Chapter 6 - 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. 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 Polar spacecraft measured cusp energetic particles (CEP) in the high-altitude cusp region. Recent satellite measurements reveal that the Earth's magnetospheric cusp is in fact a broad and dynamic region having a size of several RE with charged particles of high energies from 20 keV up to 20 MeV. The increased intensities of the charged particles were associated with the deep diamagnetic cavities. 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 nT. 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.