THE LOW-LATITUDE BOUNDARY-LAYER AT MID-ALTITUDES - IDENTIFICATION BASED ON VIKING HOT PLASMA DATA

On passes through the pre-dawn to pre-noon auroral oval the Viking spa cecraft (apogee at 13,400 km) encounters often a generally well-define d precipitation region of ions with characteristic energies of several hundred eV, with neither cusp nor plasma mantle characteristics. Furt hermore, these ions are clearly separable from ions originating from t he plasma sheet. Partial ion number densities in this plasma precipita tion region strongly correlate with solar wind densities, supporting t he view that it constitutes the mid-altitude projection of the magneto spheric low-latitude boundary layer (LLBL). The LLBL projection thus i dentified 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 patt ern can be divided into precipitation originating from the plasma shee t and LLBL. i.e. generally no region was encountered with properties s uggesting that it projects to the magnetospheric plasma sheet boundary layer (PSBL).