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| Titel |
Nonlinear low-frequency wave aspect of foreshock density holes |
| VerfasserIn |
N. Lin, E. Lee, F. Mozer, G. K. Parks, M. Wilber, H. Rème |
| 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 ; 26, no. 12 ; Nr. 26, no. 12 (2008-11-25), S.3707-3718 |
| Datensatznummer |
250016303
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| Publikation (Nr.) |
 copernicus.org/angeo-26-3707-2008.pdf |
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| Zusammenfassung |
| Recent observations have uncovered short-duration density holes in the
Earth's foreshock region. There is evidence that the formation of density
holes involves non-linear growth of fluctuations in the magnetic field and
plasma density, which results in shock-like boundaries followed by a
decrease in both density and magnetic field. In this study we examine in
detail a few such events focusing on their low frequency wave
characteristics. The propagation properties of the waves are studied using
Cluster's four point observations. We found that while these density
hole-structures were convected with the solar wind, in the plasma rest frame
they propagated obliquely and mostly sunward. The wave amplitude grows
non-linearly in the process, and the waves are circularly or elliptically
polarized in the left hand sense. The phase velocities calculated from four
spacecraft timing analysis are compared with the velocity estimated from
δE/δB. Their agreement justifies the plane electromagnetic wave nature of the
structures. Plasma conditions are found to favor firehose instabilities.
Oblique Alfvén firehose instability is suggested as a possible energy
source for the wave growth. Resonant interaction between ions at certain
energy and the waves could reduce the ion temperature anisotropy and thus
the free energy, thereby playing a stabilizing role. |
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