DPS 2001 meeting, November 2001
Session 9. Cassini and Galileo at Jupiter Posters
Displayed, 9:00am Tuesday - 3:00pm Saturday, Highlighted, Tuesday, November 27, 2001, 5:00-7:00pm, French Market Exhibit Hall

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[9.07] Cassini UVIS Observation of Occultation of Sigma Leo by Jupiter

J. E. Colwell (LASP, University of Colorado), R. A. West (JPL), D. E. Shemansky (University of Southern California), L. W. Esposito, W. E. McClintock (LASP, University of Colorado)

The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed the occultation of the star sigma Leo (B9.5V, V=4.05) on January 7, 2001, with its Far Ultraviolet spectrograph (FUV) and High Speed Photometer (HSP). The ingress occultation occurred at a latitude of 12.8 degrees south and a local time of 13:15 hours, and the egress occultation was at 19.0 degrees south latitude at a local time of 00:20 hours. The speed of the occultation impact parameter was 8.8 km/s. The HSP integration period was 2 msec for a sampling resolution of only 18 m in altitude. The star signal is ~2000 counts/second which allows for a signal to noise ratio of greater than 10 at a vertical resolution of 800 m. The FUV recorded spectra of sigma Leo during the occultation from 110 to 190 nm with a spectral resolution of 0.3 nm. The FUV recorded a total stellar count rate of ~100 counts/second with an integration period of 2 seconds corresponding to a vertical resolution of approximately 17 km with signal to noise comparable to that of the HSP at a resolution of 800 m. The duration of the transition from full signal to fully occulted star in the HSP data is 25 seconds on ingress and 20 seconds on egress where the background is much lower because it is on the night side of Jupiter. The half-power points of the HSP light curves occur at altitudes of 2320 km and 36 km relative to a spheroid with equatorial radius 71492 km and polar radius 66854 km. We will discuss fitting the four UVIS occultation light curves with a model atmosphere including extinction by molecular and atomic species, refraction, temperature profiles, and zonal wind gradients.


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