Saturn in Lyman α: A comparison of Cassini and Voyager observations

We analyze Saturn Lyman α system scans observed by the Cassini/UVIS instrument probing non-auroral airglow emissions from Saturn's disk, the ring atmosphere and the Saturn hydrogen torus. We constrain variations in disk brightness with latitude and local time, relating them to models of Saturn's upper atmosphere. We compare these results with Voyager observations of similar viewing geometry. The Cassini disk brightness is almost four time lower than the Voyager brightness. This difference cannot be explained by changes in the solar Lyman α flux or atmospheric structure. The change in solar flux is modest and atmospheric structure retrieved from occultation data is relatively stable over time. The disk brightness observed by Cassini can be explained by resonant scattering of solar Lyman α radiation and the best-fit hydrogen abundance in the thermosphere is consistent with atmospheric structure retrieved from the occultation data as well as photochemical model predictions. Our results support the recently proposed recalibration of the Voyager/UVS instruments that would bring the Voyager disk brightness in line with our model predictions and provide a consistent interpretation of Saturn Lyman α emissions over time. We also explore the extended emission in the Saturn system, centered near the ring plane in edge-on viewing. The extended emission can be explained by a distribution of atomic hydrogen that is symmetric about the ring plane. We do not find evidence for a plume of hot hydrogen atoms escaping from Saturn's sunlit atmosphere that was proposed previously. Instead, the structure earlier interpreted as a plume is most simply explained by solar illumination from below the ring plane during southern summer combined with the edge-on viewing geometry.