J-E Wahlund, M W Morooka, L Z Hadid, A M Persoon, W M Farrell, D A Gurnett, G Hospodarsky, W S Kurth, S-Y Ye, D J Andrews, N J T Edberg, A I Eriksson, E Vigren

The ionized upper layer of Saturn's atmosphere, its ionosphere, provides a closure of currents mediated by the magnetic field to other electrically charged regions (e.g., rings) and hosts ion - molecule chemistry. In 2017, the Cassini spacecraft passed inside the planet's rings, allowing in situ measurements of the ionosphere. The Radio and Plasma Wave Science (RPWS) instrument detected a cold, dense and dynamic ionosphere at Saturn that interacts with the rings. Plasma densities reached up to 1000 cm-3 and electron temperatures were below 1160 K near closest approach. The density varied between orbits by up to 2 orders of magnitude. Saturn's A- and B-rings cast a shadow on the planet that reduced ionization in the upper atmosphere, causing a North-South asymmetry.