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| Titel |
Time evolution of electric fields and currents and the generalized Ohm's law |
| 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 ; 23, no. 4 ; Nr. 23, no. 4 (2005-06-03), S.1347-1354 |
| Datensatznummer |
250015228
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| Publikation (Nr.) |
 copernicus.org/angeo-23-1347-2005.pdf |
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| Zusammenfassung |
| Fundamentally, the time derivative of the electric field is given by the
displacement-current term in Maxwell's generalization of Ampère's law, and
the time derivative of the electric current density is given by
the generalized Ohm's law. The latter is derived by summing the accelerations
of all the plasma
particles and can be written exactly, with no approximations,
in a (relatively simple) primitive form containing no
other time derivatives. When one is dealing with time
scales long compared to the inverse of the electron
plasma frequency and spatial scales large compared to
the electron inertial length, however, the time derivative
of the current density becomes negligible in comparison to
the other terms in the generalized Ohm's law, which then becomes the equation
that determines the electric field itself. Thus,
on all scales larger than those of electron plasma oscillations, neither
the time evolution of J nor that of E can be calculated
directly. Instead, J is determined by B through Ampère's law
and E by plasma dynamics through the generalized
Ohm's law. The displacement current may still be non-negligible if
the Alfvén speed is comparable to or larger than the speed of light,
but it no longer determines the time evolution of E, acting instead
to modify J. For theories of substorms, this implies that,
on time scales appropriate
to substorm expansion, there is no equation from which the time evolution of
the current could be calculated, independently of ∇xB.
Statements about change (disruption, diversion, wedge formation, etc.)
of the electric current are merely descriptions of change in the magnetic
field and are not explanations. |
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