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| Titel |
The mechanical advantage of the magnetosphere: solar-wind-related forces in the magnetosphere-ionosphere-Earth system |
| VerfasserIn |
V. M. Vasyliūnas  |
| 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 ; 25, no. 1 ; Nr. 25, no. 1 (2007-02-01), S.255-269 |
| Datensatznummer |
250015775
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| Publikation (Nr.) |
 copernicus.org/angeo-25-255-2007.pdf |
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| Zusammenfassung |
| Magnetosphere-ionosphere interactions involve electric currents that
circulate between the two regions; the associated Lorentz forces,
existing in both regions as matched opposite pairs,
are generally viewed as the primary mechanism by which linear momentum,
derived ultimately from solar wind flow,
is transferred from the magnetosphere to the ionosphere, where it is further
transferred by collisions to the neutral atmosphere. For a given total
amount of current, however, the total force is proportional to
ℒB and in general,
since ℒ2B~ constant by flux conservation,
is much larger in the ionosphere than in the magnetosphere
(ℒ = effective length, B = magnetic field).
The magnetosphere may be described
as possesing a mechanical advantage: the Lorentz force in it is coupled
with a Lorentz force in the ionosphere that has been amplified by a factor
given approximately by the square root of magnetic field magnitude ratio
(~20 to 40 on field lines connected to the outer magnetosphere).
The linear momentum transferred to the ionosphere (and thence to the
atmosphere) as the result of magnetic stresses applied by the magnetosphere
can thus be much larger than the momentum supplied by the solar wind through
tangential stress. The added linear momentum comes from within the Earth,
extracted by the Lorentz force on currents that arise
as a consequence of magnetic perturbation fields from the ionosphere
(specifically, the shielding
currents within the Earth that keep out the time-varying external fields).
This implies at once that Fukushima's theorem on the vanishing of ground-level
magnetic perturbations cannot be fully applicable, a conclusion confirmed by
re-examining the assumptions from which the theorem is derived.
To balance the inferred Lorentz force within the Earth's interior,
there must exist an antisunward mechanical stress there, only a small
part of which is the acceleration of the entire Earth system by the net force
exerted on it by the solar wind. The solar-wind interaction can thus
give rise to internal forces, significantly larger than the force
exerted by the solar wind itself, between the ionosphere and the neutral
atmosphere as well as within the current-carrying regions of the Earth's
interior. |
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