IDEAL MHD FLOW BEHIND INTERPLANETARY SHOCKS DRIVEN BY MAGNETIC CLOUDS

We present an ideal MHD theory to describe for the first time the ''ma gnetic barrier'' (or ''depletion layer'') of that class of interplanet ary ejecta called magnetic clouds. By ''magnetic barrier'' we mean tha t region of the sheath where the magnetic pressure is comparable to, o r larger than, the gas pressure and where, therefore, the effects of t he magnetic field on the flow are substantial, We model magnetic cloud s as cylindrical flux ropes. We consider three cases: one steady state and two nonsteady situations. The two nonsteady situations correspond to (1) a self-similarly expanding magnetic cloud, and (2) to a nonexp anding magnetic cloud which has a net bulk motion with respect to the medium at infinity. In all cases the cloud drives an interplanetary sh ock ahead of it. We describe an algorithm to integrate the MHD equatio ns in which the: behavior of the sum of the magnetic and plasma pressu re: is prescribed. We assume here that the sum of the magnetic and pla sma pressure is constant along any line normal to the magnetic cloud b oundary. We find that in steady state the cloud boundary cannot be a t angential discontinuity, that is, a finite magnetic barrier thickness can only be obtained with a reconnecting cloud boundary. In general, t he magnetic barriers of magnetic clouds are thick, that is, they are a substantial fraction of the cloud's sheath. In steady state and the. nonsteady case (situation 2, above), their width depends inversely on the Alfven Mach number. The non-steady state (situation 1) has similar ities with the problem of solar wind flow around the terrestrial magne tosphere. In particular, the. barrier thickness in this case. is propo rtional to the inverse square of tile Alfven Mach number. This work sh ould be useful in the interpretation of data from the sheath region ah ead of magnetic clouds driving interplanetary shocks.