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| Titel |
Evaluation of magnetic helicity density in the wave number domain using multi-point measurements in space |
| VerfasserIn |
Y. Narita, G. Kleindienst, K.-H. Glassmeier |
| 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 ; 27, no. 10 ; Nr. 27, no. 10 (2009-10-21), S.3967-3976 |
| Datensatznummer |
250016685
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| Publikation (Nr.) |
 copernicus.org/angeo-27-3967-2009.pdf |
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| Zusammenfassung |
| We develop an estimator for the magnetic helicity density,
a measure of the spiral geometry of magnetic field lines,
in the wave number domain as a wave diagnostic tool
based on multi-point measurements in space.
The estimator is numerically tested with a synthetic data set
and then applied to an observation of magnetic field fluctuations
in the Earth foreshock region provided by
the four-point measurements of the Cluster spacecraft.
The energy and the magnetic helicity density are
determined in the frequency and the wave number domain,
which allows us to identify the wave properties
in the plasma rest frame correcting for the Doppler shift.
In the analyzed time interval, dominant wave components
have parallel propagation to the mean magnetic field,
away from the shock at about Alfvén speed and
a left-hand spatial rotation sense of helicity with respect to
the propagation direction, which means a right-hand
temporal rotation sense of polarization. These wave properties
are well explained by the right-hand resonant beam instability
as the driving mechanism in the foreshock. Cluster observations
allow therefore detailed comparisons with various theories
of waves and instabilities. |
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