The middle and high latitude ionosphere at approximately 550 km altitude

Ambient electron density measurements made by the radio frequency capacitance probe on Ariel 4 satellite were analyzed in order to study the structures present at the satellite altitudes, i.e. between approximately 470 and 600 km, at latitudes poleward of + or - 50 degrees Lambda both during winter and equinox; data obtained on 94 days centered on the solstices, and March 1972 were used. The general morphology of and the extreme densities, the mid-latitude electron density trough, the polar cap depletions and the electron density enhancements associated with the cusp auroral zone were determined statistically. Analyses of the solstice and the equinox data acquired during quiet magnetic conditions show that the Northern and Southern Hemisphere ionospheres were significantly different. The global characteristics of the winter solstice and the equinox electron densities were similar although they differ from each other in fine details.