Long-Term And/Or Seasonal Variation of Suprathermal O2+ and N2+ at Saturn

The suprathermal minor heavy ion groups O₂⁺ and ²⁸M⁺ (mixed N₂⁺ and/or CO⁺) at~32 and ~28 amu/e, respectively, exhibit long-term spatial-temporal variations in Saturn's magnetosphere at ~3-20 Rs from 2004 to 2011. These ~83-167 keV/e ions are the second most abundant ion groups heavier than the water group W⁺ (O⁺, OH⁺, H₂O⁺, and H3O⁺) measured by the Cassini CHEMS ion spectrometer. O₂⁺ declines by order of ~6-7 relative to W⁺ from mid-2004 until equinox in mid-2009 and then remains relatively constant. ²⁸M⁺, initially ~5 times less abundant than O₂⁺, declines by order of ~2 relative to W⁺ until early-2007 and then remains at lower values with significant variation. After late-2009 ²⁸M⁺/O₂⁺ is ~0.5. The declining O₂⁺ and ²⁸M⁺ flux occurs as ring plane illumination decreases approaching Saturn's equinox. The main ring atmosphere/ ionosphere is the most likely primary source for the O₂⁺. Titan, Enceladus, Rhea, and the E-ring are candidate sources for the ²⁸M⁺. Neither O₂⁺ nor ²⁸M⁺ has begun recovery by mid-2011, arguing for reduced levels this Saturn season. Transition from higher/declining to lower/persistent levels suggests multiple competing components for both ion groups, that is, one component, probably ring-insolation related, is initially dominant and decreases gradually to be masked later by a lower-level component. The relation of the ions' flux generation and modulation by Saturn's moons and rings is addressed.