Ion composition of the topside equatorial ionosphere during solar minimum

Observations from both the Bennett ion mass spectrometer and the retarding potential analyzer on board the Atmosphere Explorer E satellite were used to study the longitudinally averaged O(+), H(+), and He(+) concentrations from 150 to 1100 km in the equatorial ionosphere during the 1975-1976 solar minimum. The results suggest that the ion mass spectrometer measurements need to be increased by a factor of 2.15 to agree with the densities from the retarding potential analyzer and with ground-based measurements. The peak H(+) concentrations are about 2.5 x 10 exp 4/cu cm during the day and 10 exp 4/cu cm at night and vary little with season. The O(+)/H(+) transition altitude lies between 750 and 825 km during the day and between 550 and 600 km at night. He(+) is a minor species at all altitudes; its concentration is highly variable with a maximum value of about 10 exp 3/cu cm during equinox daytime.