Modeling the effect of atmospheric gravity waves on Saturn’s ionosphere

Cassini’s radio occultations by Saturn reveal a highly variable ionosphere with a complex vertical structure often dominated by several sharp layers of electrons. The cause of these layers has not yet been satisfactorily explained. This paper demonstrates that the observed system of layers in Saturn’s lower ionosphere can be explained by the presence of one or more propagating gravity waves. We use a two-dimensional, non-linear, time-dependent model of the interaction of atmospheric gravity waves with ionospheric ions to model the observed periodic structures in two of Cassini’s electron density profiles (S08 entry and S68 entry). A single gravity wave is used to reproduce the magnitude, the location, and the shape of the observed peaks in the region dominated by H+ ions. We also use an analytical model to study small-amplitude variations in the S56 exit electron density profile. We identify three individual wave modes and achieve a good fit to the data. Both models are used to derive the properties (horizontal and vertical wavelengths, period, amplitude, and direction of propagation) of the forcing waves present at the time of the occultations.