@article{Lacombe:2017:1538-4357/aa8c06,
author = {Lacombe, C and Alexandrova, O and Matteini, L},
doi = {1538-4357/aa8c06},
journal = {ASTROPHYSICAL JOURNAL},
title = {Anisotropies of the Magnetic Field Fluctuations at Kinetic Scales in the Solar Wind: Cluster Observations},
url = {http://dx.doi.org/10.3847/1538-4357/aa8c06},
volume = {848},
year = {2017}
}

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TY  - JOUR
AB  - 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 proton to sub-electron scales. We consider 93 ten-minute intervals of the Cluster/STAFF measurements. We find that the fluctuations $\delta {B}_{\perp }^{2}$ are not gyrotropic at a given frequency f, a property already observed at larger scales ($\parallel /\perp $ means parallel/perpendicular to the average magnetic ${{\boldsymbol{B}}}_{0}$). This non-gyrotropy gives indications of the angular distribution of the wave vectors ${\boldsymbol{k}}$: at $f\lt $ 10 Hz, we find that ${k}_{\perp }\gg {k}_{\parallel }$, mainly in the fast wind; at $f\,\gt $ 10 Hz, fluctuations with a non-negligible k  are also present. We then consider the anisotropy ratio $\delta {B}_{\parallel }^{2}/\delta {B}_{\perp }^{2}$, which is a measure of the magnetic 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 at electron scales, for $f\geqslant 50\,\mathrm{Hz}$. From 1 to 15–20 Hz, 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 plasma β. For $f\gt 15\mbox{--}20\,\mathrm{Hz}$, the observed compressibility is larger than expected for KAWs, and it is stronger in the slow wind: this could be an indication of the presence of a slow-ion acoustic mode of fluctuations, which is more compressive and is favored by the larger values of the electron to proton temperature ratio generally observed in the slow wind.
AU  - Lacombe,C
AU  - Alexandrova,O
AU  - Matteini,L
DO  - 1538-4357/aa8c06
PY  - 2017///
SN  - 0004-637X
TI  - Anisotropies of the Magnetic Field Fluctuations at Kinetic Scales in the Solar Wind: Cluster Observations
T2  - ASTROPHYSICAL JOURNAL
UR  - http://dx.doi.org/10.3847/1538-4357/aa8c06
UR  - http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000412819100005&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=1ba7043ffcc86c417c072aa74d649202
UR  - http://hdl.handle.net/10044/1/62060
VL  - 848
ER  - 

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