3D Anisotropy of Solar Wind Turbulence, Tubes, or Ribbons?

TL;DR

This study investigates the anisotropy of solar wind turbulence, revealing that structures are tubes or ribbons depending on expansion conditions, challenging previous assumptions and aligning with certain turbulence models.

Contribution

It demonstrates that solar wind turbulence anisotropy varies with expansion, showing tube-like structures under strong expansion and ribbon-like under weak expansion, with implications for turbulence theories.

Findings

Strong expansion leads to axisymmetric anisotropy consistent with critical balance.

Weak expansion results in 3D anisotropy with ribbon structures as predicted by Boldyrev.

Cross-helicity does not increase at small scales in the solar wind.

Abstract

We study the anisotropy with respect to the local magnetic field of turbulent magnetic fluctuations at magnetofluid scales in the solar wind. Previous measurements in the fast solar wind obtained axisymmetric anisotropy, despite that the analysis method allows nonaxisymmetric structures. These results are probably contaminated by the wind expansion that introduces another symmetry axis, namely, the radial direction, as indicated by recent numerical simulations. These simulations also show that while the expansion is strong, the principal fluctuations are in the plane perpendicular to the radial direction. Using this property, we separate 11 yr of Wind spacecraft data into two subsets characterized by strong and weak expansion and determine the corresponding turbulence anisotropy. Under strong expansion, the small-scale anisotropy is consistent with the Goldreich & Sridhar critical balance. As in previous works, when the radial symmetry axis is not eliminated, the turbulent structures are field-aligned tubes. Under weak expansion, we find 3D anisotropy predicted by the Boldyrev model, that is, turbulent structures are ribbons and not tubes. However, the very basis of the Boldyrev phenomenology, namely, a cross-helicity increasing at small scales, is not observed in the solar wind: the origin of the ribbon formation is unknown.