Long-lived Magnetic-Tension-Driven Modes in a Molecular Cloud

TL;DR

This paper investigates long-lived magnetic-tension-driven MHD wave modes in molecular clouds, showing that in flux-freezing conditions, these modes persist indefinitely and may explain observed supersonic motions.

Contribution

It demonstrates that magnetic-tension-driven modes can remain nonlinear and long-lived in molecular clouds under flux-freezing conditions, providing a new perspective on turbulence persistence.

Findings

Persistent nonlinear MHD modes driven by magnetic tension.

Modes can sustain large-scale motions without dissipation.

Potential explanation for observed supersonic turbulence in clouds.

Abstract

We calculate and analyze the longevity of magnetohydrodynamic (MHD) wave modes that occur in the plane of a magnetic thin sheet. Initial turbulent conditions applied to a magnetically subcritical cloud are shown to lead to relatively rapid energy decay if ambipolar diffusion is introduced at a level corresponding to partial ionization primarily by cosmic rays. However, in the flux-freezing limit, as may be applicable to photoionized molecular cloud envelopes, the turbulence persists at "nonlinear" levels in comparison with the isothermal sound speed $\cs$, with one-dimensional rms material motions in the range of $\approx 2\,\cs -5\,\cs$ for cloud sizes in the range of $\approx 2\,\pc - 16\,\pc$. These fluctuations persist indefinitely, maintaining a significant portion of the initial turbulent kinetic energy. We find the analytic explanation for these persistent fluctuations. They are magnetic-tension-driven modes associated with the interaction of the sheet with the external magnetic field. The phase speed of such modes is quite large, allowing residual motions to persist without dissipation in the flux-freezing limit, even as they are nonlinear with respect to the sound speed. We speculate that long-lived large-scale MHD modes such as these may provide the key to understanding observed supersonic motions in molecular clouds.