Alfvenic Turbulence Beyond the Ambipolar Diffusion Scale

TL;DR

This study explores how Alfvénic turbulence behaves beyond the ambipolar diffusion scale in two-fluid MHD simulations, revealing conditions under which turbulence persists or is damped, and characterizing the energy spectrum in different regimes.

Contribution

It demonstrates that Alfvénic turbulence can exist past the ambipolar diffusion scale in super-Alfvénic conditions and characterizes the damping behavior and energy spectrum in two-fluid MHD turbulence.

Findings

Turbulence resembles single fluid MHD above the ambipolar diffusion scale.

Alfvénic turbulence persists past the scale in super-Alfvénic regimes.

Damped turbulence exhibits a $k^{-4}$ energy spectrum.

Abstract

We investigate the nature of the Alfv\'enic turbulence cascade in two fluid MHD simulations in order to determine if turbulence is damped once the ion and neutral species become decoupled at a critical scale called the ambipolar diffusion scale (L$_{AD}$). Using mode decomposition to separate the three classical MHD modes, we study the second order structure functions of the Alfv\'en mode velocity field of both neutrals and ions in the reference frame of the local magnetic field. On scales greater than L$_{AD}$ we confirm that two fluid turbulence strongly resembles single fluid MHD turbulence. Our simulations show that the behavior of two fluid turbulence becomes more complex on scales less than L$_{AD}$. We find that Alfvenic turbulence can exist past L$_{AD}$ when the turbulence is globally super-Alfv\'enic, with the ions and neutrals forming separate cascades once decoupling has taken place. When turbulence is globally sub-Alfvenic and hence strongly anisotropic with a large separation between the parallel and perpendicular decoupling scales, turbulence is damped at L$_{AD}$. We also find that the power spectrum of the kinetic energy in the damped regime is consistent with a $k^{-4}$ scaling (in agreement with the predictions of Lazarian, Vishniac & Cho 2004).