A Multi-Wavelength Study of Io's Atomic Oxygen and Sulfur Emission

Electrons impacting Io's atmosphere produce line emission via direct excitation of atoms, and via the dissociative excitation of Io's molecular atmosphere. We report the first observations of O I multiplets at 7774A & 8446A, as well as an S I triplet near 9225A, seen in Jupiter's shadow. Past laboratory studies have predicted that these lines appear as prompt emissions from dissociative excitation of SO₂. Such features offer a new tracer to understand the breakdown of Io's bulk atmosphere by superthermal electrons in the plasma torus. They also offer a metric for the importance of the dissociative excitation contribution to the FUV emissions, since they cascade to produce the intense O I 1356A, 1304A and S I 1900A multiplets, respectively. However, further work is needed to fully disentangle the relative roles of electron impact excitation versus dissociative excitation and/or recombination of molecular ions. Synthetic spectra show that direct excitation by thermal electrons with densities and energies characteristic of the torus' ribbon can reasonably fit the FUV sulfur line ratios observed by HST/COS. Extending such models to previously unexplored wavelengths predicts that the forbidden [S I] 25.245μm line should easily stand out above Io's thermal continuum. Non-detection in our preliminary analysis of SOFIA/EXES data consequently indicates either collisional quenching, or a surprising absence of cold electrons within a few hundred km altitudes.