The effect of a magnetic field on an adiabatic explosion

Abstract

As a paradigm for various explosive processes in the interstellar medium we consider the problem of an adiabatic explosion into a uniform magnetic field which is frozen to the gas. A typical numerical run is described and reveals the following features. The outer shock becomes oblate with respect to the field lines whereas the inner hot gas prolate density and temperature contours. During the later stages the shock weakens and the explosion comes into equilibrium with the interstellar medium. The dominant feature at this stage is a concentration of accelerated material at each pole. We then try to interpret this analytically, considering the magnetic field as a small perturbation to a spherical explosion. This enables us to derive a formula for the eccentricity, which is proportional tot ^6/5. However, the linear perturbation is singular at the centre and needs to be matched to a self-similar flow there, for which we give an approximation. This similarity solution is eventually important outside the region occupied by the material initially responsible for the explosion. We give some discussion of the various asymptotic regimes involved.