A Survey of Visible S+ Emission in Io's Plasma Torus During the Hisaki Epoch

To complement Hisaki's ultraviolet monitoring of the Io plasma torus, a ground-based campaign assembled a comprehensive data set at visible wavelengths with the ARC 3.5-m telescope at Apache Point Observatory. This work concentrates on the bright S⁺ emissions in these data. Recurrent traits in the intensity and location are discerned as a function of Jovian longitude (λ_III). The longitudinal brightness structure differs from past data sets in the visible but is broadly consistent with that of Io's auroral footprint and concurrent Hisaki extreme ultraviolet measurements. Positions of the ribbon feature with λ_III confirm that it exhibits a radial wobbling motion that is lesser than the centrifugal limit along a given L-shell. We also find that the radial separation between the cold torus and the ribbon is modulated as Jupiter rotates. The torus is displaced 0.13 R_J dawnward, on average, by an electric field with mean strength 3.8 mV/m, consistent with that inferred by analysis of Hisaki's dawn-dusk brightness asymmetry. The lesser visible brightness asymmetry is uncorrelated to this field strength, however. S⁺ emissions are enhanced downstream of Io, principally at Jovian dawn. Io interacts with the densest region at specific Jovian longitudes and local times; near λ_III 130° and dawn the ribbon is farthest from Jupiter but still passes radially interior to the satellite. The electron density sweeping past Io and/or its radial proximity to the passing ribbon may also influence brightness, but S⁺ emissions are not governed by a single predominant driver.