Observations of molecular oxygen ions in Saturn's inner magnetosphere

We present an analysis of molecular oxygen ion (O 2 + ) abundance in Saturn's inner magnetosphere based on observations made with the Cassini Plasma Spectrometer (CAPS). Using data summed over 23 orbits, we resolve and isolate O 2 + counts from the tail of the water group ion mass distribution (W + [O + , OH + , H 2 O + , H 3 O + ]) and from background sources. O 2 + was initially estimated at ∼1–2% of the total ion population in the inner magnetosphere based on CAPS data from the Saturn orbital insertion pass. Through refined analysis techniques, we have found O 2 + to account for just above 0.3% of all W + ions at L = 4.5, with only minor fluctuations in relative density out to L = 7.5 − 8.0. Beyond L = 7.5, the relative O 2 + /W + abundance exhibits a statistically significant increase through L = 10.5, which may indicate a neutral O 2 source in the vicinity of Rhea.