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 dis k model is presented. The disk model uses the observed stratifications for the gas density and gravitational acceleration at the solar neighborhood, a nd in this sense it can be called a realistic model. The distribution of th e total gas pressure is defined by these observed stratifications, and the gaseous disk is assumed isothermal. The initial magnetic field is taken par allel to the disk, with a midplane value of 5 mu G, and its stratification along the z-axis is derived from the condition of magnetohydrostatic equili brium in an isothermal atmosphere. The resulting isothermal sound speed is similar to 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 press ure ratio decreases with Ttl, and the warm model is Parker unstable. The di spersion relations show that the fastest growing mode has a wavelength of a bout 3 kpc, for both symmetric and antisymmetric perturbations, and the cor responding growth timescales are of about 3 x 10(7) yr. The structure of th e final equilibrium stage is also derived, and we find that the midplane an tisymmetric (MA) mode gathers more gas in the magnetic valleys. The resulti ng MA gas condensations have larger densities, and the column density enhan cement is a factor of about 3 larger than the value of the initial stage. T he unstable wavelengths and growth times for the multicomponent disk model are substantially larger than those of a thin-disk model, and some of the i mplications of these results are discussed.