Anisotropies of the Magnetic Field Fluctuations at Kinetic Scales in the Solar Wind: Cluster Observations
- 10 October 2017
- journal article
- Published by American Astronomical Society in The Astrophysical Journal
- Vol. 848 (1), 45
- https://doi.org/10.3847/1538-4357/aa8c06
Abstract
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 are not gyrotropic at a given frequency f, a property already observed at larger scales ( means parallel/perpendicular to the average magnetic ). This non-gyrotropy gives indications of the angular distribution of the wave vectors : at 10 Hz, we find that , mainly in the fast wind; at 10 Hz, fluctuations with a non-negligible k ∥ are also present. We then consider the anisotropy ratio , 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 . 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 , 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.This publication has 87 references indexed in Scilit:
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