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[34.01] Detection of Chlorine Ions in the FUSE Spectrum of the Io Plasma Torus

P. D. Feldman, T. B. Ake, A. F. Berman, H. W. Moos, D. J. Sahnow, D. F. Strobel, H. A. Weaver (JHU), P. R. Young (CfA), FUSE Solar System Team

The spectrum of the Io plasma torus in the range 995--1087 Å\ was recorded at ~0.3 Å\ resolution by the Far Ultraviolet Spectroscopic Explorer (FUSE) on January 20, 2000. Five orbits of data were obtained in point-and-shoot mode (no tracking of the moving target) with the East ansa of the torus initially centered in the 30'' \times 30'' aperture of the FUSE LiF spectrographs yielding a total observation time of 3405 seconds. The spectral resolution exceeds by a factor of ten that of the data obtained by the Hopkins Ultraviolet Telescope (HUT) during the Astro-1 mission (Moos et al., ApJ 382, L105, 1991). This region of the spectrum is dominated by resonance multiplets of \ion{S}{3} at 1018 Å\ and \ion{S}{4} at 1070 Å, whose multiplet structure is nearly completely resolved, as well as numerous \ion{S}{2} multiplets originating on the ²\!D^o state of the ground configuration. Weak emission from the resonance multiplets of \ion{Cl}{3} at 1011 Å\ and \ion{Cl}{2} at 1071 Å\ are seen, the former being stronger with two components roughly one-tenth the brightness of the main components of \ion{S}{3} \lambda1018. Although collision strengths for Cl ions are not readily available, the isoelectronic relationship between S and Cl ions suggests an abundance of Cl⁺² of a few percent relative to S⁺, similar to the result found by Küppers and Schneider (GRL 27, 513, 1999) for Cl⁺ from optical spectra. The ratio of \ion{S}{4} to \ion{S}{3} brightness is about twice that observed by HUT, which when the different slit geometries are accounted for supports the analysis by Hall et al. (ApJ 420, L45, 1994) that \ion{S}{4} emissions originate from a region more extended out of the centrifugal plane than the \ion{S}{3} emissions. We also note the detection of weak \ion{He}{2} emission at 1025.3 Å, on the blue wing of geocoronal Lyman-\beta. The origin of the helium ions is not clear at this time.

This work is based on data obtained for the Guaranteed Time Team by the NASA-CNES-CSA FUSE mission operated by the Johns Hopkins University. Financial support to U. S. participants has been provided by NASA contract NAS5-32985.

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