Detection of chlorine ions in the Far Ultraviolet Spectroscopic Explorer spectrum of the Io plasma torus

The spectrum of the Io plasma torus in the range of 995-1187 Angstrom was recorded at 0.26 Angstrom resolution by the Far Ultraviolet Spectroscopic Explorer (FUSE) on 2000 January 20. 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 " x 30 " aperture of the FUSE LiF spectrographs yielding a total observation time of 3405 s. The spectral resolution exceeds by a factor of 10 that of the data obtained by the Hopkins Ultraviolet Telescope (HUT) during the Astro-1 mission. This region of the spectrum is dominated by resonance multiplets of S III and S IV, whose multiplet structures are nearly completely resolved, as well as numerous S II multiplets originating on the D-2(o) state of the ground configuration. Weak emission from a few lines of the resonance multiplets of Cl III and Cl II is detected at or above the 3 sigma level, Cl III being stronger with two components roughly one-tenth the brightness of the main components of S III lambda 1018. We derive an abundance of Cl+2 of 3% relative to S+2, leading to an overall chlorine ion abundance in the torus of approximate to1%. The ratio of S IV to S III brightness is about twice that observed by HUT, which, when the different slit geometries are accounted for, supports the earlier analysis that S IV emissions originate from a region more extended out of the centrifugal plane than the S III emissions.