Mathematical Aspects of Geophysical and Astrophysical Fluid Dynamics: Magnetic buoyancy instability in galaxies

Abstract

We study the nonlinear evolution of the magnetic buoyancy instability in rotating and non-rotating gas layers using numerical solutions of non-ideal, isothermal MHD equations. The unstable magnetic field is either imposed through the boundary conditions or generated by an imposed $α$-effect. In the case of an imposed field, we solve for the deviations from the background state with periodic boundary conditions. We also include cosmic rays as a weightless fluid which exerts a dynamically significant pressure and amplifies magnetic buoyancy, known as the Parker instability. Without rotation, systems with an imposed magnetic field evolve to a state with a very weak magnetic field, very different from the marginally stable eigenfunction, where the gas layer eventually becomes very thin as it is supported by thermal and turbulent pressures alone. However, this does not happen when the $α$-effect maintains the magnetic field. Rotation fundamentally changes the development of the instability. A rotating system develops nonlinear oscillations, and the magnetic field direction changes even with an imposed magnetic field. We demonstrate that the cause is a secondary $α$-effect at large altitudes as the gas flow produced by the instability becomes helical. The secondary $α$-effect has an anomalous sign with the $α$-coefficient being negative in the northern hemisphere, whereas the Coriolis force produces a positive $α$. The mean-field dynamo action outside the original gas layer can also lead to a switch in the magnetic field parity from quadrupolar to dipolar. Altogether, the magnetic buoyancy instability and the mean-field dynamo action become separated as distinct physical effects in a nonlinear rotating system. We show that none of the assumptions used in analytic studies of the Parker instability is corroborated by numerical results.