SEASONAL MEAN STRUCTURE OF THE NIGHT-TIME F2-REGION OVER ARECIBO

Seasonal mean night-time variations of ion and electron temperatures, electron density, ion drift velocity, and light ion composition of the F2 region are derived from incoherent scatter observations at Arecibo based on 19 nights of observation over the latest sunspot minimum years 1974-1976. It is shown that the downward flux of ionization is sufficient to maintain the nocturnal F2 region against recombination at low latitudes. The difference in the electron density decay rate from summer to winter is consistent with the seasonal variation in magnitude of the ionization flux. The mean eastward electric field, which is responsible for any vertical component perpendicular to B, is very small throughout the night. However, the southward electric field, i.e. east-west ion drifts, shows a substantial systematic variation during the night, being southward (eastward ion drifts) before midnight and northward after midnight, with a mean amplitude of 1-2 mVm-1. The H+ ion concentration shows a marked seasonal variation. The mean relative concentration of H+ ion to electron density at 500 km sometimes exceeds 50% before sunrise in winter. A strong anti-correlation of H+ ion concentration with magnetic activity is observed. The observed ion temperatures average about 20-30 K higher than the prediction of the Jacchia (1971) neutral model for the observed range of the 10.7 cm solar flux. © 1979.