IMF Changes and Polar-Cap Electric Fields and Currents

Abstract

The polar cap, here defined as the region poleward of the auroral oval, is magnetically connected to the solar wind. It is generally true that currents may flow easily between the two regions, and that polar-cap electric fields and currents respond sensitively to variations in the interplanetary magnetic field (IMF). The effects of the solar-magnetospheric x, y, and z components of the IMF have proved to be quite distinct. Strong southward components are associated with a larger polar cap and stronger electric fields and field-aligned currents. Strong northward components lead to a smaller polar cap and perhaps to the occasional appearance of sunward convection within it. Positive IMF B_y components intensify dawnside antisunward convection in the northern hemisphere and may displace the cusp convection “throat” toward dusk. Negative B_y components intensify duskside antisunward convection and may displace the northern-hemisphere throat toward dawn. These displacements and intensifications respond in the opposite sense in the southern hemisphere. Variations of the x-component induce interhemispheric asymmetries in polar-cap particle access and acceleration, both phenomena increasing significantly in the hemisphere for which the tail magnetic field and the IMF have x components of opposite signs. These effects and other recent results on the field-aligned currents flowing at the edge of the polar cap and in the cusp region are reviewed in this paper.