Energetic Particle Observations in the Low‐Latitude Boundary Layer

Abstract

We present three‐dimensional energetic (≥ 24 keV) particle distribution observations for two traversals of the magnetospheric low‐latitude boundary layer (LLBL). One traversal occurred during local morning hours (∼0515 LT; December 19, 1977, 0800–1600 UT), and the second during local evening hours (∼1845 LT; July 7, 1978, 0400–1200 UT). Both cases were for a northward magnetosheath field. In the dawnside LLBL the data show trapped electron distributions and tailward convecting ion distributions that display a small but observable bidirectional field‐aligned component. In the duskside LLBL the data reveal tailward convecting, trapped ion distributions and no measurable electron fluxes. LLBL electric fields inferred from the convecting energetic ion distributions are ∼ 3–5 mV/m. Earthward of the dusk LLBL is a region of enhanced fluxes in which the convective anisotropies of the energetic ion distributions become too small to measure. Earthward of this region is the plasma sheet where nearly isotropic ion fluxes are seen with a small but steady sunward convection anisotropy. From an analysis of the data we conclude that for these two cases of northward magnetosheath magnetic field, the flank LLBL is contained on closed geomagnetic field lines, the plasma sheet is the main source of energetic particles in the flank LLBL, and it is possible that two large energetic particle circulation cells (electrons in the dawnside and protons in the duskside) exist in the geomagnetic tail.