Saturn suprathermal O_2 ~+ and mass-28~+ molecular ions: Long-term seasonal and solar variation

摘要

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_2H_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_2O~+, and H_3O~+). 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. Key Points Energized Saturn ring ionosphere O2+ ions show seasonal and solar variation. The long-term O2+/W+ variation is not as anticipated after equinox. Energized local-origin Mass-28 ions initially show likely seasonal variation.