Small-Scale Behavior of Decaying Hall Magnetohydrodynamic Turbulence

Abstract:

With the advent of new multi-spacecraft missions such as the Magnetospheric Multiscale (MMS) just launched, examining the small scales of collisionless space plasmas, it is increasingly important to understand the behavior of turbulence in the kinetic regime where the fluid approximation breaks down. One way to begin examining the role of kinetic effects is to consider more accurate fluid approximations, such as Hall magnetohydrodynamics (HMHD). In this study, decaying HMHD and magnetohydrodynamic (MHD) turbulence are examined using 3D direct numerical simulations with grids up to 768^3 points and with different initial conditions. It is found that the small scales of HMHD turbulence are magnetically dominated and this behavior can be associated with nonlinear advection becoming subdominant to dissipation. Current structures in HMHD are found to be narrower than in MHD whereas vorticity structures are broader. In the simulations examined herein, an association between strong current structures and alignment between the current and magnetic field is present, which may be important in collisionless plasmas where field-aligned currents can be unstable.