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| Titel |
Diurnal variations of Titan |
| VerfasserIn |
J. Cui, M. Galand, R. V. Yelle, V. Vuitton, J.-E. Wahlund, P. P. Lavvas, I. C. F. Mueller-Wodarg, W. T. Kasprzak, J. H. Waite |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250023585
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| Zusammenfassung |
| We present our analysis of the diurnal variations of Titan’s ionosphere (between 1,000 and
1,400Â km) based on a sample of Ion Neutral Mass Spectrometer (INMS) measurements in
the Open Source Ion (OSI) mode obtained from 8 close encounters of the Cassini
spacecraft with Titan. Though there is an overall ion depletion well beyond the
terminator, the ion content on Titan’s nightside is still appreciable, with a density
plateau of ~700Â cm-3 below ~1,300Â km. Such a plateau is associated with the
combination of distinct diurnal variations of light and heavy ions. Light ions (e.g.
CH5+, HCNH+, C2H5+) show strong diurnal variation, with clear bite-outs in
their nightside distributions. In contrast, heavy ions (e.g. c-C3H3+, C2H3CNH+,
C6H7+) present modest diurnal variation, with significant densities observed on
the nightside. We propose that the distinctions between light and heavy ions are
associated with their different chemical loss pathways, with the former primarily
through "fast" ion-neutral chemistry and the latter through "slow" electron dissociative
recombination. The INMS data suggest day-to-night transport as an important source
of ions on Titan’s nightside, to be distinguished from the conventional scenario
of auroral ionization by magnetospheric particles as the only ionizing source on
the nightside. This is supported by the strong correlation between the observed
night-to-day ion density ratios and the associated ion lifetimes. We construct a
time-dependent ion chemistry model to investigate the effects of day-to-night transport on the
ionospheric structures of Titan. The predicted diurnal variation has similar general
characteristics to those observed, with some apparent discrepancies which could be
reconciled by imposing fast horizontal thermal winds in Titan’s upper atmosphere. |
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