Magnetic turbulence in space plasmas: Scale-dependent effects of anisotropy

The presence of a background magnetic field induces anisotropy in magnetic turbulence. Understanding properties of anisotropy is important to characterize turbulence power spectrum. This paper presents a case study of anisotropy by using a minimum variance analysis, in three different regions of the heliosphere, namely in the solar wind, and in the Earth's foreshock and magnetosheath behind a quasiparallel bow shock. A strong anisotropy is found in all cases, with very interesting cross-scale effects at the ion cyclotron frequency. In particular, (1) the eigenvalues of the variance matrix have a strong intermittent behavior, with very high localized fluctuations below the ion cyclotron scale. As a consequence the probability distribution functions are almost Gaussian5 above the ion cyclotron scale and become power laws at smaller scales; (2) the minimum variance direction is almost parallel to the background magnetic field at scales larger than the ion cyclotron scale in the solar wind and in the foreshock, while their probability density functions become broader at smaller scales. In the magnetosheath the minimum variance direction exhibits a tendency to become nearly perpendicular to the large-scale magnetic field below the ion cyclotron scale.