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 CO ₂ ⁺ and N ₂ ⁺ , 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 O ₂ ⁺ 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 N ₂ ⁺ 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 F ₂ “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( ¹ D ), and O( ¹ S ).