Simulations of Optical Emissions in Io's Plasma Torus

Abstract

The Io plasma torus in Jupiter's magnetosphere, dominated by sulfur and oxygen ions from Io's volcanism, exhibits complex radial and azimuthal structures and significant temporal variability. This study analyzes ground-based optical emission data from the Dual Imaging Spectrograph on the 3.5 m telescope at Apache Point Observatory (APO). We deduce variability and determine steady-state radial conditions by combining 30 nights of observations from 2013 to 2018 and co-adding dawn and dusk profiles. The results of a “Cubic-cm” spectral emission model are compared with forward modeling techniques to account for projection effects and line-of-sight (LOS) integration. This study provides the first detailed characterization of local conditions within the ribbon, gap region, and cold torus from remote sensing of the major Io plasma torus species ($S^{+}$, $S^{++}$, and $O^{+}$). We find that electron and ion densities in the cold torus are significantly lower than previous Voyager 1 PLS measurements, consistent with later studies. Electron temperatures in the cold torus align with Voyager PLS data, while those in the warm torus are lower than expected from Cassini UVIS and more consistent with contemporaneous Hisaki observations. Key findings include a shift of the cold torus toward Jupiter and a larger gap region, with ribbon locations remaining stable. The electron density profiles show a shallower decline with radial distance in the warm torus than previously reported, highlighting the variability within the Io plasma torus and the challenges of non-uniqueness in fitting plasma parameters determined via remote sensing.