Electron to Light Ion Density Ratios During Cassini’s Grand Finale: Addressing Open Questions About Saturn’s Low-latitude Ionosphere

During Cassini's Grand Finale in 2017, the number densities of electron and light ions in Saturn's low-latitude ionosphere were measured in situ. This region is strongly influenced by the influx of ring material from Saturn's D ring.

The electron data from the onboard Langmuir probe (LP) and light ion densities from the Ion and Neutral Mass Spectrometer (INMS) correlate very well even on short timescales after correcting the INMS timestamps, whereas prior the correlation was limited to broader scales. We analyze the electron-to-ion ratios for the proximal orbits and identify three distinct regions in Saturn's ionosphere:

1) For altitudes above ∼2500 km and latitudes between -20° and 20°, the electron-to-light-ion ratios for the four analysed orbits are generally <1. This essentially suggests that either the INMS light ion densities are overestimated or the measured electron densities are underestimated. Our ongoing analysis may also provide additional constraints on the electron temperature profile by comparing changes between LP fixed-bias, RPWS wave, and INMS ion data.

2) At altitudes below ∼2500 km, we can further utilise the electron-to-light-ion ratios to estimate the abundance of heavier ions around closest approaches for orbits 288 and 292. Our results broadly match those of recent models.

3) At latitudes poleward of ±20° (altitudes >8000 km) the ratios increase rapidly. This may indicate the presence of heavier ions, such as O⁺ and water group species, spiralling in from the C ring.