Dynamical Effects of the Parker Instability in the Interstellar Medium

We follow numerically the nonlinear evolution of the Parker instability in the interstellar medium in two spatial dimensions. The response of an initially unstable equilibrium state to small, random velocity perturbations is investigated. The horizontal wavelength with maximum growth rate in the linear theory grows preferentially and dominates the nonlinear evolution of the system during timescales of interest in the interstellar medium. The nonlinear evolution of the system also favors modes whose symmetry allows motions that cross the galactic plane. In a separate calculation, we investigate the dominant mode in more detail and discuss the effect of the nonlinear evolution on the structure of the interstellar medium.