The inner edge of the plasma sheet and the diffuse aurora

Three-dimensional measurements from the ISEE 2 low energy electron spectrometer are used to map the location of the inner edge of the plasma sheet and study the anisotropies in the electron distribution function associated with this boundary. At lower energies the plasma sheet electrons have inner edge closer to the earth than higher energies with the separations at different energies being larger near dawn and after dusk than at midnight. Lowest energy inner edges are frequently located adjacent to the plasmapause in the dawn sector but are often separated from the average plasmapause in the dusk sector by a gap of at least several R/sub E/. The energy dispersion is minimal in the afternoon quadrant where the inner edge is near the magnetopause and frequently oscillating on a time scale of minutes. The location of the inner edge is probably determined primarily by the motion of electrons in the existing electric and magnetic fields rather than by strong diffusion, as has sometimes been supposed. Evidence against strong diffusion is (1) boundaries closer to the earth than would occur if strong diffusion were operating and (2) the frequency and persistent occurrence of anisotropies that would be rapidly smoothed if pitch angle scattering proceeded at the strong diffusion rate. These anisotropies include 900 pitch angle depletions at the inner edge and 900 enhancements in the surrounding regions. Both these anisotropies are predicted by calculations of single particle motion; the former are apparently due to more field-aligned particles having inner edges slightly closer to the earth then 900 particles, and the latter are apparently due to the preferential adiabatic acceleration of 900 particles drifting in cross-tail electric and dipolelike magnetic fields