Linewidth Differences of Neutrals and Ions Induced by MHD Turbulence

TL;DR

This paper investigates how MHD turbulence affects the linewidth differences between neutrals and ions in molecular clouds, focusing on Alfvénic turbulence and its damping mechanisms, to better understand magnetic field diagnostics.

Contribution

It identifies the specific MHD turbulence modes influencing linewidth differences and derives expressions to estimate magnetic fields from these differences.

Findings

Fast modes do not affect linewidth differences.

Slow modes can influence line widths under certain conditions.

In super-Alfvénic turbulence, linewidth difference is independent of magnetic field strength.

Abstract

We address the problem of the difference of line widths of neutrals and ions observed from molecular clouds and explore whether this difference can arise from the effects of magnetohydrodynamic (MHD) turbulence acting on partially ionized gas. Among the three fundamental modes of MHD turbulence, we find fast modes do not contribute to linewidth differences, whereas slow modes can have an effect on different line widths for certain parameters. We focus on Alfv\'{e}nic component because they contain most of the turbulent energy, and consider the damping of this component taking into account both neutral-ion collisions and neutral viscosity. We consider different regimes of turbulence corresponding to different media magnetizations and turbulent drivings. In the case of super-Alfv\'{e}nic turbulence, when the damping scale of Alfv\'{e}nic turbulence is below $l_A$, where $l_A$ is the injection scale of anisotropic GS95-type turbulence, the linewidth difference does not depend on the magnetic field strength. While for other turbulent regimes, the dependence is present. For instance, the difference between the squares of the neutral and ion velocity dispersions in strong sub-Alfv\'{e}nic turbulence allows evaluation of magnetic field. We discuss earlier findings on the neutral-ion linewidth differences in the literature and compare the expressions for magnetic field we obtain with those published earlier.