Radar spectrographic estimates of ionic composition from 225 to 1400 kilometers for solar minimum winter and summer conditions

Observed diurnal variations of the ionic composition in the ionosphere over Arecibo are presented in the 225–1400-km altitude range for a typical winter and summer day under solar minimum geomagnetically undisturbed conditions. He+ remains a minor constituent (about 20% or less) for all the times observed. In winter it is found to maximize at about 450–500 km at night and at about twice this altitude in the day; the transition from strongly dominant O+ to strongly dominant H+ takes place in a corresponding region whose extent is about 300 km at night but more like triple this in the day. In summer the transition region is some what higher and thicker. Sunrise and sunset rates of change of composition are greater in summer than winter, and the minimum altitude of the contours of constant composition is shifted from local sunrise in summer to roughly midnight in winter. These seasonal differences are ascribed to the effect of photoelectrons streaming from the magnetically conjugate ionosphere.