Identification and observations of the plasma mantle at low altitude

Abstract

The direct injection of magnetosheath plasma into the cusp produces at low altitude a precipitation regime with an energy‐latitude dispersion—the more poleward portion of which we herein term the “cusp plume.” An extensive survey of the Defense Meteorological Satellite Program (DMSP) F7 and F9 32 eV to 30 keV precipitating particle data shows that similar dispersive signatures exist over much of the dayside, just poleward of the auroral oval. Away from noon (or more precisely, anywhere not immediately poleward of the cusp) the fluxes are reduced by a factor of about 10 as compared to the cusp plume, but other characteristics are quite similar. For example, the inferred temperatures and flow velocities, and the characteristic decline of energy and number flux with increasing latitude is essentially the same in a longitudinally broad ring of precipitation a few degrees thick in latitude over much of the dayside. We conclude that the field lines on which such precipitation occurs thread the magnetospheric plasma mantle over the entire longitudinally extended ring. Besides the location of occurrence (i.e., immediately poleward of the dayside oval), the identification is based especially on the associated very soft ion spectra, which have densities from a few times 10−2 to a few times 10−1/cm³; on the temperature range, which is from a few tens of eV up to about 200 eV; and on the characteristic gradients with latitude. Further corroborating evidence that the precipitation is associated with field lines which thread the plasma mantle includes drift meter observations which show that regions so identified based on the particle data consistently lie on antisunward convecting field lines. Our observations indicate that some dayside high‐latitude auroral features just poleward of the auroral oval are embedded in the plasma mantle.