Formation of a compound slow shock-rotational discontinuity structure

Whang et al. [1996, 1997] reported observations of a new double discontinui ty in the interplanetary space and magnetotail. The double discontinuity is a compound structure of a slow shock (SS) followed by an adjoining rotatio nal discontinuity (RD), In a pressure isotropic plasma the SS-RD compound s tructure cannot be maintained, because the normal flow speed downstream of an SS is less than the Alfven speed and the downstream RD will overtake the SS. However, the Alfven speed in a pressure anisotropic plasma may be high ly reduced by an increased value? of P-parallel to/P-perpendicular to, We s uggest that the observed SS-RD compound structure may be related to this an isotropic effect. In the hybrid simulations of a slow shock, the back strea ming ions will result in high P-parallel to and relatively low P-perpendicu lar to in the foreshock region, This leads to a region with highly reduced local intermediate wave speed. A rotational discontinuity propagating: towa rd the slow shock from the downstream region would be slowed down in the hi gh P-parallel to region and attached to the SS transit;ion region to form a structure resembling a double discontinuity. We present hybrid simulations to examine possible results of the interaction between an no and an SS, an d the formation of a compound discontinuity. Our simulations show that ther e are two different types of intel actions: (1) An RD passes through and pr opagates upstream of an SS, when a weak SS cannot protide enough high aniso tropy (P-parallel to/P-perpendicular to > 1); and (2) a compound SS-RD-SS s tructure is formed in which an no is trapped inside the transition region o f a strong SS. In this compound structure, the SS is divided into two porti ons by the no. However, this SS-RD-SS structure is somewhat different from the observed double discontinuity in which the no is attached to the downst ream part of a slow shock.