Saturn suprathermal O 2 + and mass‐28 + molecular ions: Long‐term seasonal and solar variation

Suprathermal singly charged molecular ions, O 2 + (at ~32 Da/e) and the Mass‐28 ion group 28 M + (ions at ~28 Da/e, with possible contributions from C 2 H 5 + , HCNH + , N 2 + , 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 + , H 2 O + , and H 3 O + ). O 2 + /W + , initially ~0.05, declined steadily until equinox in mid‐2009 by a factor of ~6, and 28 M + /W + , initially ~0.007, declined similarly until early‐2007 by a factor of ~2. The O 2 + /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 O 2 and O 2 + . The water ice‐dominated main rings and Enceladus plume depositions thereon are the two most likely O 2 + sources. Enceladus' dynamic plumes, though, have no known long‐term dependence. After declining, O 2 + /W + and 28 M + /W + levels remained low until late‐2011 when O 2 + /W + increased, but 28 M + /W + did not. The O 2 + /W + increase was steady and became statistically significant by mid‐2012, indicating a clear increase after a decline, that is, a possibly delayed O 2 + “seasonal” recovery. Ring insolation is driven by solar UV flux which itself varies with the sun's 11 year activity cycle. The O 2 + /W + and 28 M + /W + declines are consistent with seasonal ring insolation. No O 2 + /W + response to the late‐2008 solar‐cycle UV minimum and recovery is evident. However, the O 2 + /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 O 2 + observations can be understood.