Electron/ion precipitation differences in relation to region 2 field-aligned currents

The question of the mechanism producing the premidnight region 2 currents is investigated using magnetic field and precipitating particle data from the Defense Meteorological Satellite Program F7 satellite. In this study, the variations in the extent and intensity of the region 2 currents on the nightside, in the darkened hemisphere, are determined for a small, isolated substorm on January 18, 1984, and the variations related to the characteristics of the latitudinal profile in electron and ion precipitation. The region 2 currents are found to collocate with a steep slope in the ion energy density. The ion energy density decreases with decreasing latitude. The region 2 current intensity increases proportionally to the increase in the ion energy density gradient. In the premidnight sector the equatorward boundary of auroral ion precipitation generally extends to lower latitudes than that for electrons, suggesting charge imbalance in the right sense to drive region 2 currents. We find no correlation between the extent of the latitudinal separation of the two boundaries and the strength of the region 2 currents. In fact there was one pass with a clear region 2 current where this boundary behavior reversed. The observations are consistent with an azimuthal gradient in the ion energy density near the inner edge of the plasma sheet being an important factor in the generation of the premidnight region 2 currents if the increase in the azimuthal gradient is proportional to the increase in the radial gradient there. copyright American Geophysical Union 1989