Interpreting Magnetic Variance Anisotropy Measurements in the Solar Wind

TL;DR

This paper critically examines the use of magnetic variance anisotropy ($A_m$) in solar wind turbulence analysis, offering new interpretation methods and validating them against theory, simulations, and measurements.

Contribution

It introduces a more informative approach for interpreting $A_m$ in spacecraft data and compares linear theory predictions with turbulence simulations and observations.

Findings

Linear theory aligns well with measurements and simulations.

Solar wind in studied interval is predominantly Alfvénic.

$A_m$ effectively constrains turbulence mode composition.

Abstract

The magnetic variance anisotropy ($\mathcal{A}_m$) of the solar wind has been used widely as a method to identify the nature of solar wind turbulent fluctuations; however, a thorough discussion of the meaning and interpretation of the $\mathcal{A}_m$ has not appeared in the literature. This paper explores the implications and limitations of using the $\mathcal{A}_m$ as a method for constraining the solar wind fluctuation mode composition and presents a more informative method for interpreting spacecraft data. The paper also compares predictions of the $\mathcal{A}_m$ from linear theory to nonlinear turbulence simulations and solar wind measurements. In both cases, linear theory compares well and suggests the solar wind for the interval studied is dominantly Alfv\'{e}nic in the inertial and dissipation ranges to scales $k \rho_i \simeq 5$.