Anisotropy of Magnetic Field and Velocity Fluctuations in the Solar Wind

Abstract:

The anisotropy of the solar wind and/or interplanetary magnetic field fluctuations contains important information about the solar wind origin and on the evolution of the turbulent cascade during the solar wind expansion. In this paper, we perform a large statistical study of the fluctuation anisotropy in minimum variance frames of the magnetic field and the solar wind velocity. We use 20-minute intervals of simultaneous magnetic field (Wind) and velocity (Spektr-R) observations but also study the length-scale dependence by separately analyzing 2, 10, and 40-minute intervals. Our study confirms the expectation that magnetic turbulence may be a composite of fluctuations with a slab geometry (wave vector aligned with the mean magnetic field) and a 2D geometry (wave vectors perpendicular to the mean field). Regardless of the length scale, ≈85% of the observed magnetic-field fluctuations can be classified as 2D, ≈15% as slab, and a negligible portion of fluctuations as quasi-isotropic. On the other hand, the velocity fluctuations tend to be more isotropic. The wave vectors of both fluctuating quantities are oriented predominantly parallel or perpendicular to the mean magnetic field. The proportion between Alfvénic and slow-mode-like fluctuations depends on the plasma beta and length scale. We do not find any influence of the solar wind expansion on the formation of the turbulent cascade at the studied ranges of scales. Finally, the experimental results are supported by numerical 3D MHD simulations.