THE LOW-LATITUDE BOUNDARY-LAYER AND THE BOUNDARY PLASMA SHEET AT LOW ALTITUDE - PRENOON PRECIPITATION REGIONS AND CONVECTION REVERSAL BOUNDARIES

The dayside zone of soft precipitation can be divided into four distinct types of plasma regimes, each corresponding to the respective magnetospheric source region: the cusp, the mantle, the low-latitude boundary layer (LLBL), and the dayside extension of the BPS. Based on a detailed spectral study, including comparisons with nonsimultaneous ISEE 1 satellite LLBL data, we identify regions of LLBL-type plasma in the DMSP data set and compare these plasma boundaries with convection reversal boundaries (CRBs) as determined by either Sondrestrom or the drift meter instrument on board the DMSP F9 spacecraft. The nine cases considered are all in the prenoon local time sector. We find that in eight of the nine cases the CRB occurs within the LLBL as expected, generally near to, but not coincident with, the equatorward edge of the LLBL-type plasma. In our sample set, chosen for cases with latitudinally wide, easily identifiable LLBL signatures, the average latitudinal width was 1.85-degrees magnetic latitude. The CRB, defined as the onset of steady antisunward convection, occurred about 30% of this width beyond the equatorward onset of LLBL-type particles. The most equatorward portion of the region with LLBL-type plasma usually had near-zero or erratic convection and may correspond to the "stagnation region" reported from ISEE observations. The potential drop observed across the low-altitude LLBL is roughly estimated to be typically approximately 5 keV. A summary is given on how the various high-altitude sources can be identified when plasma regions are observed at low altitude in the dayside auroral oval.