ALFVENIC TURBULENCE BEYOND THE AMBIPOLAR DIFFUSION SCALE

We investigate the nature of the Alfvenic 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 (L-AD). Using mode decomposition to separate the three classical MHD modes, we study the second-order structure functions of the Alfven 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-Alfvenic, 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 et al.).