Solar primary and secondary ionization at Saturn

Solar EUV and soft X‐ray photons are a prime source of energy upon Saturn's upper atmosphere. In particular, they represent a significant source of ionization yielding the creation of an ionosphere, a region probed by the Cassini Radio Science Subsystem (RSS) experiment. During the past decade the only ionization process modeled at Saturn under solar illumination has been photoionization. We present the first detailed calculation of the ionization rate by suprathermal electrons (photoelectrons and their secondaries), applied to Saturn using realistic solar flux and neutral atmospheric input. The energy degradation model, describing the absorption of solar photons and the transport of suprathermal electrons, is self‐consistently coupled with an ionospheric model providing the electron and ion densities and temperatures. The coupled model is applied to equinox conditions at 30°N latitude. We assess and compare the photoionization and electron‐impact ionization rates. We discuss their sensitivity with solar flux models as well as solar activity. We find that the secondary production rate affects not only the bottom side of the ionosphere, as previously predicted, but also the main ionospheric peak. The electron density is increased by up to 30% at the peak with a decrease in the peak altitude by several hundreds of kilometers around local noon. Above the homopause, the largest increase in electron density is found after sunrise below the ionospheric peak. Finally, we compare the energy degradation of photoelectrons with auroral electrons and discuss the relevance of this study to the interpretation of observations by the Cassini RSS and Cassini Plasma Science/Electron Spectrometer instruments.