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| Titel |
Corotation-driven magnetosphere-ionosphere coupling currents in Saturn’s magnetosphere and their relation to the auroras |
| VerfasserIn |
S. W. H. Cowley, E. J. Bunce |
| 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 ; 21, no. 8 ; Nr. 21, no. 8, S.1691-1707 |
| Datensatznummer |
250014672
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| Publikation (Nr.) |
 copernicus.org/angeo-21-1691-2003.pdf |
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| Zusammenfassung |
We calculate the latitude
profile of the equatorward-directed ionospheric Pedersen currents that are
driven in Saturn’s ionosphere by partial corotation of the magnetospheric
plasma. The calculation incorporates the flattened figure of the planet, a
model of Saturn’s magnetic field derived from spacecraft flyby data, and
angular velocity models derived from Voyager plasma data. We also employ an
effective height-integrated ionospheric Pedersen conductivity of 1 mho,
suggested by a related analysis of Voyager magnetic field data. The Voyager
plasma data suggest that on the largest spatial scales, the plasma angular
velocity declines from near-rigid corotation with the planet in the inner
magnetosphere, to values of about half of rigid corotation at the outer
boundary of the region considered. The latter extends to ~ 15–20 Saturn radii
(RS) in the equatorial plane, mapping along magnetic field lines to
~ 15° co-latitude in the ionosphere. We find in this case that the ionospheric
Pedersen current peaks near the poleward (outer) boundary of this region, and
falls toward zero over ~ 5°–10° equator-ward of the boundary as the plasma
approaches rigid corotation. The peak current near the poleward boundary,
integrated in azimuth, is ~ 6 MA. The field-aligned current required for
continuity is directed out of the ionosphere into the magnetosphere essentially
throughout the region, with the current density peaking at ~ 10 nA m-2 at
~ 20° co-latitude. We estimate that such current densities are well below the
limit requiring field-aligned acceleration of magnetospheric electrons in
Saturn’s environment ( ~ 70 nAm-2), so that no significant auroral
features associated with this ring of upward current is anticipated. The
observed ultraviolet auroras at Saturn are also found to occur significantly
closer to the pole (at ~ 10°–15° co-latitude), and show considerable
temporal and local time variability, contrary to expectations for corotation-related
currents. We thus conclude that Saturn’s ‘main oval’ auroras are not
associated with corotation-enforcing currents as they are at Jupiter, but
instead are most probably associated with coupling to the solar wind as at
Earth. At the same time, the Voyager flow observations also suggest the
presence of radially localized ‘dips’ in the plasma angular velocity
associated with the moons Dione and Rhea, which are ~ 1–2 RS in
radial extent in the equatorial plane. The presence of such small-scale flow
features, assumed to be azimuthally extended, results in localized several-MA
enhancements in the ionospheric Pedersen current, and narrow bi-polar
signatures in the field-aligned currents which peak at values an order of
magnitude larger than those associated with the large-scale currents. Narrow
auroral rings (or partial rings) ~ 0.25°
co-latitude wide with intensities ~ 1 kiloRayleigh may be formed in the regions
of upward field-aligned current under favourable circumstances, located at
co-latitudes between ~ 17° and ~
20° in the north, and ~ 19°
and ~22° in the south.
Key words. Magnetospheric physics
(current systems; magnetosphere-ionosphere interactions; planetary
magnetospheres) |
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