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W.B. Hubbard, R. Hill, M.J. Rieke (U Ariz), P. Drossart, F. Roques, B. Sicardy, T. Widemann (Obs Paris), R. Doyon, D. Nadeau (U Montreal), M. Marley (NMSU)
The magnitude-7 star was occulted by Jupiter's high northern atmosphere on 10 October 1999. We report results from high-quality observations obtained at VLT (Chile), Kitt Peak and Catalina Station (Arizona), and Mt. Megantic (Canada). Our observations cover a jovian latitude range from 55 deg N to 73 deg N, and, when combined with data sets from stellar occultations in 1971 and 1989, with Voyager and Galileo Probe data, and with independent astrometry, enable us to investigate the overall oblateness of the jovian mesosphere at pressures of ~ 1 \mubar. Our high-jovian-latitude data sets probe a portion of the jovian atmosphere that may be subjected to auroral heating -- we compare our observed lightcurves with lightcurves predicted for auroral temperature-pressure profiles.
For the Galileo Probe temperature (T) vs. pressure (P) profile, applicable to low latitudes, a single-parameter fit to a stellar occultation lightcurve yields an effective refractivity scale height (H) of 24 km. If this same T-P profile is applied to high latitudes, the effective H is about 21 km. With auroral heating, the inferred H increases to about 28 km. There is considerable scatter in the H values obtained from the 1999 occultation data, but some indication of the higher H values consistent with auroral heating.
The oblateness e for the jovian atmosphere that we infer from the data sets lies between the dynamical oblateness e=0.06493 of Lindal et al. (JGR 86, 8721-8727, 1981), and the value derived from the 1971 \beta Sco occultation by Hubbard and Van Flandern, e=0.060 ±0.001 (AJ 77, 65-74, 1972). Thus, there is evidence that the polar jovian atmosphere at microbar pressures is ~100 km farther from the center of mass than would be the case for a barotropic planet in hydrostatic equilibrium.
Supported in part by NASA Planetary Astronomy Program.