Identifying Shadowing Signatures of C Ring Ringlets and Plateaus in Cassini Data from Saturn’s Ionosphere

Abstract

For orbits 288 and 292 of Cassini’s Grand Finale, clear dips (sharp and narrow decreases) are visible in the H 2 + densities measured by the Ion and Neutral Mass Spectrometer (INMS). In 2017, the southern hemisphere of Saturn was shadowed by its rings and the substructures within. Tracing a path of the solar photons through the ring plane to Cassini’s position, we can identify regions in the ionosphere that were shadowed by the individual ringlets and plateaus (with increased optical depths) of Saturn’s C ring. The calculated shadowed altitudes along Cassini’s trajectory line up well with the dips in the H 2 + data when adjusting the latter based on a detected evolving shift in the INMS timestamps since 2013, illustrating the potential for verification of instrument timings. We can further estimate the mean optical depths of the ringlets/plateaus by comparing the dips to inbound H 2 + densities. Our results agree well with values derived from stellar occultation measurements. No clear dips are visible for orbits 283 and 287, whose periapsides were at higher altitudes. This can be attributed to the much longer chemical lifetime of H 2 + at these higher altitudes, which in turn can be further used to estimate a lower limit for the flow speed along Cassini’s trajectory. The resulting estimate of ∼0.3 km s−1 at an altitude of ∼3400 km is in line with prior suggestions. Finally, the ringlet and plateau shadows are not associated with obvious dips in the electron density, which is expected due to their comparatively long chemical (recombination) lifetime.

Utilizing Helium Ion Chemistry to Derive Mixing Ratios of Heavier Neutral Species in Saturn's Equatorial Ionosphere

“…The INMS data is initially corrected for the time shift identified and described by Dreyer et al. (2022).…”

“…To test our starting hypothesis, we set out to compare the He + densities derived from a simple atmospheric photoionization model, assuming loss only via H 2 , with those measured by the INMS. The INMS data is initially corrected for the time shift identified and described by Dreyer et al (2022).…”

Cassini Radio Occultation Observations of Saturn's Ionosphere: Electron Density Profiles From 2005 to 2013

“…The planet's rings are a source of material that falls into the atmosphere, undergoes chemical reactions with ionospheric species that enhance plasma loss rates, and reduces plasma densities (Liu & Ip, 2014; L. Moore & Mendillo, 2007; L. Moore et al., 2015; Moses & Bass, 2000; O’Donoghue et al., 2013, 2019; Persoon et al., 2019; Wahlund et al., 2018; Waite et al., 2018). Ring shadowing, which reduces the flux of ionizing solar irradiance incident upon the atmosphere, and ring‐planet electrodynamic connections along magnetic field lines, which can enhance ion outflow, are also significant factors (Cravens, Morooka, et al., 2019; Dreyer et al., 2022; Farrell et al., 2018; Hadid et al., 2018; Mendillo et al., 2005; L. E. Moore et al., 2004). The planet's magnetic field focuses the precipitation of energetic magnetospheric charged particles into the high‐latitude atmosphere, enhancing plasma production rates and plasma densities there (Cowley et al., 2004; Galand et al., 2011; Müller‐Wodarg et al., 2012; O’Donoghue et al., 2016; Ray et al., 2012; Sakai & Watanabe, 2016).…”

“…Stallard et al, , 2008; T. S. T. S. Stallard, Masters, et al, 2012), in situ measurements by Cassini during its proximal orbits (Grand Finale) (Agiwal et al, 2021;Cravens, Morooka, et al, 2019;Dreyer et al, 2022;Farrell et al, 2018;Hadid et al, 2019;Johansson et al, 2022;Morooka et al, 2019;Persoon et al, 2019;Shebanits et al, 2020;Sulaiman et al, 2017;Wahlund et al, 2018;Waite et al, 2018), and radio occultations Kliore et al, 2009Kliore et al, , 2014Lindal et al, 1985;Nagy et al, 2006).…”

“…Observations have shown that Saturn's ionosphere is extremely variable, which presents challenges to theoretical understanding of ionospheric processes and properties. Available observations of Saturn's ionosphere include Saturn electrostatic discharges (SEDs) (Fischer et al., 2007, 2008; Fischer, Gurnett, et al., 2011; Fischer, Kurth, et al., 2011; L. Moore et al., 2012), ground‐based infrared measurements (Chowdhury et al., 2019, 2022; Melin et al., 2016; O’Donoghue et al., 2013, 2014, 2016, 2017, 2019; T. Stallard et al., 1999, 2008; T. S. Stallard, Melin, et al., 2012; T. S. Stallard, Masters, et al., 2012), in situ measurements by Cassini during its proximal orbits (Grand Finale) (Agiwal et al., 2021; Cravens, Moore, et al., 2019; Cravens, Morooka, et al., 2019; Dreyer et al., 2022; Farrell et al., 2018; Hadid et al., 2019; Johansson et al., 2022; L. Moore et al., 2018; Morooka et al., 2019; Persoon et al., 2019; Shebanits et al., 2020; Sulaiman et al., 2017; Wahlund et al., 2018; Waite et al., 2018), and radio occultations (Kliore, Lindal, et al., 1980; Kliore, Patel, et al., 1980; Kliore et al., 2009, 2014; Lindal et al., 1985; Nagy et al., 2006).…”