Distribution of SO2 content at the night side of Venus' upper mesosphere

International audience; Venus has a dense CO2 atmosphere with a thick cloud layer (50-70 km) consisting of sulfuric acid (H2 SO4) aerosols. Sulfur oxides (SOx) are directly associated with those aerosols and plays an important role in chemistry of the atmosphere. Any change of its content within or above the clouds has an influence on photochemical processes in the mesosphere (70-100 km). Recent ground-based observations [1-4] and continuous monitoring from the Venus Express orbiter [5-7] have shown high temporal and spatial variability of SO2 abundance mostly on the day side: from 20 to 500 ppbv above the clouds. There is a lack in the detailed analysis at the night-time mesosphere where photo dissociation of sulfur dioxide is replaced by interaction with the global subsolar/antisolar circulation and chemical reactions with atoms of Cl, OH, O etc. In this paper we present vertical distribution of SO2 content at the night side of Venus upper meso-sphere that resulted from stellar occultations made by the SPICAV UV spectroscopy. This mode of occultation occurred for the entire VEx mission, and it gave us possibility to observe yearly variations for period 2006-2014 at altitudes 85-100 km. In parallel, we have reprocessed the terminator dataset from the UV solar occultations at the same altitude range [5] up to 2014. Like this, we have got whole the night-time coverage of SO2 distribution from the sunset to the sunrise twilights of the upper mesosphere. Experiment: SPICAV UV channel operated in the spectral range 118-320 nm with a resolution 1-2 nm at nadir or stellar/solar occultation modes [8]. Here we deal with measurements of SO2 atmospheric absorption in stellar and solar occultation modes. In the case of stellar occultation the instrument observes night-side mesosphere while in solar occultation it probes evening/morning twilights at altitude range 85-100 km. SPICAV can register SO2 absorption bands at 190-220 and 270-300 nm and CO2 bands at 120-210 nm. In the occultation mode an instrument registers the solar (or a stellar) flux out of planetary atmosphere and a flux, having passed through different levels of the atmosphere. The ratio of the second flux to the first one determines the atmospheric transmission at a fixed tangent altitude. This transmission (a relative quantity) is interpreted as due to the extinction from aerosols and gases that can be identified by their spectral signature, and their quantity along the line of sight (LOS) of the instrument (Fig. 1).