Night O2 (a1Δg) airglow spatial distribution and temporal behavior on Venus based on SPICAV IR/VEx nadir dataset

The infrared O2 (a1Δg) airglow driven by the subsolar to antisolar circulation occurs between two global circulation regimes on Venus: the zonal super-rotation below 90 km and the subsolar to antisolar circulation over 120 km. Here we report the complete global results of SPICAV IR/Venus Express observations of O2 (a1Δg) nightglow from 2006 till the end of the mission in 2014. The spectrometer resolving power of ∼1000 at the considered spectral range was the highest among Venus Express instruments, and it was sufficient to resolve O2 (a1Δg) airglow and thermal emission signals. The nadir spectra were fitted to a model representing these two phenomena ensuring robust separation of the thermal emission, and so retrieval of the airglow intensity. Its spatial distribution in the night hemisphere, averaged among 8 years of observations, shows the maximum intensity of 1.4 ± 0.8 MR located at 23H30 of local time and 0–5° N of latitude. A high variability of the airglow was observed. “Bright spots”, i.e., those with an intensity above the global maximum, were observed over a latitude range of 70°S-70°N with the majority occurring around the antisolar point. Lack of imaging capability was an obstacle to track the short-term variations by SPICAV IR. A single observation sequence of 20 days covering neighboring locations exhibits indications of airglow intensity fluctuations with a 5.4-day periodicity. SPICAV IR working period covered the end of the 23rd Solar cycle and the solar activity increase of the 24th Solar cycle. A correlation of the O2 (a1Δg) with the EUV solar flux increase predicted by global circulation model simulations was not found.