Detection of Chlorine Ions in the [ITAL]Far Ultraviolet Spectroscopic Explorer[/ITAL] Spectrum of the Io Plasma Torus

This is the authors' abstract. We don't add key points for this paper.

The spectrum of the Io plasma torus in the range of 995-1187 Å was recorded at 0.26 Å 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'' × 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 2 D 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 σ level, Cl III being stronger with two components roughly one-tenth the brightness of the main components of S III λ1018. We derive an abundance of Cl +2 of 3% relative to S +2 , leading to an overall chlorine ion abundance in the torus of ≈1%. 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.