Anisotropy of Solar Wind Turbulence between Ion and Electron Scales

TL;DR

This study directly observes the anisotropy of solar wind turbulence between ion and electron scales, revealing spatially anisotropic fluctuations and spectral behaviors consistent with certain wave theories, but not others.

Contribution

It provides the first direct multispacecraft measurements of turbulence anisotropy between ion and electron scales in the solar wind.

Findings

Perpendicular fluctuations are more intense than parallel ones, with S_perp/S_par ratios of 5 and >3.

Spectral index of perpendicular component varies from -2.6 at large angles to -3 at small angles.

Parallel component spectral index is shallower than -1.9, differing from kinetic Alfven wave predictions.

Abstract

The anisotropy of turbulence in the fast solar wind, between the ion and electron gyroscales, is directly observed using a multispacecraft analysis technique. Second order structure functions are calculated at different angles to the local magnetic field, for magnetic fluctuations both perpendicular and parallel to the mean field. In both components, the structure function value at large angles to the field S_perp is greater than at small angles S_par: in the perpendicular component S_perp/S_par = 5 +- 1 and in the parallel component S_perp/S_par > 3, implying spatially anisotropic fluctuations, k_perp > k_par. The spectral index of the perpendicular component is -2.6 at large angles and -3 at small angles, in broad agreement with critically balanced whistler and kinetic Alfven wave predictions. For the parallel component, however, it is shallower than -1.9, which is considerably less steep than predicted for a kinetic Alfven wave cascade.