Supporting Evidence for a Galactic Lyman-alpha Background from Cassini UVIS Data

Cassini UVIS interplanetary hydrogen Lyman-alpha measurements from 2003-2004, obtained from a heliospheric downwind to sidewind location on approach to Saturn and during the first Saturn orbit, are presented and fit by a heliospheric hot density model with solar illumination (Pryor et al. 2013, 2020). Adding the recently proposed 43 +/- 3 Rayleigh (R) isotropic galactic hydrogen Lyman-alpha signal derived from New Horizons data (Gladstone et al. 2021) improves our model's ability to fit the observed Cassini ``27-day" signal modulations from both upwind and downwind directions. Our modeling of the UVIS data favors a galactic Lyman-alpha background of ~40-100 R over a model with no significant galactic background (Pryor et al., 2022, The Astronomical Journal, Volume 164, Issue 2, id.46, 6 pp., DOI: 10.3847/1538-3881/ac7570 ).