Improved calibrations of the stellar occultation data accumulated by the SPICAV UV onboard Venus Express
Résumé
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 O3) and aerosols. It was also able to register different UV emissions around Venus (nitric oxide airglow, Lyman-α) 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).