Synoptic maps of polar caps for stable interplanetary magnetic field intervals during January 1992 geospace environment modeling campaign

Abstract

Observations from January 27–28, 1992, when four polar‐orbiting DMSP satellites were in operation, are used to construct synoptic maps of convective flows and particle regions within the polar ionosphere. We chose periods of relative steady interplanetary magnetic field (IMF) and use data from multiple satellite passes during each period to obtain unprecedented two‐dimensional coverage. For strongly negative IMF By, convection patterns are similar to those that have been previously obtained but also show significant IMF Bz effects. For Bz » |By|, we see strong sunward flow near the pole, but antisunward convection on open field lines at lower latitudes on the nightside. The particle observations reveal the two‐dimensional structure of the cusp/mantle region and the existence of a strong polar arc within the circular convection cell that has a negative electric field divergence on open field lines. The particle observations also readily identify the magnetic separatrix when a satellite crosses the inner edge of the cusp or the boundary between the plasma sheet and polar rain. However, on the morningside and often on the afternoonside, we find a distinct and well‐defined region of soft‐electron and magnetosheath‐like ion precipitation [(the “soft‐electron zone” (SEZ)] lying between the plasma sheet and the region of polar rain. Separatrix identification is uncertain at local times where the SEZ is present; however, it appears that large portions of the SEZ are on open field lines. Dawn‐dusk displacements of the open field line regions in the two polar caps are seen that are consistent with IMF By effects. Additionally, the dayside separatrix is observed to be further poleward for positive than for negative IMF Bz, whereas Bz is not observed to have a significant effect on the location of the nightside separatrix. Also, we consistently find larger cross polar‐cap potential drops in the southern hemisphere than in the northern hemisphere. This suggests that there are significant (tens of kV) magnetic‐field‐aligned potential drops along high‐altitude regions of open field lines where the magnetic field is weak and particle motion strongly violates the guiding center approximation.