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Saturn suprathermal O2+ and mass‐28+ molecular ions: Long‐term seasonal and solar variation
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| Zeitschriftentitel: | Journal of Geophysical Research: Space Physics |
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| Personen und Körperschaften: | , , , , , |
| In: | Journal of Geophysical Research: Space Physics, 118, 2013, 6, S. 3446-3463 |
| Format: | E-Article |
| Sprache: | Englisch |
| veröffentlicht: |
American Geophysical Union (AGU)
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| Schlagwörter: |
| Zusammenfassung: | <jats:p>Suprathermal singly charged molecular ions, O<jats:sub>2</jats:sub><jats:sup>+</jats:sup> (at ~32 Da/e) and the Mass‐28 ion group <jats:sup>28</jats:sup>M<jats:sup>+</jats:sup> (ions at ~28 Da/e, with possible contributions from C<jats:sub>2</jats:sub>H<jats:sub>5</jats:sub><jats:sup>+</jats:sup>, HCNH<jats:sup>+</jats:sup>, N<jats:sub>2</jats:sub><jats:sup>+</jats:sup>, and/or CO<jats:sup>+</jats:sup>), 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<jats:sup>+</jats:sup> (O<jats:sup>+</jats:sup>, OH<jats:sup>+</jats:sup>, H<jats:sub>2</jats:sub>O<jats:sup>+</jats:sup>, and H<jats:sub>3</jats:sub>O<jats:sup>+</jats:sup>). O<jats:sub>2</jats:sub><jats:sup>+</jats:sup>/W<jats:sup>+</jats:sup>, initially ~0.05, declined steadily until equinox in mid‐2009 by a factor of ~6, and <jats:sup>28</jats:sup>M<jats:sup>+</jats:sup>/W<jats:sup>+</jats:sup>, initially ~0.007, declined similarly until early‐2007 by a factor of ~2. The O<jats:sub>2</jats:sub><jats:sup>+</jats:sup>/W<jats:sup>+</jats:sup> 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<jats:sub>2</jats:sub> and O<jats:sub>2</jats:sub><jats:sup>+</jats:sup>. The water ice‐dominated main rings and Enceladus plume depositions thereon are the two most likely O<jats:sub>2</jats:sub><jats:sup>+</jats:sup> sources. Enceladus' dynamic plumes, though, have no known long‐term dependence. After declining, O<jats:sub>2</jats:sub><jats:sup>+</jats:sup>/W<jats:sup>+</jats:sup> and <jats:sup>28</jats:sup>M<jats:sup>+</jats:sup>/W<jats:sup>+</jats:sup> levels remained low until late‐2011 when O<jats:sub>2</jats:sub><jats:sup>+</jats:sup>/W<jats:sup>+</jats:sup> increased, but <jats:sup>28</jats:sup>M<jats:sup>+</jats:sup>/W<jats:sup>+</jats:sup> did not. The O<jats:sub>2</jats:sub><jats:sup>+</jats:sup>/W<jats:sup>+</jats:sup> increase was steady and became statistically significant by mid‐2012, indicating a clear increase after a decline, that is, a possibly delayed O<jats:sub>2</jats:sub><jats:sup>+</jats:sup> “seasonal” recovery. Ring insolation is driven by solar UV flux which itself varies with the sun's 11 year activity cycle. The O<jats:sub>2</jats:sub><jats:sup>+</jats:sup>/W<jats:sup>+</jats:sup> and <jats:sup>28</jats:sup>M<jats:sup>+</jats:sup>/W<jats:sup>+</jats:sup> declines are consistent with seasonal ring insolation. No O<jats:sub>2</jats:sub><jats:sup>+</jats:sup>/W<jats:sup>+</jats:sup> response to the late‐2008 solar‐cycle UV minimum and recovery is evident. However, the O<jats:sub>2</jats:sub><jats:sup>+</jats:sup>/W<jats:sup>+</jats:sup> 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<jats:sub>2</jats:sub><jats:sup>+</jats:sup> observations can be understood.</jats:p> |
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| Umfang: | 3446-3463 |
| ISSN: |
2169-9380
2169-9402 |
| DOI: | 10.1002/jgra.50383 |