Using the effective recombination coefficient to constrain the positive ion composition in Saturn's ionosphere

We combine RPWS/LP and INMS data from Cassini's Grand Finale orbits into Saturn's lower ionosphere to calculate the effective recombination coefficient α₃₀₀ at a reference electron temperature of 300 K. Assuming photochemical equilibrium at altitudes below 2500 km and using an established method to determine the electron production rate, we derive upper limits for α₃₀₀ of ∼ 2.5∗10^-7 cm³ s^-1, which suggest that Saturn's ionospheric positive ions are dominated by species with low recombination rate coefficients.
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. The results do not give constraints on the nature of the negative ions.