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| Titel |
Anisotropies of the magnetic field fluctuations at kinetic scales in the solar wind : Cluster observations |
| VerfasserIn |
Catherine Lacombe, Olga Alexandrova, Lorenzo Matteini |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250152940
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| Publikation (Nr.) |
 EGU/EGU2017-17850.pdf |
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| Zusammenfassung |
| We present the first statistical study of the anisotropy of the magnetic field turbulence in the
solar wind between 1 and 200 Hz, i.e. from ion to sub-electron scales. We consider a sample
of 93 intervals of 10 minutes of STAFF measurements, on a single Cluster spacecraft. We
find that the fluctuations δB⊥2 (perpendicular to the average magnetic field B0) are not
gyrotropic at a given frequency f, a property already observed at larger scales. This
non-gyrotropy of the frequency spectra gives indications on the shape of the angular
distribution of the wave vectors k: below about 10 Hz, we find that the components k⊥
(perpendicular to B0) of the wave vectors are much larger than the components k∥ (parallel to
B0), mainly in the fast wind; above 10 Hz, fluctuations with a non-negligible k∥ are also
present. We then consider the anisotropy ratio δB∥2∕δB⊥2 between the compressive
fluctuations parallel to B0 and δB⊥2, which is a measure of the compressibility
of the fluctuations. This ratio, always smaller than 1, increases with f. It reaches
a value showing that the fluctuations are more or less isotropic for f ≥ 50 Hz.
From 1 to 15-20 Hz, i.e. down to scales ten times smaller than the proton inertial
length, there is a strong correlation between the observed compressibility and the one
expected for the kinetic Alfvén waves (KAWs), which only depends on the total β. At
f > 20 Hz, the observed compressibility is larger than expected for KAWs; and
it is stronger in the slow solar wind: this could be an indication of the presence
of a slow-ion acoustic type of fluctuations which is favoured by the larger values
of the electron to proton temperature ratio generally observed in the slow wind. |
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