The Downward Flux of O+ over the Nightside of Venus

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We have constructed a map of the downward flux of O+ over the nightside of Venus at high and low solar activities through a combination of modeling and analysis of Pioneer Venus ion mass spectrometer data. O+density profiles were obtained for almost 400 inbound or outbound segments of orbits from the first 2 years of the mission and about 40 more in the recent reentry phase of the mission. We have determined the nearly linear relationship between the O+ maximum density and the downward O+ flux for several solar zenith angles and local times by constructing models of the nightside ionosphere of Venus for a range of downward ion fluxes at the upper boundaries. We find that the largest downward fluxes occur near the terminators, and the fluxes fall off sharply toward the antisolar point. Although the standard deviations in the data are large, there is a suggestion of a local maximum near 155°, and the location of this maximum correlates fairly well with structure in the peak electron density as a function of solar zenith angle reported for the Pioneer Venus radio occultation experiment. The average downward ion flux is inferred to be about 1.7 × 108 cm-2 sec-1 over the nightside hemisphere at solar maximum and a factor of 7 less at moderately low solar activity.