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| Titel |
On the generation of solitary waves observed by Cluster in the near-Earth magnetosheath |
| VerfasserIn |
J. S. Pickett, L.-J. Chen, S. W. Kahler, O. Santolík, M. L. Goldstein, B. Lavraud, P. M. E. Décréau, R. Kessel, E. Lucek, G. S. Lakhina, B. T. Tsurutani, D. A. Gurnett, N. Cornilleau-Wehrlin, A. Fazakerley, H. Rème, A. Balogh |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1023-5809
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| Digitales Dokument |
URL |
| Erschienen |
In: Nonlinear Processes in Geophysics ; 12, no. 2 ; Nr. 12, no. 2 (2005-02-02), S.181-193 |
| Datensatznummer |
250010481
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| Publikation (Nr.) |
 copernicus.org/npg-12-181-2005.pdf |
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| Zusammenfassung |
| Through case studies involving Cluster waveform observations, solitary waves
in the form of bipolar and tripolar pulses have recently been found to be
quite abundant in the near-Earth dayside magnetosheath. We expand on the
results of those previous studies by examining the distribution of solitary
waves from the bow shock to the magnetopause using Cluster waveform data.
Cluster's orbit allows for the measurement of solitary waves in the
magnetosheath from about 10 RE to 19.5 RE. Our results clearly
show that within the magnetosheath, solitary waves are likely to be observed
at any distance from the bow shock and that this distance has no dependence
on the time durations and amplitudes of the solitary waves. In addition we
have found that these same two quantities show no dependence on either the
ion velocity or the angle between the ion velocity and the local magnetic
field direction. These results point to the conclusion that the solitary
waves are probably created locally in the magnetosheath at multiple
locations, and that the generation mechanism is most likely not solely
related to ion dynamics, if at all. To gain insight into a possible local
generation mechanism, we have examined the electron differential energy flux
characteristics parallel and perpendicular to the magnetic field, as well as
the local electron plasma and cyclotron frequencies and the type of bow
shock that Cluster is behind, for several time intervals where solitary
waves were observed in the magnetosheath. We have found that solitary waves
are most likely to be observed when there are counterstreaming (~parallel and anti-parallel to the magnetic field) electrons at or below
about 100eV. However, there are times when these counterstreaming electrons
are present when solitary waves are not. During these times the background
magnetic field strength is usually very low (<10nT), implying that the
amplitudes of the solitary waves, if present, would be near or below those
of other waves and electrostatic fluctuations in this region making it
impossible to isolate or clearly distinguish them from these other emissions
in the waveform data. Based on these results, we have concluded that some of
the near-Earth magnetosheath solitary waves, perhaps in the form of electron
phase-space holes, may be generated locally by a two-stream instability
involving electrons based on the counterstreaming electrons that are often
observed when solitary waves are present. We have not ruled out the
possibility that the solitary waves could be generated as a result of the
lower-hybrid Buneman instability in the presence of an electron beam,
through the electron acoustic mode or through processes involving
turbulence, which is almost always present in the magnetosheath, but these
will be examined in a more comprehensive study in the future. |
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