Auroral‐plasma sheet electron anisotropy

Abstract

Observations from 90 passes through the high‐latitude extension of the Earth's plasma sheet made by the Hydra particle spectrometer on Polar show clear, repeatable structure in the electron pitch angle anisotropy. Two distinct regions are seen. The first region begins at the poleward edge of the plasma sheet and consists of alternating regions of field‐aligned electron beams moving away from the ionosphere and loss‐cone distributions indicative of electron acceleration and precipitation below the spacecraft. The second region begins equatorward of the first region and is characterized by a higher temperature loss‐cone distribution superimposed on lower temperature, field aligned population. With decreasing latitude, the boundary in energy between these two populations in the second region decreases until only the trapped, loss‐cone population remains. The two regions are identified as connecting to the discrete and diffuse auroral regions, respectively, and it is shown that the observed pitch angle anisotropies can be explained as arising from auroral processes below the spacecraft.