Abstract
ABSTRACT We investigate the nature of the Alfvénic turbulence cascade in two-fluid magnetohydrodynamic (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 (LAD). Using mode decomposition to separate the three classical MHD modes, we study the second-order structure functions of the Alfvén mode velocity field of both neutrals and ions in the reference frame of the local magnetic field. On scales greater than LAD 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 LAD. We find that Alfvénic turbulence can exist past LAD when the turbulence is globally super-Alfvénic, with the ions and neutrals forming separate cascades once decoupling has taken place. When turbulence is globally sub-Alfvénic and hence strongly anisotropic, with a large separation between the parallel and perpendicular decoupling scales, turbulence is damped at LAD. We also find that the power spectrum of the kinetic energy in the damped regime is consistent with a scaling (in agreement with the predictions of Lazarian et al.).