Observations of molecular oxygen ions in Saturn's inner magnetosphere

Abstract

We present an analysis of molecular oxygen ion (O₂ ⁺) 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 O₂ ⁺ counts from the tail of the water group ion mass distribution (W⁺ [O⁺, OH⁺, H₂O⁺, H₃O⁺]) and from background sources. O₂ ⁺ 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 O₂ ⁺ 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 O₂ ⁺/W⁺ abundance exhibits a statistically significant increase through L = 10.5, which may indicate a neutral O₂ source in the vicinity of Rhea.