The Parker instability in disks with differential rotation

Abstract

We present a detailed study of the growth of the Parker instability in a differentially rotating disk embedded in an azimuthal equilibrium magnetic field, such as the interstellar gas or an accretion disk. Basic properties of the instability without shear are first recalled. Differential rotation is modeled in the shearing sheet approximation, classical in the theory of spiral density waves. The action of differential rotation is reduced to two different effects, (i) a linear time-dependence of the radial wavenumber, and (ii) a radial differential force. We present both exact numerical solutions, and approximate analytical ones based on the WKB approximation in the limit of weak differential rotation. Most important are (i) a transient natural stabilization of the Parker mode due to the radial differential force (ii) the generation of magnetosonic and Alfvenic waves, and (iii) in a certain parameter range a possible ``turn-over'' of the perturbation whereby, quite surprisingly, matter which had started being elevated by the instability may end up dropping towards the disk midplane. A simplified model shows the possible observable effects of this turn-over.