The high‐latitude winter F region at 300 km: Thermal plasma observations from AE‐C

Ion composition and electron temperature data obtained from AE‐C during a magnetically quiet period centered on the June 1976 solstice have been used in a statistical study of the southern winter F region poleward of ‐50° Λ at 300‐km altitude. Prominent ionospheric features revealed by topographic maps of O + , NO + , and O 2 + concentration and T e include the nightside main trough, an ionization ‘hole’ poleward of the nightside auroral zone, and ionization and T e enhancements in the dayside auroral zone‐cusp region. The main trough, in which O + was the dominant ion, extended throughout the night between ‐60° and ‐70° Λ, the lowest trough densities, ∼1 × 10³ cm −3 , being detected near dusk. We attribute these low concentrations to the opposition, in the dusk sector, of plasma corotation and solar wind induced plasma convection velocities, leading to long plasma residence (and decay) times. That the distributions of NO + and O 2 + in the trough region exhibited little correlation with O + suggests that drift‐enhanced O + loss via reactions such as O + + N 2 → NO + + N played a minor role in the formation of the trough during this period. A band of enhanced electron temperature coincided with the trough throughout the night; this T e peak, which has been observed previously in the topside ionosphere, is attributed to heat conducted downward from the protonosphere. The ionization hole, a region poleward of the nightside auroral zone between ‐70° and ‐80° Λ, was characterized by depletions in all the measured ion densities and by a minimum in T e . The total ion concentration measured in this region exhibited extreme temporal variability, ranging from values as low as 2 × 10² to 6 × 10³ cm −3 . We have concluded that the hole forms as a result of slow antisunward plasma drift across the dark polar cap and attendant ion recombination; an average drift velocity of ∼0.1 km/s, corresponding to a convection electric field of less than 5 mV/m, could produce the deepest holes observed. The ion density variability in the hole is attributed to changes in the transpolar plasma convection configuration and the distribution of energetic particle fluxes. The dayside auroral zone‐cusp region was characterized, in general, by enhanced levels of ionization and electron temperature associated with energetic particle precipitation. On some passes through this region, however, localized O + depletions and corresponding molecular ion increases were detected; we attribute these features to the reactions O + + N 2 → NO + + N and O + + O 2 → O 2 + + O, whose rates are enhanced by the high‐speed plasma drifts observed in the cusp region.