Improved calibrations of the stellar occultation data accumulated by the SPICAV UV onboard Venus Express

Stellar occultation is a powerful method to study vertical structure of the Venus night mesosphere. The UV channel of SPICAV spectrometer, operated in 2006-2014 on board ESA's Venus Express orbiter, allowed retrieval profiles of atmospheric gases (CO2, SO2, and O-3) and aerosols. It was also able to register different UV emissions around Venus (nitric oxide airglow, Lyman-alpha) overlapping the absorption features at 120-300 nm. Several calibration steps convert the raw data to atmospheric transmission spectra used for the retrievals. The systematic errors of resulted gaseous concentrations mainly relate to: (i) an uncertainty of the wavelength to pixel assignment; (ii) a portion of emitting light contaminating the analyzed transmission spectra. In the present paper, we have tested a new method of the wavelength-to-pixel assignment based on the spectral features of measured stars. Secondly, using imaging capabilities of the instrument, we have demonstrated an accurate separation between different kinds of registered signal: extended UV nightglow, light from a point star, transmitted through the atmosphere, and, sometimes, solar light, scattered by Venus dusk. The efficiency of two approaches performing the separation was studied. As a result, corrected transmission spectra provided retrievals of gaseous concentrations with 20-40% higher precision respectively to those processed in previous SPICAV stellar occultation studies (Montmessin et al., 2011, Icarus 216, 82; Piccialli et al., 2015, Planet. Space Sci. 113-114, 321; Belyaev et al., 2017, Icarus 294, 58).