Quasi‐neutrality in the polar cusp

Data from the High Altitude Plasma Instrument on Dynamics Explorer have shown a remarkably close correspondence between the densities of suprathermal electrons and positive ions at altitudes between 16,000 and 23,000 km in the polar cusp. It is argued that this quasi‐neutrality results from an ambipolar electric field at the magnetopause, which allows the entry of magnetosheath electrons only to the extent required to balance the charge carried by the positive ions.