Evolution of the Parker instability: MHD flows, shock waves, and other consequences

We perform a two-dimensional MHD calculation of the nonlinear evolution of the Parker instability, with particular application to a galactic gas layer supported by thermal-pressure and magnetic forces. We study modes with linear perturbations having both odd and even symmetry about the galactic plane. The initial rapid expansion of the magnetic field leads to rapid flows along the arched field lines, resulting in shock fronts near the valleys of the curved field lines. Eventually, the system settles into final (two-dimensional) equilibrium states. Modes which allow the field lines to cross the galactic plane evolve more rapidly than modes which preserve reflection symmetry about the galactic plane, and enable the system to more greatly reduce its gravitational potential energy. These modes also result in diffuse gas concentrations that are spaced half as far apart as in the cases with reflection symmetry. For typical interstellar conditions, the separation of these gas concentrations is 0.5 - 1 kpc.