Grains in shocks in weakly ionized clouds - II. Magnetic field rotation in oblique shocks

Abstract

A four-fluid treatment of oblique C-type shocks in magnetized dark molecular clouds is presented. The four fluids are those consisting of neutrals, ions, electrons and negatively charged spherical grains of uniform size and composition. The charged particles couple to the neutrals by frictional interaction and their motion is described by the drift approximation. In an oblique shock, a current is set up which has a component parallel to the magnetic field component transverse to the shock velocity; this component of the current induces rotation of the magnetic field around the shock propagation direction. We give steady solutions for oblique C-type shocks for which the propagation speed is only slightly above the coupled Alfvén speed of about 2.2 km |$\text s^{-1}$|⁠. in these shocks, the magnetic field component transverse to the shock velocity rotates by |$180^\circ$| through the shock. The rotation occurs for the upstream perturbations chosen in previous studies of steady oblique shocks in molecular clouds. it is possible that the set of upstream perturbations that do not lead to large rotation in these shocks is nonempty, but we find that the set is limited. We also find that, for reasonable dark cloud parameters, the rotation ill a shock with a speed of a few km |$\text s^{-1}$| or more is sufficient to cause the velocity component parallel to the shock velocity of at least one of the charged fluids to drop sharply to zero in the shock frame.