Uranus after solstice: Results from the 1998 November 6 occultation

We observed a stellar occultation of the star U149 by Uranus from Lowell Ob servatory and the IRTF on Mauna Kea on November 6, 1998. The temperatures d erived from isothermal fits to the Lowell lightcurves are 116.7 +/- 7.9 K f or immersion, and 124.8 +/- 15.5 K for emersion. The secular increase in te mperature seen during the period 1977-1983 has reversed. Furthermore, the r ate of decrease (>1.2 K/yr) cannot be explained solely by radiative cooling . Although the temperature structure of Uranus' upper atmosphere may be rel ated to seasonal effects (e.g., the subsolar latitude) or local conditions (e.g., diurnally averaged insolation), these observations suggest nonradiat ive influences on the temperature, such as adiabatic heating/cooling or the rmal conduction. (C) 2001 Academic Press.