Birkeland currents in the plasma sheet

Abstract

Geotail particle and magnetic field measurements were combined to generate long‐term‐averaged 3‐D models of the plasma sheet. Ampere's law was used to calculate the Birkeland current j∥ in the −30 < x < −16RE, ∣z∣ > 1RE region. Current diversion, or the growth of current in a unit flux tube j∥/B, took place throughout the region studied. This suggests that electron scattering is broadly distributed. No substantial change in j∥/B could be detected between the plasma sheet boundary layer and the ionosphere. Birkeland currents were strongest and exhibited a dawn‐dusk asymmetry when the interplanetary magnetic field (IMF) was southward. This asymmetry may be associated with the formation of thin current sheets on the dusk side during disturbed periods. Symmetries were apparent above and below the neutral sheet when the IMF was northward or southward, but these symmetries were not present when the IMF pointed dawnward or duskward. For these latter cases, separate surfaces were found on which Bx = 0, By = 0, and j∥ = 0. This apparently complex structure could be understood as a consequence of the tendency for By in the neutral sheet to have the same sign as the IMF By. The observed Birkeland currents were in the region 1 sense when leaving the plasma sheet for all IMF orientations. Current diversion was analyzed in an MHD framework. The analysis suggested that the reduction of gradient and curvature guiding center drifts, and the presence of polarization currents in the diversion region can provide sources of electrons to sustain a steady j∥. It also was noted that the formation of an E∥ region in the topside ionosphere can make it appear to the conducting ionosphere as if it is being driven by a current source rather than by the plasma sheet electric field.