The dynamics of critical magnetospheric boundary regions is examined through combined particle and fluid simulations. The particle simulations focus on the dynamics of the magnetopause whose structure is shown to be determined by the differential penetration of magnetosheath ions and electrons. The magnetosheath electrons with their small gyroradius have only limited penetration and give rise to a sharp peak in the current density, producing a well-defined magnetopause current sheet. The magnetosheath ions penetrate further, where they intermix with magnetospheric plasma producing the magnetopause boundary layer. An ambipolar electric field is set up by this differential penetration which draws some of the magnetospheric ions out into the current layer and beyond. In addition, the ambipolar electric field sets up surface currents along the flanks of the magnetopause. These surface currents and their analogues in the magnetotail are shown to play a major role in (1) mapping of currents into the ionosphere and (2) determining the reconnection rate at the magnetopause and magnetotail current sheets by the effective resistivity they produce on these current sheets. These surface currents which are neglected in present MHD treatments are explicitly included in a modified set of single fluid equations, called magnetoplasma dynamics (MPD).

The MPD description is able to remove the need for assuming an artifical resistivity while, at the same time providing, a comprehensive description of the currents that map into the ionosphere as well as the various current sheets attached to the different magnetospheric boundary layers. Important results include (1) the mapping of region 1 and 2 currents into the noon and midnight sectors of the ionosphere, (2) their equatorward motion during the southward turning of the interplanetary magnetic field, (3) their temporary enhancement of pseudobreakup, and (4) their enhancement and poleward expansion at substorm onset. ¿ American Geophysical Union 1994