Field-aligned currents in the polar cap during saturation of the polar cap potential

During periods of very strong southward magnetic field in the solar wind the polar cap potential becomes saturated. Recent investigations of the saturation of the polar cap potential suggest that the Region 1 current system plays a major role in balancing the solar wind pressure, leading to the saturation effect. To do this, the Region 1 current must flow on open field lines, closing outside Earth magnetosphere, where it can exert a J×BJ×B force on the solar wind. In this paper we present observations from DMSP F13 of the low-altitude distribution of field-aligned currents with Region 1 polarity relative to the boundary between open and closed field lines. The observations show that during a period of strongly southward solar wind magnetic field, a substantial amount of field-aligned current was flowing on field lines that had merged with the interplanetary magnetic field. On the other hand, during a period of nominal southward solar wind magnetic field, much less field-aligned current with Region 1 polarity was found in the open field line region. These observations support the view that Region 1 field-aligned currents can play a role in the force balance between the magnetosphere and the solar wind during periods of strongly southward IMF.