On the MHD analogue of the Brunt/Väisälä frequency in a magnetized plasma

Abstract

The MHD analogue of the Brunt/Väisälä frequency, NB, in a magnetized, ideally conducting plasma is obtained, with the vertical component of the magnetic field, Br, taken into account. The magnetic field vector (Br, Bθ, Bϕ) is assumed to satisfy the condition B·∇B ≈ BrdB/dr, which holds in many cases of interest. The frequency NB happens to depend, generally speaking, on the magnetic field orientation relative to the direction of gravity. However, for an isentropic gas, the convective instability criterion is governed by the magnetic field strength (rather than by the orientation of B). In general, the magnetic field has a stabilizing (destabilizing) effect if B/ρ grows (decreases) along the vertical axis r. This conclusion seems not to depend on the specific magnetic field configuration.