Infalling ring material produce layers of charged dust in the ionosphere of Saturn

The Cassini Radio & Plasma Wave Science (RPWS) and Ion and Neutral Mass Spectrometer (INMS) observations of Saturn's ionosphere during the proximal orbits (Rev) 288–293 in the altitude range 1450–2500 km (above the 1-bar pressure level) are modelled with the inclusion of a dust component. Previous reports have revealed that large amounts of nm- to μm-sized dust grains precipitate from the D-ring into the atmosphere of Saturn's equatorial region. We find that the processed charged dust (< 2 nm radius) has a profound effect on the ionospheric structure, enhancing the ion number density well above photochemical equilibrium levels, while the free electrons tend to become attached to the dust population as a result of a low photo-electron detachment rate by EUV light from the Sun at the distance of Saturn. The charged dust layers are influenced by the strong near horizontal magnetic field inhibiting vertical ambipolar diffusion as most charged components are magnetized above about 1800 km. Our dust-ionosphere model calculations reveal that layers of mostly singly negatively charged dust can explain much of the observed ionospheric densities. Modelling uncertainties include sticking coefficients, recombination rates, dust distributions and the presence of negative ions as well as variations thereof along the spacecraft trajectory. Our models generate overall acceptable fits to the observed RPWS electron and ion densities. The observations are therefore compatible with charged grains (and/or cluster ions) being present and playing a dominant role in the Kronian ionosphere structure and chemistry. Similar processes may be active at gas giants with associated ring systems.