Quantitative estimates of the slab and 2-D power in solar wind turbulence using multispacecraft data

A quantitative estimate of the field-aligned anisotropy of magnetohydrodynamic inertial range turbulence is obtained by comparing multispacecraft data with a fully three-dimensional turbulent magnetic field numerical model. The simulated turbulence is a superposition of slab fluctuations parallel to the mean field and 2-D fluctuations perpendicular to it. This model is fitted to spatial autocorrelation functions, computed from measurements made by the four Cluster spacecraft in the solar wind, by varying the slab-2-D fraction. The mean power in the 2-D fluctuations, 81 +/- 3%, is consistent with solar wind fluctuations being anisotropic with energy mainly in wave vectors perpendicular to the large-scale mean magnetic field. Eight intervals of multipoint magnetic field data are analyzed, and a large degree of variation in the anisotropy estimates about the mean is observed. The origin of this variability is unclear. However, variations in the anisotropy estimates are correlated between different field components.