Reanalysis of voyager 2 UVS occultations at Uranus: Hydrocarbon mixing ratios in the equatorial stratosphere

Abstract

Application of noise filtering and inversion techniques to single-channel UVS lightcurves obtained during the Voyager 2 solar occultation at Uranus has yielded tighter constraints on the structure and composition of the upper equatorial stratosphere at the time of the encounter. Specifically, atmospheric pressure and temperature profiles in the altitude region bracketted by total number densities 2 x 1015 cm-3 and 5 x 1016 cm-3 have been derived, based on the observed H2 Rayleigh scattering opacity profiles (wavelengths > 153 nm) with an assumed helium mixing ration of 0.15: at the density level 3.3 (+/-0.2) x 1016 cm-3, the pressure is 0.60 (+/-0.01) mbar with a temperature 133 (+/-8) K. Lightcurves obtained at shorter wavelengths reveal the mixing ratio of accetylene to be increasing (0.7 x 10-8-1.5 x 10-8) with increasing pressure in the pressure interval 0.10-0.30 mbar. Ethane is also present at these pressures with a mixing ratio of roughly 10-8; if methane is present at mixing ratios in excess of roughly 3 x 10-7, then the ethane estimate may need to be halved. Comparison with photochemical models indicates values of 103-10-4 cm2 sec-1 for the eddy mixing coefficient at the methane homopause, depending on the manner in which eddy mixing is assumed to vary with atmospheric number density. It has not been possible to obtain meaningful results from the stellar occultations, which are characterized by poor signal-to-noise ratios.