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Cassini UVIS observations of the Io plasma torus. II. Radial variations | | Andrew J. Steffl
; Fran Bagenal
; A. Ian F. Stewart
; | | Date: |
16 Jan 2013 | | Abstract: | On January 14, 2001, shortly after the Cassini spacecraft’s closest approach
to Jupiter, the Ultraviolet Imaging Spectrometer (UVIS) made a radial scan
through the midnight sector of Io plasma torus. The Io torus has not been
previously observed at this local time. The UVIS data consist of 2-D spectrally
dispersed images of the Io plasma torus in the wavelength range of
561{AA}-1912{AA}. We developed a spectral emissions model that incorporates
the latest atomic physics data contained in the CHIANTI database in order to
derive the composition of the torus plasma as a function of radial distance.
Electron temperatures derived from the UVIS torus spectra are generally less
than those observed during the Voyager era. We find the torus ion composition
derived from the UVIS spectra to be significantly different from the
composition during the Voyager era. Notably, the torus contains substantially
less oxygen, with a total oxygen-to-sulfur ion ratio of 0.9. The average ion
charge state has increased to 1.7. We detect S V in the Io torus at the
3{sigma} level. S V has a mixing ratio of 0.5%. The spectral emission model
used in can approximate the effects of a non-thermal distribution of electrons.
The ion composition derived using a kappa distribution of electrons is
identical to that derived using a Maxwellian electron distribution; however,
the kappa distribution model requires a higher electron column density to match
the observed brightness of the spectra. The derived value of the kappa
parameter decreases with radial distance and is consistent with the value of
{kappa}=2.4 at 8 RJ derived by the Ulysses URAP instrument (Meyer-Vernet et
al., 1995). The observed radial profile of electron column density is
consistent with a flux tube content, NL^2, that is proportional to r^-2. | | Source: | arXiv, 1301.3813 | | Services: | Forum | Review | PDF | Favorites | |
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