The relative orientation between the magnetic field and gas density structures in non-gravitating turbulent media

TL;DR

This study uses numerical simulations to analyze how magnetic field orientation relative to gas density structures varies in non-gravitating turbulent media, revealing dependence on initial magnetic strength and turbulence mode.

Contribution

It demonstrates that strong magnetic fields cause a transition in magnetic-gas alignment with density, influenced by turbulence type, a novel insight into magnetic field behavior in turbulent gas.

Findings

Strong magnetic fields lead to a change from parallel to perpendicular orientation with density.

Solenoidal turbulence produces a sharper transition than compressive turbulence.

The relative orientation remains stable over time after initial turbulence effects.

Abstract

Magnetic fields are a dynamically important agent for regulating structure formation in the interstellar medium. The study of the relative orientation between the local magnetic field and gas (column-) density gradient has become a powerful tool to analyse the magnetic field's impact on the dense gas formation in the Galaxy. In this study, we perform numerical simulations of a non-gravitating, isothermal gas, where the turbulence is driven either solenoidally or compressively. We find that only simulations with an initially strong magnetic field (plasma-$\beta<1$) show a change in the preferential orientation between the magnetic field and isodensity contours, from mostly parallel at low densities to mostly perpendicular at higher densities. Hence, compressive turbulence alone is not capable of inducing the transition observed towards nearby molecular clouds. At the same high initial magnetisation, we find that solenoidal modes produce a sharper transition in the relative orientation with increasing density than compressive modes. We further study the time evolution of the relative orientation and find that it remains unchanged by the turbulent forcing after one dynamical timescale.