Saturn's Dusty Ionosphere

Abstract

Measurements of electrons and ions in Saturn's ionosphere down to 1,500-km altitudes as well as the ring crossing region above the ionosphere obtained by the Langmuir probe onboard the Cassini spacecraft are presented. Five nearly identical deep ionosphere flybys during the Grand Finale orbits and the Final plunge orbit revealed a rapid increase in the plasma densities and discrepancies between the electrons and ions densities (N e and N i ) near the closest approach. The small N e /N i ratio indicates the presence of a dusty plasma, a plasma which charge carrier is dominated by negatively charged heavy particles. Comparison of the Langmuir probe obtained density with the light ion density obtained by the Ion and Neutral Mass Spectrometer confirmed the presence of heavy ions. An unexpected positive floating potential of the probe was also observed when N e /N i ≪ 1. This suggests that Saturn's ionosphere near the density peak is in a dusty plasma state consisting of negatively and positively charged heavy cluster ions. The electron temperature (T e ) characteristics in the ionosphere are also investigated and unexpectedly high electron temperature value, up to 5000 K, has been observed below 2,500-km altitude in a region where electron-neutral collisions should be prominent. A well-defined relationship between T e and N e /N i ratio was found, implying that the electron heating at low altitudes is related to the dusty plasma state of the ionosphere.Plain Language Summary Cassini Langmuir probe measurements revealed ion densities in excess of the electron densities, indicative of a dusty plasma, in Saturn's ionosphere below 2,500-km altitude. Comparison of the Langmuir probe measurements with those of the Ion and Neutral Mass Spectrometer, sensitive to only lighter ions during this period, showed that heavy ions dominate in this region. Positive spacecraft potentials were also found, suggesting that Saturn's ionosphere contains dusty plasma of negatively and positively charged heavy ions.

Saturn's Ionosphere: Electron Density Altitude Profiles and D‐Ring Interaction From The Cassini Grand Finale

“…Assuming the dominance of H + and T i ≈ T e ≈ 1, 160 K (Wahlund et al, 2017), we obtain H theo ≈ 1, 917 km. This is significantly larger than the values of 840 and 260 km derived in regions D and C, respectively, possibly related to the thermal equilibrium assumption we have made (T i ≈ T e ) and/or hinting toward a significant component of molecular ions and a mean positive ion mass, in region D closer to 2-3 amu (mainly H + and H + 3 ) and in region C ≳ 5-10 amu (Morooka et al, 2019;Waite et al, 2018). In Figure 4b, we present the FP orbit, which covers almost the same altitude range from at least 5,500 km down to 1,300 km.…”

“…Layer C, the region below 2,200 km, represents a chemical equilibrium region, dominated by chemical processes. Morooka et al (2019) observe the dominance of negatively and positively charged heavy cluster ions in this altitude range, and Cravens et al (2019) interpret it as a nondiffusive layer of heavy molecular ions. Moreover, recently the Ion-Neutral Mass Spectrometer observations implied the presence of organic ions at those low altitudes , and the densities of positive dust grains measured by the Charge Energy Mass Spectrometer, designed to measure only the positive dust, are shown to be directly proportional to the heavy positive ion population (Mitchell et al, 2018) below 1,700 km.…”

“…The estimated scale heights in this C layer (260 km and 200 km, Figures 4a and 4b, respectively) probably represent the true neutral scale heights. Noting that the profiles in layer D are dominated by H + , and because of the gravity, the density, the mass, and temperature gradients (Morooka et al, 2019), an ambipolar electric field will generate in the plasma within regions D and C. As a consequence, this would result in the h, n e variability at this boundary region. This transition from chemical to diffusion dominance is similar to the sub-F 2 region in the terrestrial ionosphere (Schunk & Nagy, 2009).…”

Magnetic Field Effect on Antenna Signals Induced by Dust Particle Impacts

“…The low and positive potential was possibly the consequence of the presence of negative ions or secondary electron emission from the SC induced by ions or fast neutrals (Morooka et al, 2019;Wahlund et al, 2017). The low and positive potential was possibly the consequence of the presence of negative ions or secondary electron emission from the SC induced by ions or fast neutrals (Morooka et al, 2019;Wahlund et al, 2017).…”

The Ion Composition of Saturn's Equatorial Ionosphere as Observed by Cassini

“…The total ion density (heavy plus light species) must equal the electron density, N e , plus the densities of any negative ion species or grains (see Morooka et al, 2019) in order to enforce quasi-neutrality. The total ion density (heavy plus light species) must equal the electron density, N e , plus the densities of any negative ion species or grains (see Morooka et al, 2019) in order to enforce quasi-neutrality.…”