ANOMALOUS MAGNETOSHEATH PROPERTIES DURING EARTH PASSAGE OF AN INTERPLANETARY MAGNETIC CLOUD

The aim of this paper is to model for the first time the variation of field and flow parameters in the magnetosheath during Earth passage of an interplanetary magnetic cloud. Under typical solar wind conditions , magnetohydrodynamic (MHD) effects on the flow of plasma in the terre strial magnetosheath are important only in a layer adjacent to the mag netopause which is a few thousand kilometers thick (''depletion layer' ' or ''magnetic barrier''). During the passage of an interplanetary ma gnetic cloud, however, conditions upstream of the bow shock depart str ongly from the norm. In this case, interplanetary parameters vary slow ly over a wide range of values. Values of the upstream Alfven,Mach num ber are much lower than those otherwise sampled (similar to 3 versus 8 -10). Together with the magnetic shear across the magnetopause, this p arameter plays a central role in determining the structure of the magn etosheath. close to the magnetopause. As a consequence of sustained lo w values of the: upstream Alfven Mach number, the magnetic field exert s a strong influence on the flow over a very substantial fraction of t he magnetosheath throughout the duration of cloud passage, i.e., for a time period of the order of 1-2 days. We apply an algorithm to integr ate. the ideal MHD equations, using a boundary layer technique, and co mpute the variations of held and flow parameters along the stagnation streamline. We choose as our example the magnetic cloud which passed E arth on January 14-15, 1988. The interaction of this cloud with the ma gnetosphere, as regards the resulting ionospheric flow patterns and th e substorm activity, has been the subject of various investigations. U sing information from these studies, we obtain results on the magnetos heath when the magnetopause is modeled, first as a tangential disconti nuity and then as a rotational discontinuity. Our results are in good general agreement with recent observations on the behavior of field an d flow quantities in the magnetosheath region adjacent to the magnetop ause. In addition, we predict the existence of a magnetic barrier when the upstream Alfven Mach number is low, irrespective of the magnetic shear across the magnetopause.