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

We have investigated the physical processes occurring in order to maintain charge neutrality at the equatorward edge of the cusp by examining data fro m 200 Polar cusp crossings. The significant differences in magnetosheath io n and electron velocities could potentially create a region at the equatorw ard edge of the cusp where only solar wind electrons have access. Our calcu lations suggest that Polar should encounter this region at least 3 min befo re the first solar wind ions are observed. Only six Polar cusp crossings we re identified, however, where a clear separate electron edge was observed. We then used state-of-the-art plasma instruments on Polar to examine partic le features at the separate electron and ion edges. We find evidence that i n the electron only region, electrons have been retarded by an electric pot ential above the spacecraft. We discuss the validity of examining electron spectra for evidence of retarding potentials, and we conclude that quantita tive values cannot be obtained from single spacecraft measurements. We also fmd evidence of intermittent transverse ion acceleration in the electron o nly region. There are distinct differences in the ion conic distributions o bserved within the different regions at the equatorward edge of the cusp in terms of energy, pitch angle, and ion species. O+ conics are observed on c losed field lines equatorward of the H+ cusp, while H+ and He conics are se en at higher latitudes. In the electron only region, and He+ conics have pi tch angles between 90 degrees and 120 degrees. Further into the cusp, after the first solar wind ions are detected, higher-energy H+ and He+ conies wi th pitch angles extending to 180 degrees are measured. We suggest that proc esses occurring to maintain charge neutrality at the equatorward edge of th e cusp may play a role in the generation of the ion conics observed.