The Plasma Proton Environment Within Saturn's F-G Ring Gap as Observed by the Cassini Plasma Spectrometer Ion Mass Spectrometer During Saturn Orbit Insertion

Abstract

We report on the detection of protons and the potential detection of H₂⁺ between Saturn's F and G rings based on Cassini Plasma Spectrometer (CAPS) Ion Mass Spectrometer (IMS) time-of-flight (TOF) composition measurements acquired during Saturn Orbit Insertion (SOI) outbound pass. The range in dipole L shell is 2.3 < L < 2.8. Initial results based on TOF data were presented in E. C. Sittler et al. (2017). Here we present the latest results of our analysis. During the SOI outbound pass between the F and G rings the CAPS IMS was in a mode of reduced post-acceleration voltage at −6 kV instead of the usual −14.6 kV. This reduced voltage still allows the analysis of protons since 6 keV protons are minimally scattered by the instrument's ultrathin carbon foils when compared to heavier ions O⁺ and O₂⁺. Background noise from penetrating radiation and ghost peaks produced by foil-scattered O⁺ ions within the instrument were considered in our analysis. The analysis allowed an determination of the proton density, temperature and flow velocity, accounting for spacecraft potential by assuming a convected Maxwellian for the proton velocity distribution function. We find average proton density n_P = 3.4 ± 1.2 #/cm³, proton temperature T_P = 1.74 ± 0.12 eV, proton corotation flow speed V_P = 24 ± 1.5 km/s in spacecraft reference frame and spacecraft potential Φ_SC = −0.8 ± 1.5 V. These results are compared with previous theoretical estimates of H⁺ and H₂⁺ ions within Saturn's inner magnetosphere.