Non Axi-symmetric Anisotropy of Solar Wind Turbulence

TL;DR

This study investigates the non-axisymmetric anisotropy observed in solar wind turbulence, suggesting it may result from sampling effects rather than intrinsic plasma dynamics, supported by in-situ data and simulations.

Contribution

It demonstrates that observed non-axisymmetry in solar wind turbulence can be explained by sampling effects, challenging previous assumptions about plasma behavior.

Findings

In-situ observations show non-axisymmetry across inertial and dissipation ranges.

Sampling of MHD turbulence simulations reproduces observed non-axisymmetry.

Linear superposition of waves with axisymmetric fluctuations explains the trend in non-axisymmetry.

Abstract

A key prediction of turbulence theories is frame-invariance, and in magnetohydrodynamic (MHD) turbulence, axisymmetry of fluctuations with respect to the background magnetic field. Paradoxically the power in fluctuations in the turbulent solar wind are observed to be ordered with respect to the bulk macroscopic flow as well as the background magnetic field. Here, non- axisymmetry across the inertial and dissipation ranges is quantified using in-situ observations from Cluster. The observed inertial range non- axisymmetry is reproduced by a 'fly through' sampling of a Direct Numerical Simulation of MHD turbulence. Furthermore, 'fly through' sampling of a linear superposition of transverse waves with axisymmetric fluctuations generates the trend in non- axisymmetry with power spectral exponent. The observed non-axisymmetric anisotropy may thus simply arise as a sampling effect related to Taylor's hypothesis and is not related to the plasma dynamics itself.