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

Abstract

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.

Auroral Processes at the Giant Planets: Energy Deposition, Emission Mechanisms, Morphology and Spectra

“…With the decrease in solar illumination with latitude, the peak electron density and total electron content (TEC) are expected to decrease with increasing latitude. and Moore et al (2010) showed that the reverse trend is observed, as illustrated in Figure 7. The decrease in TEC from mid latitudes towards the equator is likely due to the inflow of water from the rings and icy moons (Connerney and Waite 1984;Moore et al 2006Moore et al , 2010.…”

“…Estimations have been proposed by Moses and Bass (2000), and more recent updates have been presented by Moore et al (2010 in order to best match the Cassini/RSS observations. k * 1 is expected to increase in the auroral regions (e.g.…”

“…Kim and Fox 1991;Moses and Bass 2000). Ionospheric models have been developed for the outer planets, as reviewed by with more recent models proposed for Jupiter (Achilleos et al 1998;Perry et al 1999;Grodent et al 2001;Millward et al 2002;Tao et al 2010;Barrow and Matcheva 2011) and Saturn (Moses and Bass 2000;Moore et al 2004Moore et al , 2006Moore et al , 2008Moore et al , 2010Galand et al 2009Galand et al , 2011Barrow and Matcheva 2013).…”

Comparative ionospheres: Terrestrial and giant planets

“…Using the one-dimensional version of the Saturn-ThermosphereIonosphere-Model (STIM), Moore et al (2006a) demonstrated that a water flux still provided the best match to early Cassini radio occultation observations. Further modeling of the effects of variable influxes of water on Saturn's ionosphere was presented in Moore and Mendillo (2007) and Moore et al (2010) .…”

“…These patterns for magnitudes of peak electron density and TEC are dominated by the 59 values from Cassini and summarize a trend noted earlier (Nagy et al, 2006(Nagy et al, , 2009Kliore et al, 2009)-namely, a noticeable departure from the anticipated morphology that sub-solar conditions would exhibit the most robust ionosphere. The proposed resolution of this low-latitude anomaly came from modeling: the introduction of enhanced chemical loss rates due to an influx of water from the rings and icy moons (Moses and Bass, 20 0 0; Moore et al, 2010 ). Here we do not concentrate on the issue of NmS2 magnitudes, but rather on the variability about average behavior, a topic explored in neither statistical nor modelling studies to date.…”