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M. J. Person, J. L. Elliot, J. Pate (MIT), I. Glass (SAAO), R. C. Stone (USNO), K. M. Morzinski, E. W. Dunham (Lowell Obs.)
On 2001 August 25, an occultation by Triton of the star designated Tr231 (McDonald and Elliot, Astronomical Journal, 119, 2000, 936-944) was observed from the 1.9-m telescope of the South African Astronomical Observatory. Preliminary analysis indicates that this light curve has the highest signal to noise of any Earth-based observations of Triton occultations (Olkin, et al., Icarus, 129, 1997, 178-201; Elliot, et al., Icarus, 148, 2000, 347-369). Initial inversions of this light curve indicate a thermal profile with a positive thermal gradient (dT/dR) at altitudes above ~50-60 km, but having an isothermal or negative gradient below. These results are qualitatively consistent with those from the 1997 (Tr180) HST occultation light curve (Elliot, et al., Icarus, 143, 2000, 425-428). The initial thermal gradient of those results can be explained by the downward conduction of heat deposited high in the atmosphere coupled with CO radiative cooling. The addition of a currently unidentified cooling mechanism at lower altitudes is required to explain the isothermal or sub-isothermal profile below 50km. Analysis of the Tr231 event is continuing. This work is partially supported by NASA grants NAG5-10444 and NAG5-9008.