Multicomponent theory of buoyancy instabilities in astrophysical plasma objects: The case of magnetic field perpendicular to gravity

TL;DR

This paper develops a multicomponent plasma theory to analyze buoyancy instabilities with magnetic fields perpendicular to gravity, revealing differences from MHD predictions and applicable to astrophysical objects like galaxy clusters.

Contribution

It introduces a multicomponent approach to buoyancy instabilities, deriving dispersion relations that account for collisions and electric fields, contrasting with traditional MHD models.

Findings

Collisionless dispersion relation differs from MHD predictions.

Electric field perturbations significantly influence instability criteria.

Results applicable to galaxy clusters and astrophysical plasmas.

Abstract

We develop a general theory of buoyancy instabilities in the electron-ion plasma with the electron heat flux based not upon MHD equations, but using a multicomponent plasma approach in which the momentum equation is solved for each species. We investigate the geometry in which the background magnetic field is perpendicular to the gravity and stratification. General expressions for the perturbed velocities are given without any simplifications. Collisions between electrons and ions are taken into account in the momentum equations in a general form, permitting us to consider both weakly and strongly collisional objects. However, the electron heat flux is assumed to be directed along the magnetic field that implies a weakly collisional case. Using simplifications justified for an investigation of buoyancy instabilities with the electron thermal flux, we derive simple dispersion relations both for collisionless and collisional cases for arbitrary directions of the wave vector. The collisionless dispersion relation considerably differs from that obtained in the MHD framework and is similar to the Schwarzschild's criterion. This difference is connected with simplified assumptions used in the MHD analysis of buoyancy instabilities and with the role of the longitudinal electric field perturbation which is not captured by the ideal MHD equations. The results obtained can be applied to clusters of galaxies and other astrophysical objects.