CUSP CURRENTS FROM IONOSPHERIC VORTICITY GENERATED BY GASDYNAMIC AND MERGING FLOW-FIELDS AT THE MAGNETOPAUSE

Cusp currents that arise from ionospheric vorticity generated by the c ombined merging outflow and gasdynamic flow fields at the magnetopause , as proposed by Saunders (1989), are quantified and compared with tho se calculated by Mei et al. (1994) from vorticity, generated by mappin g the solar wind electric field into a limited cusp region of the pola r cap, as proposed in the synthesis view of Banks et al. (1984). The r esults are essentially identical for strong interplanetary magnetic fi eld (IMF) B-y, thus demonstrating equivalence,between mechanical and e lectrical descriptions of reconnection-driven convection. For southwar d IMF, however, the mechanical description yields weak cusp currents w ith dawn-dusk bipolarity, as deduced from early observations by Iijima and Potemra (1976), whereas the electrical description yields none. T he bipolar currents arise from the diverging pattern of gasdynamic flo w. The currents become unipolar as B-y increases and the asymmetry of the merging outflow dominates. Additional cusp currents in both models arise at kinks in the flow contours (additional ionospheric vorticity ) around the border of the cusp region, owing to limiting the area of mapping from the magnetopause. The border currents form a bipolar pair that rotates around the circumference of the cusp as the IMF rotates in clock angle. They dominate the currents arising from vorticity with in the cusp.