Solar Activity Variations of the Venus Thermosphere/Ionosphere

We present models of the low and high solar activity thermospheres and ionospheres of Venus for a background atmosphere based largely on the VTS3 model of Hedin et al. [1983]. Our background model consists of 12 neutral species, and we compute the density profiles of 13 ions and 7 minor neutrals. We find that the peak production rates of some ions, such as CO2+ and N2+, vary approximately as the solar flux and that some, whose parent neutrals are photochemically produced, such as O+, N+, and C+, show variations that are amplified over that of the solar flux. The solar cycle variation of the O2+ density at its peak is about a factor of 1.6, in good agreement with the radio occultation measurements and previous models. The peak density of N2+ varies by a factor of ∼ 2, but that of CO+ varies by a larger factor because the mixing ratio of CO is also correlated with solar activity. The atomic ions O+, N+, and C+, which peak at high altitudes, exhibit larger density enhancements at high solar activity of factors of 5–18. Thus there is a solar cycle variation in the overall composition of the ionosphere, with a relatively larger proportion of atomic ions at high solar activity. In both measurements and models the solar activity variation of the electron density profile is altitude-dependent, with variations of only ∼60% near the peak but up to an order of magnitude near 300 km. The high solar activity electron density profile exhibits an F2 “shoulder,” which is rarely seen in the radio occultation data and may indicate that the VTS3 atomic O mixing ratios are too large at high solar activity. We also discuss the solar activity variations of N, NO, C, O(1D), and O(1S)