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| Titel |
Spectra and anisotropy of magnetic fluctuations in the Earth's magnetosheath: Cluster observations |
| VerfasserIn |
O. Alexandrova, C. Lacombe, A. Mangeney |
| 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. 11 ; Nr. 26, no. 11 (2008-11-17), S.3585-3596 |
| Datensatznummer |
250016295
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| Publikation (Nr.) |
 copernicus.org/angeo-26-3585-2008.pdf |
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| Zusammenfassung |
| We investigate the spectral shape, the anisotropy of the wave vector
distributions and the anisotropy of the amplitudes of the magnetic
fluctuations in the Earth's magnetosheath within a broad range of frequencies
[10−3, 10] Hz which corresponds to spatial scales from ~10 to
105 km. We present the first observations of a Kolmogorov-like inertial
range of Alfvénic fluctuations δB2⊥}~f−5/3 in the
magnetosheath flanks, below the ion cyclotron frequency fci. In the
vicinity of fci, a spectral break is observed, like in solar wind
turbulence. Above the break, the energy of compressive and Alfvénic
fluctuations generally follows a power law with a spectral index between −3
and −2. Concerning the anisotropy of the wave vector distribution, we
observe a clear change in its nature in the vicinity of ion characteristic
scales: if at MHD scales there is no evidence for a dominance of a slab
(kł>>k⊥) or 2-D (k⊥>>kł) turbulence, above the
spectral break, (f>fci, kc/ωpi>1) the 2-D turbulence
dominates. This 2-D turbulence is observed in six selected one-hour intervals
among which the average ion β varies from 0.8 to 10. It is
observed for both the transverse and compressive magnetic fluctuations,
independently on the presence of linearly unstable modes at low frequencies
or Alfvén vortices at the spectral break. We then analyse the anisotropy of
the magnetic fluctuations in a time dependent reference frame based on the
field B and the flow velocity V directions. Within the range
of the 2-D turbulence, at scales [1,30]kc/ωpi, and for any β
we find that the magnetic fluctuations at a given frequency in the plane
perpendicular to B have more energy along the B×V direction. This non-gyrotropy of the fluctuations at a fixed frequency is
consistent with gyrotropic fluctuations at a given wave vector, with
k⊥>>kł, which suffer a different Doppler shift along and
perpendicular to V in the plane perpendicular to B. |
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