Anisotropies in Compressible MHD Turbulence: Probing Magnetic Fields and Measuring Magnetization

TL;DR

This paper introduces the Structure-Function Analysis (SFA), a new method to measure magnetic field orientation and strength in the interstellar medium by analyzing turbulence anisotropies in compressible MHD simulations.

Contribution

The paper presents SFA as a novel approach to infer magnetic field properties from turbulence anisotropy, applicable to multi-phase interstellar media.

Findings

SFA reveals significant differences in turbulence structure functions parallel and perpendicular to magnetic fields.

The ratio of structure functions follows a power-law dependence on the Alfven Mach number.

SFA can estimate magnetic field orientation and strength from turbulent velocity data.

Abstract

Probing magnetic fields in the interstellar medium (ISM) is notoriously challenging. Motivated by the modern theories of magnetohydrodynamic (MHD) turbulence and turbulence anisotropy, we introduce the Structure-Function Analysis (SFA) as a new approach to measure the magnetic field orientation and estimate the magnetization. We analyze the statistics of turbulent velocities in three-dimensional compressible MHD simulations through the second-order structure functions in both local and global reference frames. In the sub-Alfvenic turbulence with the magnetic energy larger than the turbulent energy, the SFA of turbulent velocities measured in the directions perpendicular and parallel to the magnetic field can be significantly different. Their ratio has a power-law dependence on the Alfven Mach number $M_A$, which is inversely proportional to the magnetic field strength. We demonstrate that the anisotropic structure functions of turbulent velocities can be used to estimate both the orientation and strength of magnetic fields. With turbulent velocities measured using different tracers, our approach can be generally applied to probing the magnetic fields in the multi-phase interstellar medium.