Observations of He+concentrations in the topside ionosphere under equinox conditions near solar maximum: Morningside morphology

Abstract

[1] Average He+ concentrations are examined for solar maximum conditions at equinox using data from Defense Meteorological Satellite Program (DMSP) flights F13 and F15. The DMSP spacecraft have Sun-synchronous orbits at 800 km altitude. F13 data were taken under presunrise conditions, while F15 data were taken at midmorning local times. Data are averaged for 31 days centered on the northern spring and fall equinoxes between ±40° magnetic latitude. Both the He+ concentration and He+ fraction show significant longitudinal and latitudinal variation, which can be accounted for by neutral wind induced ion drag. The He+ fraction can reach as much as 40% of the total plasma density in the presunrise hours, but 10% or less of the total plasma density at midmorning. While the He+ concentrations before sunrise and at midmorning have comparable magnitudes, the midmorning values of the He+ fraction are much more uniform than in the presunrise data. Examination of the F10.7 cm solar flux and geomagnetic activity implies solar flux has a much greater influence on He+ concentrations. The level of solar flux as well as the amount of variability in the solar flux appear to influence the average He+ concentrations and fractions.