The EUV spectrum of the Venus dayglow: observations during the Cassini Venus flyby

We present the analysis of EUV spatially-resolved dayglow spectra obtained at 0.37 nm resolution by the UVIS instrument during the Cassini flyby of Venus, a period of high solar activity level. Emissions from OI, OII, NI, CI and CII and CO have been identified and their disc average intensity has been determined. They are generally somewhat brighter than those determined from the observations made with the HUT spectrograph at a lower activity level, We analyze the brightness distribution along the foot track of the UVIS slit of the OII 83.4 nm, OI 98.9 nm, Lyman-ß + OI 115.2 nm and NI 120.0 nm multiplets, and the CO C-X and B-X Hopfield-Birge bands. We make a detailed comparison of the intensities of the 834 nm, 989 nm, 120.0 nm multiplets and CO B-X band measured along the slit foot track on the disc with those predicted by a detailed airglow model. This model includes the treatment of multiple scattering for the optically thick OI, OII and NI multiplets. It is found that the calculated intensity of the OII emission at 83.4 nm is somewhat larger and the limb brightening more pronounced than predicted by the model. The calculated intensity variation of the CO B-X emission along the track of the UVIS slit is in fair agreement with the observations. The calculated brightness of the NI 120 nm multiplet is larger by a factor of ~2-3 than observed.