ON ROTATIONAL DISCONTINUITIES IN BOTH 2-FLUID AND HYBRID MODELS

Rotational discontinuities are studied in a two-fluid model that includes finite ion inertia dispersion and in a hybrid model in which the full ion dynamics is retained while the electrons are treated as a massless fluid. We show that as in previous dissipative MHD studies, a rotational discontinuity is unstable in both models and evolves to a 2 --> 3 intermediate shock, a slow rarefaction wave, and other waves. (However, due to the limitation in computational resources, the hybrid calculations are limited to low-to-medium ion beta.) In addition, we show that the so-called Walen relation, which holds exactly for rotational discontinuities, can also be well satisfied by tran-Alfvenic intermediate shocks. Thus intermediate shocks can be candidates for those observed structures that satisfy the Walen relation.