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M. J. Person, J. L. Elliot, S. W. McDonald (MIT), M. W. Buie, E. W. Dunham, R. L. Millis, R. A. Nye, C. B. Olkin, L. H. Wasserman (Lowell), L. A. Young (SWRI), W. B. Hubbard, R. Hill (LPL), H. J. Reitsema (Ball), J. M. Pasachoff, B. A. Babcock (Williams), T. M. McConnochie (Cornell), R. C. Stone (USNO)
An occultation by Triton of a star denoted as Tr176 by McDonald & Elliot (AJ 109, 1352), was observed on 1997 July 18 from various locations in Australia and North America. After an extensive prediction effort, two complete chords of the occultation were recorded by our PCCD portable data systems. These chords were combined with three others recorded by another group (Sicardy et al., BAAS 30, 1107) to provide an overall geometric solution for Triton's atmosphere at the occultation pressure. A simple circular fit to these five chords yielded a half-light radius of 1439 ±10 km, however least squares fitting revealed a significant deviation from the simple circular projection of a spherical atmosphere. The best fitting ellipse (a first order deviation from the circular solution) yielded a mean radius of 1440 ±6 km and an ellipticity of 0.040 ±0.003.
To further characterize the non-spherical solutions to the geometric fits, methods were developed to analyze the data assuming both circular and elliptical profiles. Circular and elliptically focused light curve models corresponding to the best fitting circular and elliptical geometric solutions were fit to the data. Using these light curve fits, the mean pressure at the 1400 km radius (48 km altitude) derived from all the data was 2.23 ±0.28 microbar for the circular model and 2.45 ±0.32 microbar for the elliptical model. These pressures agree with those for the Tr180 occultation (which occurred a few months later), so these results are consistent with the conclusions of Elliot et al. (Icarus 143, 425) that Triton's surface pressure has increased from 14.0 microbar at the time of the Voyager encounter to 19.0 microbar in 1997. The mean equivalent-isothermal temperature at 1400 km was 43.6 ± 3.7 K for the circular model and 42.0 ±3.6 K for the elliptical model. Within their calculated errors, the equivalent-isothermal temperatures were the same for all Triton latitudes probed.