Io plasma torus ion composition: Voyager, Galileo, and Cassini

Abstract

The Io torus produces ultraviolet emissions diagnostic of plasma conditions. We revisit data sets obtained by the Voyager 1, Galileo, and Cassini missions at Jupiter. With the latest version (8.0) of the CHIANTI atomic database we analyze UV spectra to determine ion composition. We compare ion composition obtained from observations from these three missions with a theoretical model of the physical chemistry of the torus by Delamere et al. (2005). We find ion abundances from the Voyager data similar to the Cassini epoch, consistent with the dissociation and ionization of SO2, but with a slightly higher average ionization state for sulfur, consistent with the higher electron temperature measured by Voyager. This reanalysis of the Voyager data produces a much lower oxygen:sulfur ratio than earlier analysis by Shemansky (1988), which was also reported by Bagenal (1994). We derive fractional ion compositions in the center of the torus to be S+/Ne ~ 5%, S++/Ne ~ 20%, S+++/Ne ~ 5%, O+/Ne ~ 20%, O++/Ne ~ 3%, and Σ(On+)/Σ(Sn+) ~ 0.8, leaving about 10–15% of the charge as protons. The radial profile of ion composition indicates a slightly higher average ionization state, a modest loss of sulfur relative to oxygen, and Σ(On+)/Σ(Sn+) ~ 1.2 at about 8 RJ, beyond which the composition is basically frozen in. The Galileo observations of UV emissions from the torus suggest that the composition in June 1996 may have comprised a lower abundance of oxygen than usual, consistent with observations made at the same time by the EUVE satellite.

Energetic Oxygen and Sulfur Charge States in the Outer Jovian Magnetosphere: Insights From the Cassini Jupiter Flyby

“…Meanwhile, the relative composition of sulfur ion charge states observed by Cassini ( Figure 4C) has an enhanced relative abundance of S 3+ than at Juno (33% versus 9% of sulfur ions), while S + and S 2+ decreased in relative flux abundance for sulfur ions at Cassini compared to at Juno (25% versus 29% for S + and 42% versus 62% for S 2+ ). shown Bodisch et al, 2017;Nerney et al, 2017), along with a forward model of Juno JADE-I observations (Kim et al, 2019). While each of these studies makes its own assumptions (e.g., assuming the density of O 2+ is 10% that of O + in Yoshioka et al, 2017) and has its own uncertainties, caveats, spatial coverages, and energy ranges, the results are seen to qualitatively agree in regard to the relative order of abundance of the species.…”

“…Meanwhile, both Yoshioka et al () and Steffl et al (2004b) used remote sensing (Hisaki EUV and Cassini UVIS, respectively). Lastly, three studies of reanalysis from forward modeling of Voyager PLS observations are shown (Bagenal et al, 2017; Bodisch et al, ; Nerney et al, ), along with a forward model of Juno JADE‐I observations (Kim et al, ). While each of these studies makes its own assumptions (e.g., assuming the density of O 2+ is 10% that of O + in Yoshioka et al, ) and has its own uncertainties, caveats, spatial coverages, and energy ranges, the results are seen to qualitatively agree in regard to the relative order of abundance of the species.…”

Proton Acceleration by Io's Alfvénic Interaction

“…At the center of the torus, we estimate the timescale for energy loss from a 100 eV beam would be approximately 250 h (Huba, 2009;Nerney et al, 2017). At the center of the torus, we estimate the timescale for energy loss from a 100 eV beam would be approximately 250 h (Huba, 2009;Nerney et al, 2017).…”

Distribution of Plasma in the Io Plasma Torus as Seen by Radio Occultation During Juno Perijove 1

“…We modify Model A to account for a recent reanalysis of the Voyager data by Bagenal, Dougherty, et al (). Model‐scale heights are updated due to composition and temperature changes found by Bagenal, Dougherty, et al () and Nerney et al (). Model densities are updated due to reanalysis of Voyager data by Bagenal, Dougherty, et al ().…”