The Polar Ionosphere of Venus Near the Terminator from Early Pioneer Venus Orbiter Radio Occultations

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Abstract
Fourteen profiles of electron density in the ionosphere of Venus were obtainecd by the dual-frequency radio occulation method with the Pioneer Venus orbiter between 5 and 30 December 1978. The solar zenith angles for these measurements were between about 85° and 92°, and the latitudes ranged from about 81° to 88° (ecliptic north). In addition to the expected decreasein peak electron density from about 1.5 x 103 to 0.5 x 103 per cubic centimeter with increasing solar zenith angle, a region of almost constant electron density above about 250 kilometers was observed. The ionopause height varies from about 300 to 700 kilometers and seems to be influenced by diurnal changes in solar wind conditions. The structures of the profiles are consistent with models in which O2+ dominates near the ionization peak and is replaced by O+ at higher altitudes.