Measurement of Intermittency of Anisotropic Magnetohydrodynamic Turbulence in High-speed Solar Wind

Abstract

We investigate the intermittency of anisotropic magnetohydrodynamic (MHD) turbulence in high-speed solar wind. Using the data recorded by the Ulysses spacecraft, we apply the Castaing function to model the probability density functions of the fluctuating magnetic field and calculate the magnetic structure functions (SFs) Sp of the order p in the coordinates (r, Θ), with r being the length scale and Θ the direction of the local mean field. The scaling exponent ζ, from Sp (r, Θ)vpropr ζ(p, Θ), has an anomalous nonlinear dependence on p, implying the intermittent scaling of solar wind turbulence, which has been observed for decades. Furthermore, we study the anisotropy of solar wind turbulence introduced by the strong mean magnetic field. From Sp (Θ = 0)vpropSp (Θ = π/2), we obtain r bottomvpropr α p par with α p = ζpar/ζbottom denoting the perpendicular-parallel spatial correlation of the moment of the pth order. For the magnetic field difference \delta \bf {B}, we find α2 = 1.78 ± 0.26, consistent with recent theories and observations. However, when the contribution from the intermittent fluctuations begins to dominate the scaling, α is not a constant but increases with p, e.g., α5 = 1.97 ± 0.41 and α8 ≈ 2.42 ± 0.64. This complication of the perpendicular-parallel spatial correlation due to the intermittency raises new questions for MHD turbulence theory.