Extreme Ultraviolet Explorer spectra of the Io plasma torus: Improved spectral resolution and new results
Spectral images of the Io plasma torus obtained from ~165 hours of observation by the Extreme Ultraviolet Explorer (EUVE) spacecraft during the I24 encounter of the Galileo spacecraft with the torus and at an earlier epoch in 1996 are analyzed by a spectral-resolution-enhancing deconvolution algorithm in order to estimate averaged torus properties. Because of our analysis's improved spectral resolution, we are better able to separate lines arising from different ion species as compared to earlier EUVE studies. We exploit this capability to estimate emission-weighted whole-torus averages of electron temperature and ion fraction. Comparing the 1996 results for the torus to those of 1999, we estimate that (1) the electrons were hotter (5.4 versus 4.6 eV), (2) the luminosity was higher (375 versus 245 GW in the 350-730 Å region), (3) the mass was lower (by 10-15%), (4) the sulfur/oxygen ratio was higher (0.8 versus 0.5), and (5) the ionization level was greater (1.2 versus 0.8 for [S++]/[S+], 0.22 versus 0.15 for [O++]/[O+]) in the 1996 observation than during I24. Considering the constraints of formation, chemistry, transport, and mass loss, the torus has fewer degrees of freedom than observables, so these two snapshots give us clues as to how torus characteristics may covary. For example, our data are consistent with the hypothesis that electron temperature and density are anticorrelated. We also confirm that the Io-correlated enhancement of torus brightness discovered by Voyager still persists. However, we raise new doubts that the torus's dawn-dusk brightness asymmetry is the result of torus mass loss that is controlled by the total mass of the torus. We also estimate corrections to the electron impact collision strengths for many EUV multiplets of interest for understanding the torus and other sulfur and oxygen plasmas of astrophysical interest, finding, for example, that the O+/S++ 484 Å feature is about an order of magnitude brighter than that predicted by available electron collision strength data.