Venusian Upper Haze Properties: Detection of a Multimodal Distribution at High Altitude by Spicav/soir

The SPICAV suite of instruments is composed of three separate channels: UV (110 to 320 nm), near-IR (0.7 to 1.7 μm) and the SOIR channel (2.3 to 4.4 μm). On specific opportunities, all three channels have operated simultaneously during solar occultations and have provided almost contiguous spectral information from 0.11 to 4.4 μm. Occultation observations provide several advantages, in particular it does not require cross-calibrating the channels together as scientific analysis is based on relative measurements; i.e. atmospheric transmissions which are the ratio of spectra obtained at a given altitude where the atmosphere produces some attenuation onto that collected outside the atmosphere where the sun can be observed free of any absorption. Haze opacities are readily retrieved using Beer-Lambert's law and vertical distribution from 65 to 120 km is inferred using regular onion peeling technique. Over the interval covered by SPICAV/SOIR, the spectral behavior of haze particles can be fully and robustly evaluated since the size parameter varies by more than one order of magnitude. Extraction of extinction coefficients have been performed for all three channels, allowing derivation of size distribution parameters. Details on the observations made for each channel will be presented. Profiles exhibit peculiar wavy structures that suggest gravity wave vertical propagations or localized destruction processes. Size distribution results will be discussed, in particular the possibility for a multi-modal distribution potentially implying different processes of formation and destruction at work in the mesosphere of Venus.