The Parker Instability in a Thick Galactic Gaseous Disk. I. Linear Stability Analysis and Nonlinear Final Equilibria

A linear stability analysis of a multicomponent and magnetized Galactic disk model is presented. The disk model uses the observed stratifications for the gas density and gravitational acceleration at the solar neighborhood, and in this sense it can be called a realistic model. The distribution of the total gas pressure is defined by these observed stratifications, and the gaseous disk is assumed isothermal. The initial magnetic field is taken parallel to the disk, with a midplane value of 5 μG, and its stratification along the z-axis is derived from the condition of magnetohydrostatic equilibrium in an isothermal atmosphere. The resulting isothermal sound speed is ~8.4 km s-1, similar to the velocity dispersion of the main gas components within 1.5 kpc from the midplane. The thermal-to-magnetic pressure ratio decreases with [z], and the warm model is Parker unstable. The dispersion relations show that the fastest growing mode has a wavelength of about 3 kpc, for both symmetric and antisymmetric perturbations, and the corresponding growth timescales are of about 3 × 107 yr. The structure of the final equilibrium stage is also derived, and we find that the midplane antisymmetric (MA) mode gathers more gas in the magnetic valleys. The resulting MA gas condensations have larger densities, and the column density enhancement is a factor of about 3 larger than the value of the initial stage. The unstable wavelengths and growth times for the multicomponent disk model are substantially larger than those of a thin-disk model, and some of the implications of these results are discussed.