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 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.

本研究联合广播和等离子体波科学/朗缪尔探针和离子和中性质谱数据从卡西尼的最后四个轨道到土星低电离层来约束有效重组系数α ₃₀₀从测量数密度和电子温度在300的基准电子温度K. 先前的研究表明，由于颗粒充电，环材料的流入会导致电子耗尽状态，这将随后影响电离层化学。考虑谷物装料的要求限制了α ₃₀₀的推导上限。假设光化学平衡并使用既定方法计算电子产生速率，我们推导出α 的上限 ₃₀₀ of ≲ 3 10 ^-7 cm ³ s ^-1对于海拔低于 2000 公里。这表明土星的电离层正离子主要由具有低复合率系数的物种主导，如 HCO ⁺。如前所述，由水基离子或复杂碳氢化合物主导的电离层与该结果不相容，因为这些物种在 300 K 的电子温度下具有> 5 10 ^-7 cm ³ s ^{-1 的}复合率系数。