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

• Published in: Journal of geophysical research. Space physics 2013-06, Vol.118 (6), p.3446-3463
• Main Authors: Christon, S. P., Hamilton, D. C., DiFabio, R. D., Mitchell, D. G., Krimigis, S. M., Jontof-Hutter, D. S.
• Format: Article
• Language: English
• Subjects: dust; Equinoxes; ion; Ionosphere; Ions; molecule; Photolysis; Saturn; season; Seasonal variations; Ultraviolet radiation
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1002/jgra.50383

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. 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.