ELECTRIC-FIELD GENERATION AT THE MAGNETOSPHERIC BOUNDARY FOR NORTHWARD IMF

We discuss three different processes which generate electric fields at the magnetopause during northward interplanetary magnetic field (IMF) conditions. These are (1) Petschek-type magnetic field reconnection, (2) magnetic field diffusion, and (3) viscous-like interaction resulti ng from the Kelvin Helmholtz instability. For northward IMF all three processes lead to the formation of a boundary layer on closed magnetic field lines adjacent to the magnetospheric boundary. The thickness of the boundary layer depend on Petschek's parameter in the first case, the magnetic Reynolds number in the second case, and an effective Reyn olds number in the third case. In each case coupling between the bound ary layer and the ionosphere occurs via field-aligned currents. These field-aligned currents result from the penetration into the polar iono sphere of the electric field generated at the magnetospheric boundary. These currents are closed by a transverse current in the boundary lay er and the associated Lorentz force causes a decrease of the kinetic e nergy of the solar wind plasma inside the boundary layer. As a result of this velocity decrease the thickness of the boundary layer increase s on both flanks of the magnetosphere near the equatorial plane. The c onvergence of the boundary layer on the dawn and dusk sides leads to a ntisunward plasma flow in the magnetospheric tail.