# On the formation of density filaments in the turbulent interstellar medium

## TL;DR

This paper explains how turbulence and magnetic fields in the interstellar medium create elongated density filaments, with their properties influenced by ionization and shock processes, aligning with recent astronomical observations.

## Contribution

It provides a theoretical framework linking MHD turbulence, ionization effects, and shock compression to the formation and characteristics of interstellar density filaments.

## Key findings

- Elongated filaments align with magnetic fields due to turbulent mixing.
- Filament width is set by neutral-ion decoupling scale.
- Both low-density and high-density filaments coexist in molecular clouds.

## Abstract

This study is motivated by recent observations on ubiquitous interstellar density filaments and guided by modern theories of compressible magnetohydrodynamic (MHD) turbulence. The interstellar turbulence shapes the observed density structures. As the fundamental dynamics of compressible MHD turbulence, perpendicular turbulent mixing of density fluctuations entails elongated density structures aligned with the local magnetic field, accounting for low-density parallel filaments seen in diffuse atomic and molecular gas. The elongation of low-density parallel filaments depends on the turbulence anisotropy. When taking into account the partial ionization, we find that the minimum width of parallel filaments in the cold neutral medium and molecular clouds is determined by the neutral-ion decoupling scale perpendicular to magnetic field. In highly supersonic MHD turbulence in molecular clouds, both low-density parallel filaments due to anisotropic turbulent mixing and high-density filaments due to shock compression exist.