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| Titel |
Saturn's polar ionospheric flows and their relation to the main auroral oval |
| VerfasserIn |
S. W. H. Cowley, E. J. Bunce, R. Prangé |
| 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 ; 22, no. 4 ; Nr. 22, no. 4 (2004-04-02), S.1379-1394 |
| Datensatznummer |
250014845
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| Publikation (Nr.) |
 copernicus.org/angeo-22-1379-2004.pdf |
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| Zusammenfassung |
We consider the flows and currents in Saturn's polar ionosphere which are implied by a
three-component picture of large-scale magnetospheric flow driven both by planetary rotation and
the solar wind interaction. With increasing radial distance in the equatorial plane, these
components consist of a region dominated by planetary rotation where planetary plasma sub-corotates
on closed field lines, a surrounding region where planetary plasma is lost down the dusk
tail by the stretching out of closed field lines followed by plasmoid formation and pinch-off, as first
described for Jupiter by Vasyliunas, and an outer region driven by the interaction with the solar
wind, specifically by reconnection at the dayside magnetopause and in the dawn tail, first discussed
for Earth by Dungey. The sub-corotating flow on closed field lines in the dayside magnetosphere is
constrained by Voyager plasma observations, showing that the plasma angular velocity falls to
around half of rigid corotation in the outer magnetosphere, possibly increasing somewhat near the
dayside magnetopause, while here we provide theoretical arguments which indicate that the flow
should drop to considerably smaller values on open field lines in the polar cap. The implied
ionospheric current system requires a four-ring pattern of field-aligned currents, with distributed
downward currents on open field lines in the polar cap, a narrow ring of upward current near the
boundary of open and closed field lines, and regions of distributed downward and upward current
on closed field lines at lower latitudes associated with the transfer of angular momentum from the
planetary atmosphere to the sub-corotating planetary magnetospheric plasma. Recent work has
shown that the upward current associated with sub-corotation is not sufficiently intense to produce
significant auroral acceleration and emission. Here we suggest that the observed auroral oval at
Saturn instead corresponds to the ring of upward current bounding the region of open and closed
field lines. Estimates indicate that auroras of brightness from a few kR to a few tens of kR can be
produced by precipitating accelerated magnetospheric electrons of a few keV to a few tens of keV
energy, if the current flows in a region which is sufficiently narrow, of the order of or less than ~1000 km
(~1° latitude) wide. Arguments are also given which indicate that the auroras should typically be
significantly brighter on the dawn side of the oval than at dusk, by roughly an order of magnitude,
and should be displaced somewhat towards dawn by the down-tail outflow at dusk associated with
the Vasyliunas cycle. Model estimates are found to be in good agreement with data derived from
high quality images newly obtained using the Space Telescope Imaging Spectrograph on the
Hubble Space Telescope, both in regard to physical parameters, as well as local time effects. The
implication of this picture is that the form, position, and brightness of Saturn's main auroral oval
provide remote diagnostics of the magnetospheric interaction with the solar wind, including
dynamics associated with magnetopause and tail plasma interaction processes.
Key words. Magnetospheric physics (auroral phenomena,
magnetosphere-ionosphere interactions, solar windmagnetosphere
interactions) |
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