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Analysis and interpretation of observations of airglow at 297 nm in the Venus thermosphere

Limb profiles obtained by the Pioneer Venus orbiter ultraviolet spectrometer (PVOUVS) are analyzed. The analysis is confined to those data obtained when the spacecraft was near periapsis (∼150-200 km altitude) and the PVOUVS grating is set to a center-of-band-pass wavelength of 297 nm. Sources are identified which give rise to the observed signal as they fall within the 1.3-nm instrumental band pass: Rayleigh scattering, [O I] 2972 A, and C I 2967 A. Additionally, contamination from off-axis scattering and the cosmic ray background are evaluated. Below 175 km, O(¹S) is produced through the photo-dissociative excitation of CO2. Above 175 km, O(¹S) produced through the dissociative recombination of O2+ and photoelectron impact excitation of C(5S°) become important sources of airglow emissions. Structure in the [O I] 2972-A emission profile in the lower thermosphere is explained in terms of large temperature excursions which alter the rate of quenching of O(¹S) by CO2. This phenomenon suggests a new technique for remotely sensing Venus lower thermosphere temperatures. The deduced temperature profile is consistent with in situ measurements which have been interpreted as evidence of the propagation of atmospheric gravity waves.