The enigmatic abundance of atomic hydrogen in Saturn's upper atmosphere
A planet's Lyman-α (Lyα) emission is sensitive to its thermospheric structure. Here, we report joint Hubble Space Telescope (HST) and Cassini cross-calibration observations of the Saturn Lyα emission made two weeks before the Cassini grand finale. To investigate the long-term Saturn Lyα airglow observed by different ultraviolet instruments, we cross-correlate their calibration, finding that while the official Cassini/UVIS sensitivity should be lowered by ~75%, the Voyager 1/UVS sensitivities should be enhanced by ~20% at the Lyα channels. This comparison also allowed us to discover a permanent feature of the Saturn disk Lyα brightness that appears at all longitudes as a brightness excess (Lyα bulge) of ~30% (~12σ) extending over the latitude range ~5-35N compared to the regions at equator and ~60N. This feature is confirmed by three distinct instruments between 1980 & 2017 in the Saturn north hemisphere. To analyze the Lyα observations, we use a radiation transfer (RT) model of resonant scattering of solar and interplanetary Lyα photons, and a latitude-dependent photochemistry model of the upper atmosphere constrained by occultation and remote-sensing observations. For each latitude, we show that the Lyα observations are sensitive to the temperature profile in the upper stratosphere and lower thermosphere, thus providing useful information in a region of the atmosphere that is difficult to probe by other means. In the Saturn Lyα bulge region, at latitudes between ~5 to ~35°, the observed brightening and line broadening support seasonal effects, variation of the temperature vertical profile, and potential superthermal atoms that require confirmation.