Night side distribution of SO₂ content in Venus’ upper mesosphere

Abstract : In this paper we present the first night side distribution of SO₂ content in Venus’ upper mesosphere (altitudes from 85 to 105 km). The dataset is based on the SPICAV UV stellar occultation experiment which took place onboard ESA's Venus Express (VEX) orbiter in 2006-2014. The UV channel of SPICAV spectrometer detected absorption bands of SO₂ and CO₂ in the spectral range 180-300 nm with a resolution of 1-2 nm while stellar light was occulted by the mesosphere. Altitude profiles of sulfur dioxide's volume mixing ratio (VMR) could be retrieved in the upper part of the mesosphere covering the whole night side on Venus. In parallel, we have reprocessed the terminator UV solar occultations dataset (Belyaev et al. (2012). Icarus 217, 740-751) in the same altitude range and extended its statistics until 2014. On average the SO₂ VMR increases with altitude from 10-30 ppb at 85 km to 100-300 ppb at 100 km in both regimes of occultation. The midnight SO₂ abundance appears to be 3-4 times higher than in the terminator region: 150-200 ppbv versus 50 pppv at altitude around 95 km. These new results were compared with the distribution of oxygen atoms, which are tracers of the global subsolar-antisolar (SS-AS) circulation at ∼100 km (the data provided by Soret et al. (2012). Icarus, 217, 849–855). The night time behavior looks similar for SO₂ molecules and O atoms with a correlation coefficient Rcorr = 0.73. Moreover, the retrieved SO₂ enrichment above 85 km appears to correlate with the density of H₂SO₄ droplets (Luginin et al., 2016; Icarus 277, 154–170).