Average spatial distributions of energetic particles in the midaltitude cusp/cleft region observed by Viking

Abstract

The cusp/cleft region provides an entrance for magnetosheath particles into the magnetosphere and a sink for magnetospheric particles. In addition, strong acceleration and/or scattering of particles takes place. The Swedish satellite Viking crossed this region at midaltitudes. Measurements from this spacecraft were used to determine the average spatial distributions of H+ and He++ ions in the energy/charge range 2 keV/e ≤ E/Q ≤ 60 keV/e and of electrons with 7 keV ≤ E ≤ 97 keV. The data supply information on the structure of the midaltitude cusp/cleft region, the particle sources, and dynamical processes. Four different parts can be distinguished: (1) The cusp extends from about 76° to 82° invariant latitude (INL) and from 0800 to 1400 MLT. It is characterized by the presence of magnetosheath origin particles and important electron acceleration signatures. The magnetosheath particles presumably enter via the entry layer. (2) A smaller region inside the cusp (77°–82° INL, 1000–1330 MLT) contains magnetosheath origin ions without electron acceleration. This is regarded as the cusp proper connected to the exterior cusp. (3) Poleward of the cusp magnetosheath origin ions are still present, but no magnetosheath electrons. This region is related to the plasma mantle. (4) Another region without magnetosheath origin ions but with strong electron acceleration extends equatorward of the cusp and probably constitutes part of the cleft, likely to be connected to the low‐latitude boundary layer. The observations provide evidence for the interaction of energetic particles with the magnetopause current layer in which ions are accelerated and the particles are scattered on field lines threading through the cusp/cleft region.