Electric and magnetic field characteristics of discrete arcs in the polar cap

Abstract

Simultaneous electric field, magnetic field, and electron flux measurements from three dawn‐dusk passes of S3‐2 were analyzed for signatures of sun‐aligned arcs in the polar cap. Two of the passes were over the summer and one over the winter polar cap. In all three cases the IMF had a strong northward component. During the pass over the winter polar cap the electric fields were highly irregular but tracked with nearly one‐to‐one correspondence to the transverse magnetic fluctuations. Upward Birkeland currents were embedded in regions of negative space charge. The carriers for the upward Birkeland currents were weakly field aligned fluxes of electrons with a temperature of a few hundred eV that had been accelerated through a potential drop of ∼1 kV. The two summer polar cap passes differed from each other in their large‐scale electric field patterns. In one pass the electric field was irregular with the upward Birkeland currents in negative space charge regions. In the other summer hemisphere pass, convection was sunward in the central polar cap. Small‐scale Birkeland current systems, possibly signatures of arclike structures, were found in regions of both sunward and antisunward convection. In this pass, variations in the electric and magnetic field did not always show the strong correlations found in the other two cases, due to the effects of electrojet activity near the flanks of the polar cap. The measurements are found to be consistent with a ‘static’ model for the electrical coupling between the ionoshere and magnetosphere with the generator at the magnetopause.