Open AccessConstraining the Positive Ion Composition in Saturn’s Lower Ionosphere with the Effective Recombination Coefficient
Author(s) -
Joshua Dreyer,
Erik Vigren,
Michiko Morooka,
JanErik Wahlund,
Stephan Buchert,
Folke Johansson,
J. H. Waite
Publication year - 2021
Publication title -
the planetary science journal
Language(s) - English
Resource type - Journals
ISSN - 2632-3338
DOI - 10.3847/psj/abd6e9
Subject(s) - saturn , ionosphere , ion , electron temperature , electron , recombination , atomic physics , plasma , physics , langmuir probe , chemistry , astrophysics , planet , geophysics , plasma diagnostics , biochemistry , quantum mechanics , gene
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 α 300 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 α 300 to upper limits. Assuming photochemical equilibrium and using an established method to calculate the electron production rate, we derive upper limits for α 300 of ≲ 3 × 10 −7 cm 3 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 cm 3 s −1 at an electron temperature of 300 K.