The thermal structure and energy balance of the Uranian upper atmosphere

Two occultation observations of the upper atmosphere of Uranus are reported: (i) the 15–16 August 1980 occultation of KM 12 from Cerro Tololo Inter-American Observatory, and (ii) the 26 April 1981 occultation of KME 13 from the Anglo-Australian Telescope and the 1-m telescope of the Australian National University. Mean atmospheric temperatures of 154 ± 15°K for the 1980 event and 132 ± 15°K for the 1981 event are derived from the lightcurves. A comparison of all available Uranus occultation data since March 1977 suggests that the mean atmospheric temperature of Uranus has changed significantly, with a typical variation of 15°K year −1. We investigate plausible energy sources that might account for such large temperature variations. We conclude that molecular and eddy diffusion, and atmospheric dynamics, are potentially as important as radiation to the upper atmospheric heat balance. There is evidence that no significant radiative cooling had occurred at two suboccultation points that had been in darkness for more than 0.5 terrestrial year, suggesting upper limits to the mixing ratios of CH 4 and C 2H 2 of 6 × 10 −5 and 5 × 10 −7, respectively. The consistently close agreement of immersion and emersion mean temperatures for each occultation, in spite of apparently large secular changes in the atmospheric mean temperature, suggests that effective meridional transport occurs on Uranus. A continuing program of occultation observations in future years should reveal whether this pattern is significant.