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