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L. A. Young (B. U.), A. S. Bosh, M. W. Buie, L. H. Wasserman (Lowell Obs.), J. L. Elliot (M. I. T.)
On 8 November 1999, we observed an occultation by Uranus's atmosphere from Lowell Observatory, and derived temperatures of 116.7 ± 7.9 K for immersion and 124.8 ± 15.5 K for emersion, from isothermal fits to the lightcurves. Numerical experiments with model atmospheres demonstrate that the temperature derived from an isothermal fit corresponds to the model temperature 2.6 scale heights below half-light (10-30 microbar). These relatively low temperatures signal a reversal in the trend observed between 1977 and 1983, during which time the temperatures derived from stellar occultations were seen to increase by 8 K/year. If interpreted solely as a temporal change, the new observations imply a decrease in the temperature of 3 K/year.
This decrease cannot be due to radiative processes. Even under unrealistically optimistic conditions (LTE, optically thin, negligible heating sources, an arbitrary 15 K decrease the 1983 temperature and matching 15 K increase in the 1998 temperatures), the atmosphere would take 4000 years to cool from emission alone. The radiative cooling rate remains too slow, even with reasonable variation of the homopause level or the production rates of ethane or acetylene. Therefore, the 1998 occultation is strong evidence for a non-radiative energy sink in the Uranian upper atmosphere.
We investigated two possible sources for this energy sink. First, thermal conduction can transport energy to the stratosphere at the required rate if the gradient near 50 microbar is 3-8 K/km. Second, a vertical wind of 0.016 cm/s can provide adiabatic cooling at a sufficient rate. We will discuss the viability of these two possible sources.