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| Titel |
Azimuthal magnetic fields in Saturn’s magnetosphere: effects associated with plasma sub-corotation and the magnetopause-tail current system |
| VerfasserIn |
E. J. Bunce, S. W. H. Cowley, J. A. Wild |
| 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.1709-1722 |
| Datensatznummer |
250014673
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| Publikation (Nr.) |
 copernicus.org/angeo-21-1709-2003.pdf |
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| Zusammenfassung |
We calculate the
azimuthal magnetic fields expected to be present in Saturn’s magnetosphere
associated with two physical effects, and compare them with the fields observed
during the flybys of the two Voyager spacecraft. The first effect is associated
with the magnetosphere-ionosphere coupling currents which result from the sub-corotation
of the magnetospheric plasma. This is calculated from empirical models of the
plasma flow and magnetic field based on Voyager data, with the effective
Pedersen conductivity of Saturn’s ionosphere being treated as an essentially
free parameter. This mechanism results in a ‘lagging’ field configuration
at all local times. The second effect is due to the day-night asymmetric
confinement of the magnetosphere by the solar wind (i.e. the magnetopause and
tail current system), which we have estimated empirically by scaling a model of
the Earth’s magnetosphere to Saturn. This effect produces ‘leading’
fields in the dusk magnetosphere, and ‘lagging’ fields at dawn. Our results
show that the azimuthal fields observed in the inner regions can be reasonably
well accounted for by plasma sub-corotation, given a value of the effective
ionospheric Pedersen conductivity of ~ 1–2 mho. This statement applies to
field lines mapping to the equator within ~ 8 RS (1 RS is
taken to be 60 330 km) of the planet on the dayside inbound passes, where the
plasma distribution is dominated by a thin equatorial heavy-ion plasma sheet,
and to field lines mapping to the equator within ~ 15 RS on the dawn
side outbound passes. The contributions of the magnetopause-tail currents are
estimated to be much smaller than the observed fields in these regions. If,
however, we assume that the azimuthal fields observed in these regions are not
due to sub-corotation but to some other process, then the above effective
conductivities define an upper limit, such that values above ~ 2 mho can
definitely be ruled out. Outside of this inner region the spacecraft observed
both ‘lagging’ and ‘leading’ fields in the post-noon dayside
magnetosphere during the inbound passes, with ‘leading’ fields being
observed both adjacent to the magnetopause and in the ring current region, and
‘lagging’ fields being observed between. The observed ‘lagging’ fields
are consistent in magnitude with the sub-corotation effect with an effective
ionospheric conductivity of ~ 1–2 mho, while the ‘leading’ fields are
considerably larger than those estimated for the magnetopause-tail currents,
and appear to be indicative of the presence of another dynamical process. No
‘leading’ fields were observed outside the inner region on the dawn side
outbound passes, with the azimuthal fields first falling below those expected
for sub-corotation, before increasing, to exceed these values at radial
distances beyond ~ 15–20 RS , where the effect of the
magnetopause-tail currents becomes significant. As a by-product, our
investigation also indicates that modification and scaling of terrestrial
magnetic field models may represent a useful approach to modelling the
three-dimensional magnetic field at Saturn.
Key words. Magnetospheric physics
(current systems; magnetosphere-ionosphere interactions; solar
wind-magnetosphere interactions) |
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