Saturn suprathermal O2+ and mass-28+ molecular ions: Long-term seasonal and solar variation



[1] Suprathermal singly charged molecular ions, O2+ (at ~32 Da/e) and the Mass-28 ion group 28M+ (ions at ~28 Da/e, with possible contributions from C2H5+, HCNH+, N2+, and/or CO+), are present throughout Saturn's ~4–20 Rs (1 Saturn radius, Rs = 60,268 km) near-equatorial magnetosphere from mid-2004 until mid-2012. These ~83–167 keV/e heavy ions measured by Cassini's CHarge-Energy-Mass Spectrometer have long-term temporal profiles that differ from each other and differ relative to the dominant water group ions, W+ (O+, OH+, H2O+, and H3O+). O2+/W+, initially ~0.05, declined steadily until equinox in mid-2009 by a factor of ~6, and 28M+/W+, initially ~0.007, declined similarly until early-2007 by a factor of ~2. The O2+/W+ decline is consistent with Cassini's in situ ring-ionosphere thermal ion measurements, and with proposed and modeled seasonal photolysis of Saturn's rings for thermal O2 and O2+. The water ice-dominated main rings and Enceladus plume depositions thereon are the two most likely O2+ sources. Enceladus' dynamic plumes, though, have no known long-term dependence. After declining, O2+/W+ and 28M+/W+ levels remained low until late-2011 when O2+/W+ increased, but 28M+/W+ did not. The O2+/W+ increase was steady and became statistically significant by mid-2012, indicating a clear increase after a decline, that is, a possibly delayed O2+ “seasonal” recovery. Ring insolation is driven by solar UV flux which itself varies with the sun's 11 year activity cycle. The O2+/W+ and 28M+/W+ declines are consistent with seasonal ring insolation. No O2+/W+ response to the late-2008 solar-cycle UV minimum and recovery is evident. However, the O2+/W+ recovery from the postequinox baseline levels in late-2011 coincided with a strong solar UV enhancement. We suggest a scenario/framework in which the O2+ observations can be understood.