Velocity Anisotropy as a Diagnostic of the Magnetization of the Interstellar Medium and Molecular clouds

TL;DR

This study uses MHD simulations to analyze velocity anisotropy in turbulence, providing a method to estimate magnetic field strength and orientation in interstellar media and molecular clouds.

Contribution

It introduces a robust technique to determine magnetic field direction and magnetization degree from velocity anisotropy in synthetic observations, effective even with density fluctuations.

Findings

Velocity anisotropy reveals magnetic field direction for M_A .5.

Degree of anisotropy correlates with magnetization (M_A) for M_A .5.

Method remains robust against large-scale density fluctuations.

Abstract

We use a set of magnetohydrodynamics (MHD) simulations of fully-developed (driven) turbulence to study the anisotropy in the velocity field that is induced by the presence of the magnetic field. In our models we study turbulence characterized by sonic Mach numbers M_s from 0.7 to 7.5, and Alfven Mach numbers M_A from 0.4 to 7.7. These are used to produce synthetic observations (centroid maps) that are analyzed. To study the effect of large scale density fluctuations and of white noise we have modified the density fields and obtained new centroid maps, which are analyzed. We show that restricting the range of scales at which the anisotropy is measured makes the method robust against such fluctuations. We show that the anisotropy in the structure function of the maps reveals the direction of the magnetic field for M_A \lesssim 1.5, regardless of the sonic Mach number. We found that the degree of anisotropy can be used to determine the degree of magnetization (i.e. M_A) for M_A \lesssim 1.5. To do this, one needs an additional measure of the sonic Mach number and an estimate of the LOS magnetic field, both feasible by other techniques, offering a new opportunity to study the magnetization state of the interstellar medium.