Distribution of convection potential around the polar cap boundary as a function of the interplanetary magnetic field

Abstract

Plasma flow data from the AE‐C, AE‐D and DE 2 satellites have been used to systematically study the distribution of the convection potential around the polar cap boundary under a variety of different interplanetary magnetic field (IMF) conditions. For either a “garden hose” (BxBy<0) or “ortho‐garden hose” (BxBy>0) orientation of the IMF, the potential distribution is mainly affected by the sign of By. In the northern hemisphere, the zero potential line (which separates the dusk convection cell from the dawn cell) on the dayside shifts duskward as By changes from positive to negative. But in the southern hemisphere, a dawnward shift has been found, although the uncertainties are large. The typical range of displacement is about ±1.5 hours MLT. Note that this shift is in the opposite direction from most simple schematic models of ionospheric flow; this reflects the fact that our “polar cap” boundary is typically more poleward than the flow reversal associated with the region 1 current system, which shifts in the opposite direction. Thus the enhanced flow region typically crosses noon. In most cases a sine wave is an adequate representation of the distribution of potential around the boundary. However, in a few cases the data favors (at the 80% confidence level) a steeper gradient near noon, more indicative of a “throat.” The potential drop at the duskside boundary is almost always greater than at the dawnside boundary. The dawn‐dusk potential offset is about 20% of the total polar cap potential drop for BZ≥1 nT, 10% for BZ∼0, and 1% for BZ≤−4 nT. A slight duskward shift of the pattern is observed as the IMF changes from “garden hose” to “ortho‐garden hose” conditions. Analytic equipotential contours, given our potential function as a boundary condition, are constructed for several IMF conditions. We find they agree reasonably well with the observed flow data, despite our simplifying assumptions of uniform conductivity and no Birkeland currents within the polar cap.