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

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-perpendicular to(2) are not gyrotropic at a given frequency f, a property already observed at larger scales (parallel to/perpendicular to means parallel/perpendicular to the average magnetic B0). This non-gyrotropy gives indications of the angular distribution of the wave vectors k: at f < 10 Hz, we find that k(perpendicular to) >> k., mainly in the fast wind; at f > 10 Hz, fluctuations with a non-negligible k(parallel to) are also present. We then consider the anisotropy ratio delta B-parallel to(2)/delta B-perpendicular to(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 Alfvn waves (KAWs), which only depends on the total plasma beta 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.