Implications from secondary emission from neutral impact on Cassini plasma and dust measurements

Abstract

We investigate the role of secondary electron and ion emission from impact of gas molecules on the Cassini Langmuir probe (RPWS-LP or LP) measurements in the ionosphere of Saturn. We add a model of the emission currents, based on laboratory measurements and data from comet 1P/Halley, to the equations used to derive plasma parameters from LP bias voltage sweeps. Reanalysing several hundred sweeps from the Cassini Grand Finale orbits, we find reasonable explanations for three open conundrums from previous LP studies of the Saturn ionosphere. We find an explanation for the observed positive charging of the Cassini spacecraft, the possibly overestimated ionospheric electron temperatures, and the excess ion current reported. For the sweeps analysed in detail, we do not find (indirect or direct) evidence of dust having a significant charge-carrying role in Saturn’s ionosphere. We also produce an estimate of H2O number density from the last six revolutions of Cassini through Saturn’s ionosphere in greater detail than reported by the Ion and Neutral Mass Spectrometer. Our analysis reveals an ionosphere that is highly structured in latitude across all six final revolutions, with mixing ratios varying with two orders of magnitude in latitude and one order of magnitude between revolutions and altitude. The result is generally consistent with an empirical photochemistry model balancing the production of H+ ions with the H+ loss through charge transfer with e.g. H2O, CH4, and CO2, for which water vapour appears as the likeliest dominant source of the signal in terms of yield and concentration.

Saturn’s atmospheric response to the large influx of ring material inferred from Cassini INMS measurements

“…The dominance of HCNH + at the main peak in our Case A model is consistent with the combined INMS and RPWS data analysis that points to a heavy ion being the dominant species at the main peak (Cravens et al, 2019); it also satisfies the combined RPWS/LP and INMS analysis that suggests that the main-peak ion should have a recombination rate coefficient ≲3 × 10 −7 cm 3 s −1 if the ion densities inferred by Morooka et al (2019) are correct (Dreyer et al, 2021) (but see Johansson et al 2022 for an alternate interpretation). However, the H + /H 3 + ratio predicted by the model at 3 × 10 −7 mbar, the deepest point in the trajectory of orbit 292, is much smaller than is observed (Waite et al, 2018;Moore et al, 2018), and the observed ion and electron densities at this pressure level are inconsistent with the INMS-inferred abundance of neutral species that are able to react with H + and maintain photochemical equilibrium (Vigren et al, 2022).…”

“…However, Johansson et al (2022) recently demonstrated that secondary electron emission current can affect the Langmuir Probe bias voltage sweeps, and when such effects are considered, the data show no evidence for dust playing a significant role in charge balance in Saturn's ionosphere. Johansson et al (2022) also suggest that electron temperatures may have been overestimated by earlier analyses (e.g., Morooka et al, 2019). We assume the electron and neutral temperatures are equal in our models (for a justification of this assumption, see Moore et al, 2008).…”

“…The Ion Neutral Mass Spectrometer (INMS) instrument on the Cassini orbiter directly sampled Saturn's atmospheric composition during these last orbital passes (Waite et al, 2018), in a manner similar to the earlier close flybys of Titan and the Enceladus plumes (e.g., Waite et al, 2005Waite et al, , 2006Cravens et al, 2006Cravens et al, , 2009Yelle et al, 2006;Vuitton et al, 2006;Cui et al, 2009;Magee et al, 2009). Grand Finale data from the Radio and Plasma Wave Science/Langmuir Probe (RPWS/LP) instrument, Magnetospheric Imaging Instrument (MIMI), and Cosmic Dust Analyzer (CDA) also provided complementary in situ measurements of electron and ion densities, dust properties, and other key pieces of information about the local atmosphere and its interaction with the rings and magnetosphere (e.g., Mitchell et al, 2018;Wahlund et al, 2018;Hsu et al, 2018;Persoon et al, 2019;Hadid et al, 2019;Morooka et al, 2019;Johansson et al, 2022). The in situ results from these last Saturn orbital passes provided one of the most surprising and puzzling discoveries from the 13-year Cassini mission.…”

“…Some of the consequences of this infalling ring-derived vapor and dust on Saturn's ionosphere have been discussed previously. Briefly, the inflow of neutral molecules from the rings causes heavy molecular ions with relatively short chemical lifetimes to supplant H + and H 3 + as the dominant ions near Saturn's main ionospheric peak at pressures ∼10 −6 -10 −7 mbar (Moore et al, 2018;Waite et al, 2018;Cravens et al, 2019;Dreyer et al, 2021;Chadney et al, 2022;Vigren et al, 2022), and incoming ring dust may influence ionospheric structure and charge balance (Mitchell et al, 2018;Hadid et al, 2019;Persoon et al, 2019;Morooka et al, 2019;Vigren et al, 2022;Johansson et al, 2022). From model-data comparisons and photochemical-equilibrium arguments, Dreyer et al (2021) find that the dominant heavy ion at low latitudes near 1700 km altitude should have an effective electron-recombination rate coefficient of ≲ 3 × 10 −7 at an electron temperature of 300 K, which makes HCO + a prime candidate for the dominant ion at the main peak (see also Moore et al, 2018).…”

“…From model-data comparisons and photochemical-equilibrium arguments, Dreyer et al (2021) find that the dominant heavy ion at low latitudes near 1700 km altitude should have an effective electron-recombination rate coefficient of ≲ 3 × 10 −7 at an electron temperature of 300 K, which makes HCO + a prime candidate for the dominant ion at the main peak (see also Moore et al, 2018). However, other heavy ions not considered in the Moore et al (2018) model might also satisfy this constraint (Dreyer et al, 2021), and even H 3 O + might satisfy existing constraints if the ion densities derived by Morooka et al (2019) have been influenced by secondary electron emission currents (see Johansson et al, 2022).…”