3D anisotropy of turbulence in the solar wind

Andrea Verdini, ASAP, Università di Firenze

Phenomenologies of strong turbulence are based on the critical balance between the Alfven time and the nonlinear time and predict anisotropy with respect to the local mean field. For a fast cascade ruled by eddy-turnover time small-scale structures should be axisymmetric, resembling tubes (Goldreich & Shridar 1995). Instead, when alignment between velocity and magnetic fluctuations (B & V alignment, Boldyrev 2005,2006) or between the two Elsasser fields (Z+ & Z- alignement, Chandran et al 2015) slows down the cascade, one should find ribbon-like structures.
We use 11 year of data from Wind spectraft to discriminate between the two forms of anisotropy. By isolating intervals with weak expansion that are expected to be closer to homogenous turbulence, we find ribbon-like structures, at variance with previous works that indicate the presence of tube-like structures.
Both angles, of B & V and of Z+ & Z- fluctuations, decrease with wavenumber but the alignment stops at relatively large scales in the inertial range, indicating that they are probably not responsible for the formation of ribbons. We present further measurements and discuss their implications for the origin of ribbons in MHD turbulence.