The Parker Instability in a Thick Gaseous Disk. II. Numerical Simulations in Two Dimensions

We present two-dimensional, ideal-MHD numerical simulations of the Parker instability in a multicomponent warm disk model. The calculations were done using two numerical codes with different algorithms, TVD and ZEUS-3D. The outcomes of the numerical experiments performed with both codes are very similar and confirm the results of the linear analysis for the undular mode derived in a previous work: the most unstable wavelength is about 3 kpc and its growth timescale is between 30 and 50 Myr (the growth rate is sensitive to the position of the upper boundary of the numerical grid). Thus, the time and length scales of this multicomponent disk model are substantially larger than those derived for thin-disk models. We use three different types of perturbations, random, symmetric, and antisymmetric, to trigger the instability. The antisymmetric mode is dominant and determines the minimum time for the onset of the nonlinear regime. The instability generates dense condensations, and the final peak column density value in the antisymmetric case, as also derived by Kim and coworkers, is about a factor of 3 larger than its initial value. These wavelengths and density enhancement factors indicate that the instability alone cannot be the main formation mechanism of giant molecular clouds in the general interstellar medium. The role of the instability in the formation of large-scale corrugations along spiral arms is briefly discussed.