MHD Simulations of Parker Instability Undergoing Cosmic-Ray Diffusion

Abstract

Parker instability arises from the presence of magnetic fields in a plasma in a gravitational field such as the interstellar medium (ISM), wherein the magnetic buoyant pressure expels the gas and causes the gas to move along the field lines. The process of mixing of this instability in the ISM near the Galactic plane is investigated. The initial ISM is assumed to consist of two fluids: plasma gas and cosmic-ray particles, in hydrostatic equilibrium, coupled with a uniform, azimuthally-aligned magnetic field. The evolution of the instability is explored in two models: an isothermal exponential-declining density model and a two-layered, hyperbolic tangent temperature model. After a small perturbation, the unstable gas aggregates at the bottom of the magnetic loops and forms dense blobs. The growth rate of the instability decreases as the coupling between the cosmic rays and the plasma becomes stronger (meaning a smaller CR diffusion coefficient). The mixing is enhanced by the cosmic-ray diffusion, while the shape of the condensed gas depends sensitively on the initial equilibrium conditions. The hyperbolic tangent temperature model produces a more concentrated and round shape of clumps at the foot points of rising magnetic arches, like the observed giant molecular cloud, whereas the exponential density model gives rise to a filamentary morphology of the clumpy structure. When considering a minimum perpendicular or cross field diffusion of cosmic rays, which is often substantially smaller than the parallel coefficient $κ_{\|}$, around $2%-4%$ of $κ_{\|}$, the flow speed is significantly increased such that the magnetic loops extend to a greater altitude. We speculate that the galactic wind flow perpendicular to the galactic disk may be facilitated by Parker instabilities through the cross field diffusion of cosmic rays.