Detection of New Auroral Emissions at Io and Implications for Its Interaction with the Plasma Torus

We observed Io's optical aurora in eclipse on six nights between 2022 and 2024 using Keck I/HIRES. Spectra revealed 13 new auroral emissions not identified previously, tripling the total number of optical emissions lines detected at Io. These included the O I lines at 777.4 and 844.6 nm, the Na I lines at 818.3 and 819.5 nm, the [S I] lines at 458.9 and 772.5 nm, the S I triplet at 922.3 nm, the [O II] lines at 732.0 and 733.0 nm and the [S II] lines at 406.9, 407.6, 671.6 and 673.1 nm. We leveraged these new detections by comparing with imaging data from the 2001 Cassini flyby to better understand the distribution of atmospheric species and their contribution to the observed auroral brightnesses. Our auroral emission model showed that the observed 557.7, 777.4 and 844.6 nm oxygen emission line brightnesses could be explained by excitation by electron impact of canonical 5 eV torus electrons on an atmosphere composed of O, SO2 and an isoelectronic proxy for SO. The SO2 emission did not decrease immediately after eclipse ingress, suggesting the emitting column may be restricted to higher altitudes. The derived O/SO2 mixing ratio was typically about 10%, but it also exhibited order-of-magnitude variance during some observations. Io's 630.0 nm [O I] brightness did not strongly vary with plasma sheet distance, suggesting electron flux at Io varies substantially beyond model predictions.