding of plasma stability.

Findings

Instability occurs when temperature decreases in gravity direction.

Growth time is comparable to local dynamical time.

Applicable to galaxy clusters with outward-increasing temperatures.

Abstract

I calculate the linear stability of a stratified low collisionality plasma in the presence of a weak magnetic field. Heat is assumed to flow only along magnetic field lines. In the absence of a heat flux in the background plasma, Balbus (2000) demonstrated that plasmas in which the temperature increases in the direction of gravity are buoyantly unstable to convective-like motions (the ``magnetothermal instability''). I show that in the presence of a background heat flux, an analogous instability is present when the temperature decreases in the direction of gravity. The instability is driven by the background heat flux and the fastest growing mode has a growth time of order the local dynamical time. Thus, independent of the sign of the temperature gradient, weakly magnetized low collisionality plasmas are unstable on a dynamical time to magnetically-mediated buoyancy instabilities. The instability described in this paper is predicted to be present in clusters of galaxies at radii from ~ 0.1-100 kpc, where the observed temperature increases outwards. Possible consequences for the origin of cluster magnetic fields, ``cooling flows,'' and the thermodynamics of the intercluster medium are briefly discussed.