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| Titel |
Energy exchange and wave action conservation for magnetohydrodynamic (MHD) waves in a general, slowly varying medium |
| VerfasserIn |
A. D. M. Walker |
| 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 ; 32, no. 12 ; Nr. 32, no. 12 (2014-12-09), S.1495-1510 |
| Datensatznummer |
250121139
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| Publikation (Nr.) |
 copernicus.org/angeo-32-1495-2014.pdf |
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| Zusammenfassung |
| Magnetohydrodynamic (MHD) waves in the solar wind and
magnetosphere are propagated in a medium whose velocity is comparable to or
greater than the wave velocity and which varies in both space and time. In
the approximation where the scales of the time and space variation are long
compared with the period and wavelength, the ray-tracing equations can be
generalized and then include an additional first-order differential equation
that determines the variation of frequency. In such circumstances the wave
can exchange energy with the background: wave energy is not conserved. In
such processes the wave action theorem shows that the wave action, defined as
the ratio of the wave energy to the frequency in the local rest frame, is
conserved. In this paper we discuss ray-tracing techniques and the energy
exchange relation for MHD waves. We then provide a unified account of how to
deal with energy transport by MHD waves in non-uniform media. The wave action
theorem is derived directly from the basic MHD equations for sound waves,
transverse Alfvén waves, and the fast and slow magnetosonic waves. The
techniques described are applied to a number of illustrative cases. These
include a sound wave in a medium undergoing a uniform compression, an
isotropic Alfvén wave in a steady-state shear layer, and a transverse
Alfvén wave in a simple model of the magnetotail undergoing compression. In
each case the nature and magnitude of the energy exchange between wave and
background is found. |
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