7 - High-altitude cusps

Abstract

The Earth's magnetospheric high-altitude cusp is a region of weak magnetic fields with a funnel-shaped geometry, one in the north and one in the south centered around noon where the fields converge. The magnetospheric cusps are located where magnetosheath plasma and momentum enters into the magnetosphere. The magnetospheric cusps permit for the most direct entry of shocked solar wind plasma into the magnetosphere. Large parallel downward flows at the equatorward edge of the cusp show that plasma penetration occurs preferentially at the dayside low-latitude magnetopause for southward interplanetary magnetic field (IMF) conditions; in contrast, under northward IMF the results suggest plasma penetration from the poleward edge of the cusp, with a substantial sunward convection. For decades the cusp was considered as only a sink of energy. No significant energetic particle fluxes were expected to be detected. Therefore it came as a big surprise when the Polar spacecraft measured cusp energetic particles (CEP) in the high-altitude cusp region. Satellite measurements have revealed that the Earth's magnetospheric cusps are in fact broad and dynamic regions. In order to maintain a pressure balance with adjacent magnetospheric regions, the magnetic pressure in the exterior cusp must decrease to account for the increase in thermal pressure from the newly injected plasma. The result is a diamagnetic cavity in the exterior cusp; in some cases this can bring the magnetic field strength close to zero. The density, temperature, and velocity distributions establish the presence of three distinct boundaries surrounding the exterior cusp region: with the lobes, the dayside plasma sheet, and the magnetosheath. This external boundary is characterized by a density decrease and a temperature increase from the magnetosheath to the exterior cusp.