Abstract
The Io plasma torus in Jupiter’s magnetosphere, dominated by sulfur and
oxygen ions from Io’s volcanism, exhibits complex radial structures and
significant temporal variability. This study analyzes ground-based
optical emission data from the Dual Imaging Spectrograph on the 3.5m
telescope at Apache Point Observatory (APO). By combining 30 nights of
observations from 2013 to 2018 and co-adding dawn and dusk profiles, we
reduce variability and determine steady-state radial conditions. 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 all 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 towards 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.