SAMPEX OBSERVATIONS OF ENERGETIC ELECTRON-PRECIPITATION IN THE DAYSIDE LOW-LATITUDE BOUNDARY-LAYER

We address the problem of the magnetic topology of the dayside low-lat itude boundary layer by using conjugate observations from the Solar, A nomalous, and Magnetospheric Particle Explorer (SAMPEX) and the Defens e Meteorological Satellite Program Flight 10 (DMSP F10) satellites whi le they are in coplanar polar orbits in the 0900-2100 magnetic local t ime (MLT) meridian. In the region identified in the DMSP observations as the low-latitude boundary layer (LLBL), we observe energetic electr on precipitation at SAMPEX in detectors measuring electrons in the ene rgy ranges E > 150 keV and E > 1.05 MeV. This precipitation is in the region that would otherwise be identified as the polar cap. We find th at the equatorward edge of the energetic electron precipitation coinci des precisely with the equatorward edge of the LLBL as these boundarie s move over a wide range of invariant latitude. We investigate the hyp othesis that the energetic electron precipitation is due to pitch angl e scattering in the magnetopause current layer of magnetospheric elect rons on open drift paths and find that this hypothesis is reasonable. Additional evidence supporting this hypothesis is the dependence of th e precipitating flux on the solar wind clock angle. A consequence of t his hypothesis is that the low-latitude boundary layer in the 0900-100 0 MLT sector lies completely on open magnetic field lines except, perh aps, when the interplanetary magnetic field is predominantly northward .