Persistence of small - scale anisotropy of magnetic turbu - lence as observed in the solar wind

– The anisotropy of magnetophydrodynamic turbulence is investigated by using solar wind data from the Helios 2 spacecraft. We investigate the behaviour of the complete high-order moment tensors of magnetic field increments and we compare the usual longitudinal structure functions which have both isotropic and anisotropic contributions, to the fully anisotropic contribution. Scaling exponents have been extracted by an interpolation scaling function. Unlike the usual turbulence in fluid flows, small-scale magnetic fluctuations remain anisotropic. We discuss the radial dependence of both anisotropy and intermittency and their relationship. The idea that small scales were statistically isotropic to a high degree, in spite of the anisotropy of large scales, is at the heart of the concept of universality underlying scaling behaviour in turbulent fluid flow [1]. As experiments became increasingly sophisticated, evidence for the presence of anisotropy for both scalar fields [2] and velocity fields [3] has grown up. To what extent scaling laws extracted from experiments are approximated by the scaling laws obtained from (isotropic) cascade models [4], became an urgent issue. A workable benchmark is represented by the following conjecture [5, 6]. Suppose that for a given statistical measurable quantity, scaling features can be splitted into an isotropic and an anisotropic part. We might ask to what extent the ratio between the anisotropic and the isotropic part vanishes as the scale goes down. Of course if we are able to define quantities for which the isotropic part is exactly zero, we can measure scaling laws related entirely to the anisotropic part. A formal analysis to reveal the isotropic and anisotropic contributions to structure functions can be performed by considering their SO(3) decomposition [7]. This is based in a foliation of various j-sectors of the linear space generated by the structure functions. For a recent review on anisotropy in turbulent flows see [8].