Anisotropy of Alfv\'enic Turbulence in the Solar Wind and Numerical Simulations

TL;DR

This paper compares the anisotropy of Alfvénic turbulence in solar wind observations with numerical simulations, revealing how spectral indices vary with angle and simulation conditions, and emphasizing the importance of local magnetic field reference.

Contribution

It provides a direct comparison of anisotropic turbulence measurements between solar wind data and MHD simulations, highlighting the effects of driving and decay on spectral indices.

Findings

Perpendicular spectral index in solar wind is close to -5/3.

Forced simulation shows -5/3 for velocity and -3/2 for magnetic field.

Decaying simulation has -5/3 for both fields.

Abstract

We investigate the anisotropy of Alfv\'enic turbulence in the inertial range of slow solar wind and in both driven and decaying reduced magnetohydrodynamic simulations. A direct comparison is made by measuring the anisotropic second-order structure functions in both data sets. In the solar wind, the perpendicular spectral index of the magnetic field is close to -5/3. In the forced simulation, it is close to -5/3 for the velocity and -3/2 for the magnetic field. In the decaying simulation, it is -5/3 for both fields. The spectral index becomes steeper at small angles to the local magnetic field direction in all cases. We also show that when using the global rather than local mean field, the anisotropic scaling of the simulations cannot always be properly measured.