Latitudinal variations in Saturn's ionosphere: Cassini measurements and model comparisons

摘要

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 (N_(MAX)) 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 N_(MAX) and TEC at low latitudes and underestimate those parameters at middle and high latitudes. STIM is able to reproduce the RSS values for N_(MAX) 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, Σ_P. We find Σ_P 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.