Uranus after solstice: results from the 1998 November 6 occultation

We observed a stellar occultation of the star U149 by Uranus from Lowell Observatory and the IRTF on Mauna Kea on November 6, 1998. The temperatures derived from isothermal fits to the Lowell lightcurves are 116.7 [+ or -] 7.9 K for immersion, and 124.8 [+ or -] 15.5 K for emersion. The secular increase in temperature seen during the period 1977-1983 has reversed. Furthermore, the rate of decrease ([greater than or equal to] 1.2 K/yr) cannot be explained solely by radiative cooling. Although the temperature structure of Uranus' upper atmosphere may be related to seasonal effects (e.g., the subsolar latitude) or local conditions (e.g., diurnally averaged insolation), these observations suggest nonradiative influences on the temperature, such as adiabatic heating/cooling or thermal conduction. Key Words: Uranus, atmosphere; occultations; atmospheres, structure; atmospheres, dynamics.