Supporting Evidence for a Galactic Lyα Background from Cassini UVIS Data

Cassini UVIS interplanetary hydrogen Ly α measurements from 2003 to 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. Adding the recently proposed 43 ± 3 Rayleigh ( R ) isotropic galactic hydrogen Ly α signal derived from New Horizons data 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 Ly α background of ∼40–100 R over a model with no significant galactic background.