Interannual and short‐term variations of the Venus nighttime hydrogen bulge

Abstract

The predawn bulge in the diurnal distribution of neutral hydrogen at Venus is found to persist over the first three Venus years of the Pioneer Venus mission. Concentrations of H, derived from in situ mass spectrometer measurements of O+, H+, O, and CO2 show a diurnal bulge with peak levels near 2–5 × 107/cm³ and a night to day concentration ratio of about 200/1. The presence of short‐term fluctuations of as much as a factor of three from day to day reflects the variability in the ion and neutral concentrations often detected on the nightside. Allowing for this superimposed variability, relatively little difference in the magnitude of n(H) between 1979 and 1980 is indicated. This appears to be consistent with the fact that the solar euv flux level changed very little between successive transits of the bulge. On the other hand, there is a suggestion in some of the modulation observed in n(H) that short‐term variation in the euv flux due to solar rotation may influence the bulge characteristics. This speculation on the effectiveness of solar radiation cannot be substantiated as yet, and there is also evidence that changes in the solar wind ρυ² parameter are linked to significant changes in n(H). Although the ion and neutral variability complicates the identification of chemical equilibrium, rather similar behavior in the distributions of n(He), which is measured directly, lends confidence to the n(H) results. Overall, the attempt to understand the hydrogen and helium variability provides a stimulus for more detailed analysis of the nightside aeronomy and its relationship to both solar and solar wind inputs.