Multipoint observation of anisotropy and intermittency in solar-wind turbulence

Using high-resolution magnetic-field Cluster observations, we have investigated the magnetic-field anisotropy via the second- and fourth-order structure functions over a wide range of scales reaching below the subproton scale. The magnetic-field increments have been computed from single- and two-spacecraft measurements. The two-satellite technique allows us to study the increments as a function of an actual space lag. Both single- and two-point analyses show that the magnetic field is anisotropic even at small time/spatial scales. The single-spacecraft data also shows that the degree of anisotropy does not change with the scale at proton and subproton scales. It is also pointed out that the degree of magnetic-field anisotropy tends to be overestimated in the single-spacecraft data analysis. This is particularly evident at small scales and it depends on the angle between the spacecraft separation and the flow direction. From the fourth-order moment of the probability density function of the magnetic-field increments we have also investigated the presence of intermittency in the fluctuations. Even though to a different degree, intermittency was present over the entire range of scales, with an indication of scale invariance at subproton scales.