A feature of the behavior of He/+/ in the nightside high-latitude ionosphere during equinox

Observations of a hole of ionization in the nightside high-latitude ionosphere during equinox in which He(+) plays an important role in the relative abundance of the ionic constituents are reported. Ion abundance data were obtained during a period of very low magnetic activity from the retarding potential analyzer and magnetic ion mass spectrometer on board AE-D as the satellite passed across the polar region near midnight. Significant decreases in O(+) and H(+) concentrations are observed in the region between 1544 and 1547 UT within the polar cap, accompanied by the rise and dominance of He(+) ions to low altitudes. The dominance of He(+) is attributed to effects on photoproduction of changes in solar zenith angle as the plasma moves from the dayside to the nightside, where it stagnates or resides for periods greater than about 3 hours just poleward of the auroral zone. Model calculations of the physical processes involved in this mechanism are presented which in general tend to fit the observations.