Field‐aligned currents and electric fields observed in the region of the dayside cusp

Thirteen passes of the S3‐2 satellite, approximately along the noon‐midnight meridian, during January 1976 through the region of the dayside cusp have been studied: eight in the northern hemisphere and five in the southern hemisphere. For all passes the IMF By < 0, and for all but three passes, Kp ≤3−, indicating relatively quiet times. Examination of the east‐west component of the magnetic field deflections indicates an extra current sheet, located poleward of region 1 and carrying current in the opposite sense, on the postnoonside (prenoonside) in the northern (southern) hemisphere but no additional current prenoon (postnoon) in the northern (southern) hemisphere. The component of the convection electric field along the satellite trajectory (north‐south) shows that convection reversals occur 2°–3° equatorward of the boundary between this extra current sheet and the region 1 field‐aligned current sheet. A boundary marking the transition from quasi‐trapped to untrapped 100‐keV protons was determined. The extra field‐aligned current sheet lies almost entirely poleward of this transition boundary, suggesting that it may lie in a region of open magnetic field lines. Fine scale structure of the magnetic deflections indicates numerous field‐aligned sheet and line currents embedded in the large‐scale field‐aligned current sheets.