Abstract
We have investigated the quiet‐time azimuthal plasma pressure gradient in the plasma sheet at a radial distance of 10 RE to 12 RE using two THEMIS spacecraft that were in overlapping orbits during the 2008 THEMIS tail season. The equatorial plasma pressure is estimated by using the in‐situ measurement of the plasma pressure and magnetic pressure based on pressure balance assumption. The results show a persistent duskward pressure gradient in the entire observed nightside region, which indicates upward field‐aligned current from the ionosphere maps to this entire region. This current corresponds to the Region‐2 current system in the post‐midnight sector and the Region 1 current system in pre‐midnight sector. The pressure gradients indicate that the upward field‐aligned currents peak in the vicinity of midnight and decrease towards dusk and dawn, perhaps approaching zero near dusk. The downward Region‐2 current system in the pre‐midnight sector is expected to be located earthward of 11 RE.
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