Charge neutrality and ion conic distributions at the equatorward electron edge of the midaltitude cusp

Abstract

We have investigated the physical processes occurring in order to maintain charge neutrality at the equatorward edge of the cusp by examining data from 200 Polar cusp crossings. The significant differences in magnetosheath ion and electron velocities could potentially create a region at the equatorward edge of the cusp where only solar wind electrons have access. Our calculations suggest that Polar should encounter this region at least 3 min before the first solar wind ions are observed. Only six Polar cusp crossings were identified, however, where a clear separate electron edge was observed. We then used state‐of‐the‐art plasma instruments on Polar to examine particle features at the separate electron and ion edges. We find evidence that in the electron only region, electrons have been retarded by an electric potential above the spacecraft. We discuss the validity of examining electron spectra for evidence of retarding potentials, and we conclude that quantitative values cannot be obtained from single spacecraft measurements. We also find evidence of intermittent transverse ion acceleration in the electron only region. There are distinct differences in the ion conic distributions observed within the different regions at the equatorward edge of the cusp in terms of energy, pitch angle, and ion species. O+ conies are observed on closed field lines equatorward of the cusp, while H+ and He+ conies are seen at higher latitudes. In the electron only region, H+ and He+ conies have pitch angles between 90° and 120°. Further into the cusp, after the first solar wind ions are detected, higher‐energy H+ and He+ conies with pitch angles extending to 180° are measured. We suggest that processes occurring to maintain charge neutrality at the equatorward edge of the cusp may play a role in the generation of the ion conies observed.