Buoyancy instabilities in weakly magnetized low-collisionality plasmas

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 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 similar to 0.1-100 kpc, where the observed temperature increases outward. Possible consequences for the origin of cluster magnetic fields, cooling flows, and the thermodynamics of the intercluster medium are briefly discussed.