DPS 34th Meeting, October 2002
Session 35. Outer Planet Satellites (other than Titan)
Poster, Chair(s): , Thursday, October 10, 2002, 4:00-6:30pm, Exhibit Hall

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[35.05] The Io Plasma Torus With Cassini UVIS

A. J. Steffl, A. I. F. Stewart, F. Bagenal, P. A. Delamere (University of Colorado/LASP)

We present results from the Cassini Ultraviolet Imaging Spectrograph (UVIS) observations of the Io plasma torus during the Jupiter encounter, October 2000-March 2001. The UVIS instrument consists of two independent spectrographs, an EUV channel and an FUV channel, covering a total wavelength range of 561-1912 with a spectral resolution of approximately 4. At present, we have concentrated on data from January 14, 2001 that provide the highest spatial resolution of the torus. We present high signal-to-noise spectra from the torus in this wavelength range and identify all major features. A model, incorporating the atomic physics data contained in the CHIANTI (Dere et al., 1997, 2001) database, was used to generate synthetic spectra of the torus and then fit these spectra to the UVIS data. The effects of a non-thermal electron distribution were included in the model by using three Maxwellian electron populations to approximate a kappa distribution of electrons over the energy range of 7-100 eV. From this model we derive the densities of six ion species: O II, S III, S II, S IV, O III, and S V and the electron temperature as a function of distance from Jupiter. Using the same analysis procedure we model a Voyager 1 UVS spectrum of the torus and confirm that the Cassini-era composition is significantly different from the Voyager-era composition. The UVIS results are consistent with a neutral cloud theory model of the Io torus incorporating a low neutral atom source rate, a slow radial transport rate, a low sulfur to oxygen ratio, and a high fraction of hot electrons. We examine the time variability of the torus during the six-month observation period and find significant changes in both torus composition and total emitted power. However, we find no long-term variation with system III longitude.

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.