Field-aligned currents in the polar cap at small IMF B-z and B-y inferred from SuperDARN radar observations

Routine SuperDARN observations of the ionospheric plasma convection and fie ld-aligned currents (FACs) in the high-latitude ionosphere are used to stud y current systems established at small interplanetary magnetic field (IMF) B-z and B-y. By statistical averaging of available data sets we show that u nder this IMF condition the ionospheric convection pattern consists of two (evening and morning) convection cells that are similar in shape. The flow intensity inside the central polar cap is noticeably depressed so that plas ma entering the polar cap flows around its border, predominantly along the lines of equal magnetic latitude, so that the convection cells are of a cre scent-like shape. This global pattern of plasma flow is associated with the effect of the region 0 field-aligned currents coexisting with the region 1 and region 2 field-aligned currents. SuperDARN observations of FACs for in dividual events support this conclusion. FACs were derived by analyzing the vorticity of the SuperDARN convection maps. We show that region 0 currents for small IMF B-z and B-y can exist in time sectors way off the magnetic n oon. Thus radar observations support earlier findings from satellite magnet ometer measurements of the region 0 current system at high latitudes during both the prenoon and afternoon at small IMF intensities. Because the regio n 0 FACs occur during small IMF intensities, it is suggested that quasi-vis cous processes play a role in their generation.