Fluid approach to mirror mode structures

Abstract

There is ample observational evidence of coherent, large-amplitude, magnetic fluctuations, including fully developed nonlinear structures, in the magnetosheaths of terrestrial planets, in the free solar wind, in the vicinity of comets, and on the edges of the cold Io wake. They are commonly believed to result from the mirror instability and often appear to be dominated by significant field decreases (‘magnetic holes’). In order to contribute to a better theoretical understanding of these phenomena, this study examines the nonlinear evolution of mirror waves on the basis of an anisotropic, 1D, one-fluid, MHD plasma model, which is closed by a double-polytropic relation and which includes elements of the kinetic theory. The model calculations provide an explanation for several aspects of the observations including differences in the growth/saturation of magnetic depressions and compressions, the tendency to a bi-stable behaviour of a mirror-unstable plasma, the formation of steep gradients at the edges of magnetic decreases, and the survival of ‘magnetic holes’ in a marginal mirror-stable plasma.