We examine the structure of the flank low-latitude boundary layer (LLBL) through differential energy spectra and particle angular anisotropies for traversals of the dawn flank (December 19, 1977) and dusk flank (July 7, 1978) during periods of predominantly northward magnetosheath field orientation. The LLBL during these crossings is known to consist of two regions with distinct energetic particle signatures, known in previous work (Williams et al., 1985) as the stagnation region and the LLBL (referred to in this paper as ''outer LLBL''). We present spectra obtained from combined ISEE 1 low-energy proton and electron differential energy analyzer (LEPEDEA) and medium energy particle instrument (MEPI) data extending over the 200 eV/q to 2 MeV energy range for the plasma sheet, stagnation region, outer LLBL and magnetosheath regions. The stagnation region is shown to be a unique region on the basis of abrupt changes observed in energy spectra, as well as the previously identified changes in bulk flow velocity, and flow azimuth upon entry into the region (Williams et al., 1985). This uniqueness is suggested to be the result of different convective histories (different source populations), but not the result of distinctly different physical mechanisms.

The stagnation region and the outer LLBL are each a mixture of plasma sheet and magnetosheath populations, but the stagnation region contains a relatively higher fraction of plasma sheet particles, consistent with its placement earthward of the outer LLBL. No obvious ion heating was observed for any of the crossings of the magnetopause or of the LLBL for either event. Evidence for energization of thermal electrons (either magnetosheath or ionospheric in origin) appears during the dusk flank crossing. Bidirectional field-aligned ion distributions are observed with typically 5-to-1 enhancement of the flux along the magnetic field during certain portions of the dusk flank crossing. Bidirectional field-aligned electron distributions appear in the outer LLBL, stagnation region, and adjacent plasma sheet during both dawn and dusk flank crossings with typical ratios between field-aligned and perpendicular flux on the order of 10-to-1. We find a quantitative flux balance in opposite field directions for these electrons in the stagnation region, outer LLBL, and adjacent plasma sheet on both dawn and dusk flanks, supporting the conclusion that the magnetic field is topologically closed during these crossings. ¿ American Geophysical Union 1991