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| Titel |
Magnetosphere-ionosphere coupling currents in Jupiter’s middle magnetosphere: dependence on the effective ionospheric Pedersen conductivity and iogenic plasma mass outflow rate |
| VerfasserIn |
J. D. Nichols, S. W. H. Cowley |
| 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. 7 ; Nr. 21, no. 7, S.1419-1441 |
| Datensatznummer |
250014652
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| Publikation (Nr.) |
 copernicus.org/angeo-21-1419-2003.pdf |
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| Zusammenfassung |
The amplitude and spatial
distribution of the coupling currents that flow between Jupiter’s ionosphere
and middle magnetosphere, which enforce partial corotation on outward-flowing
iogenic plasma, depend on the values of the effective Pedersen conductivity of
the jovian ionosphere and the mass outflow rate of iogenic plasma. The values
of these parameters are, however, very uncertain. Here we determine how the
solutions for the plasma angular velocity and current components depend on
these parameters over wide ranges. We consider two models of the poloidal
magnetospheric magnetic field, namely the planetary dipole alone, and an
empirical current sheet field based on Voyager data. Following work by Hill
(2001), we obtain a complete normalized analytic solution for the dipole field,
which shows in compact form how the plasma angular velocity and current
components scale in space and in amplitude with the system parameters in this
case. We then obtain an approximate analytic solution in similar form for a
current sheet field in which the equatorial field strength varies with radial
distance as a power law. A key feature of the model is that the current sheet
field lines map to a narrow latitudinal strip in the ionosphere, at ≈ 15°
co-latitude. The approximate current sheet solutions are compared with the
results of numerical integrations using the full field model, for which a power
law applies beyond ≈ 20 RJ, and are found to agree very well
within their regime of applicability. A major distinction between the solutions
for the dipole field and the current sheet concerns the behaviour of the
field-aligned current. In the dipole model the direction of the current
reverses at moderate equatorial distances, and the current system wholly closes
if the model is extended to infinity in the equatorial plane and to the pole in
the ionosphere. In the approximate current sheet model, however, the
field-aligned current is unidirectional, flowing consistently from the
ionosphere to the current sheet for the sense of the jovian magnetic field.
Current closure must then occur at higher latitudes, on field lines outside the
region described by the model. The amplitudes of the currents in the two models
are found to scale with the system parameters in similar ways, though the
scaling is with a somewhat higher power of the conductivity for the current
sheet model than for the dipole, and with a somewhat lower power of the plasma
mass outflow rate. The absolute values of the currents are also higher for the
current sheet model than for the dipole for given parameters, by factors of
approx 4 for the field-perpendicular current intensities, ≈ 10 for the
total current flowing in the circuit, and ≈ 25 for the field-aligned
current densities, factors which do not vary greatly with the system
parameters. These results thus confirm that the conclusions drawn previously
from a small number of numerical integrations using spot values of the system
parameters are generally valid over wide ranges of the parameter values.
Key words. Magnetospheric physics
(current systems, magnetosphere-ionosphere interactions, planetary
magnetospheres) |
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