Venus Thermospheric CO and Temperature Deduced From Cassini UVIS Airglow Measurements

Abstract

Cassini UVIS measurements of Venus\u27s dayside airglow show prominent CO Fourth Positive bands, as well as other CO bands and atomic and ionic emissions from H, O, C, N, He, O+, C+, and C++. The Fourth Positive bands are excited by photon and photoelectron impact dissociative excitation of CO2, dissociative recombination of CO2+, photoelectron impact on CO, and fluorescent scattering of solar FUV (including Lyman-alpha). These mechanisms each produce a characteristic distribution of vibrational levels in the upper state, and in addition the dissociative processes yield CO molecules at higher rotational temperatures than do the processes acting directly on CO. Solar Lyman-alpha pumps the v\u27 = 14 level of the upper state, and because Lyman-alpha penetrates to the CO2 absorbing level below 120km, the v\u27 = 14 sequence is sensitive to CO densities at much lower altitudes than the bulk of the Fourth Positive emissions. We have constructed a model of the emission of these bands incorporating the above mechanisms and using the VTS3 model of thermospheric composition. We will present the preliminary results of our analysis of the relative importance of the various excitation processes and the distribution of CO in the morning equatorial thermosphere, with attention paid to the apparent wave structure (wavelength 2000 km) seen in the raw data