The low‐latitude boundary layer at mid‐altitudes: Identification based on Viking hot plasma data

Abstract

On passes through the pre‐dawn to pre‐noon auroral oval the Viking spacecraft (apogee at 13,400 km) encounters often a generally well‐defined precipitation region of ions with characteristic energies of several hundred eV, with neither cusp nor plasma mantle characteristics. Furthermore, these ions are clearly separable from ions originating from the plasma sheet. Partial ion number densities in this plasma precipitation region strongly correlate with solar wind densities, supporting the view that it constitutes the mid‐altitude projection of the magnetospheric low‐latitude boundary layer (LLBL). The LLBL projection thus identified is observed with high probability in the MLT range from near‐noon to at least 6 MLT but occasionally extends toward 4 to 3 MLT. In the dawn to pre‐noon local time sector auroral ion precipitation pattern can be divided into precipitation originating from the plasma sheet and LLBL, i.e. generally no region was encountered with properties suggesting that it projects to the magnetospheric plasma sheet boundary layer (PSBL).