Anisotropies of the Magnetic Field Fluctuations at Kinetic Scales in the Solar Wind: Cluster Observations

Abstract

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 δ B ⊥ 2 are not gyrotropic at a given frequency f, a property already observed at larger scales ( ∥ / ⊥ means parallel/perpendicular to the average magnetic B 0 ). This non-gyrotropy gives indications of the angular distribution of the wave vectors k : at f < 10 Hz, we find that k ⊥ ≫ k ∥ , mainly in the fast wind; at f > 10 Hz, fluctuations with a non-negligible k ∥ are also present. We then consider the anisotropy ratio δ B ∥ 2 / δ B ⊥ 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 ≥ 50 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 > 15 – 20 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.