Constraining the Positive Ion Composition in Saturn's Lower Ionosphere with the Effective Recombination Coefficient

Abstract

The present study combines Radio and Plasma Wave Science/Langmuir Probe and Ion and Neutral Mass Spectrometer data from Cassini's last four orbits into Saturn's lower ionosphere to constrain the effective recombination coefficient α₃₀₀ from measured number densities and electron temperatures at a reference electron temperature of 300 K. Previous studies have shown an influx of ring material causes a state of electron depletion due to grain charging, which will subsequently affect the ionospheric chemistry. The requirement to take grain charging into account limits the derivation of α₃₀₀ to upper limits. Assuming photochemical equilibrium and using an established method to calculate the electron production rate, we derive upper limits for α₃₀₀ of ≲ 3 × 10^-7 cm³ s^-1 for altitudes below 2000 km. This suggests that Saturn's ionospheric positive ions are dominated by species with low recombination rate coefficients like HCO⁺. An ionosphere dominated by water group ions or complex hydrocarbons, as previously suggested, is incompatible with this result, as these species have recombination rate coefficients > 5 × 10^-7 cm³ s^-1 at an electron temperature of 300 K.