Cassini radio occultations of Saturn's ionosphere: Model comparisons using a constant water flux

Abstract

Recent radio occultations of Saturn's equatorial ionosphere by the Cassini spacecraft provide important insight into this poorly constrained region. Twelve new electron density profiles identify a clear dawn/dusk asymmetry as well as two apparently separate electron density peaks. This study uses a 3D general circulation model along with 1D water diffusion calculations to examine the possibility that a topside flux of neutral water into Saturn's atmosphere may provide a loss mechanism-via charge exchange with protons-that is sufficient to reproduce the ionosphere observed by Cassini. Results indicate that a constant influx of water of (0.5-1.0) × 10⁷ H₂O cm^-2 sec^-1 is adequate for reproducing Cassini measurements, providing a good match to the main electron density peak at dawn and dusk. In addition, these calculations use a reduced rate for the reaction H⁺ + H₂(ν >= 4) -> H₂⁺ + H, significantly diminishing its importance in Saturn's ionospheric photochemistry.