Multipoint Observation of Anisotropy and Intermittency in the Solar Wind Turbulence

Abstract:

Anisotropy in the solar wind turbulence is introduced by the presence of a background magnetic field. Recently, it has been shown that the return to isotropy at small scales invoked in the classical fluid turbulence does not hold in magnetohydrodynamic turbulence. It is also well known that in the inertial range of solar wind turbulence the spatial inhomogeneity in the fluctuations increases towards small scales, i.e. the intermittency increases with scale. Here, the anisotropy and intermittency in the solar wind magnetic field fluctuations are investigated with Cluster spacecraft observations. The analysis is based on the computation of the magnetic structure function tensor which consists of isotropic and anisotropic terms. This allows to estimate the degree of anisotropy in the magnetic field fluctuations. The structure function tensor is computed as a function of spacecraft separation using a pair of spacecraft. Then the scale dependence of the second and fourth order moments has been studied. We have performed the same analysis up to 50 Hz frequency using single-point measurements. We have found that the anisotropy in the magnetic field is persistent towards small scales and increases up to 1 Hz (around the proton scale). In the higher frequency range the degree of both anisotropy and intermittency tends to become constant. The minimum variance analysis performed in the same frequency range suggests a 3D distribution of the fluctuations in the kinetic range.