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| Titel |
A statistical analysis of the location and width of Saturn's southern auroras |
| VerfasserIn |
S. V. Badman, S. W. H. Cowley, J.-C. Gérard, D. Grodent |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 24, no. 12 ; Nr. 24, no. 12 (2006-12-21), S.3533-3545 |
| Datensatznummer |
250015724
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| Publikation (Nr.) |
 copernicus.org/angeo-24-3533-2006.pdf |
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| Zusammenfassung |
| A selection of twenty-two Hubble Space Telescope images of Saturn's
ultraviolet auroras obtained during 1997–2004 has been analysed to determine
the median location and width of the auroral oval, and their variability.
Limitations of coverage restrict the analysis to the southern hemisphere,
and to local times from the post-midnight sector to just past dusk, via dawn
and noon. It is found that the overall median location of the poleward and
equatorward boundaries of the oval with respect to the southern pole are at
~14° and ~16° co-latitude, respectively, with a median
latitudinal width of ~2°. These median values vary only modestly
with local time around the oval, though the poleward boundary moves closer
to the pole near noon (~12.5°) such that the oval is wider in that
sector (median width ~3.5°) than it is at both dawn and dusk
(~1.5°). It is also shown that the position of the auroral
boundaries at Saturn are extremely variable, the poleward boundary being
located between 2° and 20° co-latitude, and the equatorward boundary
between 6° and 23°, this variability contrasting sharply with the
essentially fixed location of the main oval at Jupiter. Comparison with
Voyager plasma angular velocity data mapped magnetically from the equatorial
magnetosphere into the southern ionosphere indicates that the dayside aurora
lie poleward of the main upward-directed field-aligned current region
associated with corotation enforcement, which maps to ~20°–24°
co-latitude, while agreeing reasonably with the position of the
open-closed field line boundary based on estimates of the open flux in
Saturn's tail, located between ~11° and ~15°. In this
case, the variability in location can be understood in terms of changes in
the open flux present in the system, the changes implied by the Saturn data
then matching those observed at Earth as fractions of the total planetary
flux. We infer that the broad (few degrees) diffuse auroral emissions and
sub-corotating auroral patches observed in the dayside sector at Saturn
result from precipitation from hot plasma sub-corotating in the outer
magnetosphere in a layer a few Saturn radii wide adjacent to the
magnetopause, probably having been injected either by Dungey-cycle or
Vasyliunas-cycle dynamics on the nightside. |
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