The Pioneer and Voyager spacecraft made close-up measurements of Saturn’s ionosphere and upper atmosphere in the 1970s and 1980s that suggested a chemical interaction between the rings and atmosphere. Exploring this interaction provides information on ring composition and the influence on Saturn’s atmosphere from infalling material. The Cassini Ion Neutral Mass Spectrometer sampled in situ the region between the D ring and Saturn during the spacecraft’s Grand Finale phase. We used these measurements to characterize the atmospheric structure and material influx from the rings. The atmospheric He/H2 ratio is 10 to 16%. Volatile compounds from the rings (methane; carbon monoxide and/or molecular nitrogen), as well as larger organic-bearing grains, are flowing inward at a rate of 4800 to 45,000 kilograms per second.

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We present a study of latitudinal variations in Saturn's ionosphere using Cassini Radio Science Subsystem (RSS) measurements and Saturn‐Thermosphere‐Ionosphere‐Model (STIM) simulations. On the basis of Cassini RSS observations, the peak electron density (NMAX) and the total electron content (TEC) both exhibit a clear increase with latitude, with a minimum at Saturn's equator. When compared with these RSS trends, current model simulations overestimate NMAX and TEC at low latitudes and underestimate those parameters at middle and high latitudes. STIM is able to reproduce the RSS values for NMAX and TEC at low latitude when an additional low‐latitude loss process, such as a water influx, is introduced near Saturn's equator. The lack of auroral precipitation processes in the model likely explains some model/data discrepancies at high latitude; however, most of the high‐latitude RSS data are from latitudes outside of Saturn's typical main auroral oval. Using Cassini RSS electron density altitude profiles combined with ion density fractions and neutral background parameters calculated in STIM, we also present estimates of the latitudinal variations of Saturn's Pedersen conductance, ΣP. We find ΣP to be driven by ion densities in Saturn's lower ionosphere and to exhibit a latitudinal trend with a peak at mid‐latitude. Model calculations are able to reproduce low‐latitude conductances when an additional loss process is introduced, as before, but consistently underestimate most of the mid‐ and high‐latitude conductances derived from Cassini observations, perhaps indicating a missing ionization source within the model.

[1] In the high-latitude regions of Saturn, the ionosphere is strongly coupled to the magnetosphere through the exchange of energy. The influx of energetic particles from Saturn's magnetosphere enhances the ionospheric densities and temperatures, affects the electrodynamical properties of the ionosphere, and contributes to the heating of the thermosphere. It is therefore critical to accurately model the energy deposition of these magnetospheric particles in the upper atmosphere in order to evaluate key ionospheric quantities of the coupled magnetosphere-ionosphere system. We present comprehensive results of ionospheric calculations in the auroral regions of Saturn using our Saturn Thermosphere-Ionosphere Model (STIM). We focus on solar minimum conditions during equinox. The atmospheric conditions are derived from the STIM 3-D General Circulation Model. The ionospheric component is self-consistently coupled to the solar and auroral energy deposition component. The precipitating electrons are assumed to have a Maxwellian distribution in energy with a mean energy E m and an energy flux Q 0 . In the presence of hard electron precipitation (1 < E m ≤ 20 keV) with Q 0 > 0.04 mW m −2 , the ionospheric conductances are found to be proportional to the square root of the energy flux, but the response of the ionosphere is not instantaneous and a time delay needs to be applied to Q 0 when estimating the conductances. In the presence of soft electron precipitation (E m < 500 eV) with Q 0 ≤ 0.2 mW m −2 , the ionospheric conductances at noon are found to be primarily driven by the Sun. However, soft auroral electrons are efficient at increasing the ionospheric total electron content and at heating the thermal electron population.