DPS 35th Meeting, 1-6 September 2003
Session 13. Io II
Poster, Highlighted on, Wednesday, September 3, 2003, 3:00-5:30pm, Sierra Ballroom I-II

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[13.04] Variability in the Io torus as observed by Cassini UVIS

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

We present observations of the Io torus made by the Cassini Ultraviolet Imaging Spectrograph (UVIS) during the first half (4-Oct-200014-Nov-2000) of the inbound leg of the Cassini Jupiter flyby. During this period, 49 rotations of the Jovian magnetosphere were observed, resulting in nearly 2000 spectra, covering a wavelength range of 561-1182Å. The UVIS field of view encompasses the entire Io torus, allowing measurement of the total EUV power emitted by the torus and comparison of spectra obtained simultaneously from different spatial regions (e.g. the dawn and dusk ansae). Analyses of the spectra show significant compositional changes in the torus plasma over the observing period. The dawn-dusk brightness asymmetry is clearly seen in the UVIS data and is observed to be variable: the dusk-to-dawn ratio of total emitted power ranges from 0.74-2.29 with a mean value of 1.32 and a standard deviation of 0.25. Although no significant long-term variations of torus brightness with System III longitude are observed, transient System III longitude variations are seen. These longitudinal variations remain coherent for approximately 100 hours and have an amplitude of ~15%. The phase of the System III variations is observed to change from episode to episode, and the phases vary with ion species, suggesting intermittent longitudinal variations in ion composition. Several torus brightening events, lasting 5-30 hours, are observed in this time period. During these events, the torus brightness increases by up to 15% before returning to the pre-event level. The relatively short timescale for these events suggests that a change in the hot electron component of the plasma is responsible for the increased emission.

The author(s) of this abstract have provided an email address for comments about the abstract: steffl@colorado.edu

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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.