Inertial transfer and small-scale structures in magnetohydrodynamic turbulence*

In homogeneous magnetohydrodynamic (MHD) turbulence, simulation results indicate a depletion of the interscale kinetic energy flux associated with the inertial term relative to the hydrodynamic (HD) case. Here we report on an investigation of the physical mechanisms behind this depletion, analysing the role of the contractile and extensional directions of the velocity gradient tensor. Because of incompressibility, there are only two possible types of deformation than a small sphere of fluid can undergo when subject to strain. It may either flatten and become disk-shaped, due to one contractile and two extensional directions of the strain tensor, or it may elongate in one direction and become cigar-shaped, due to two contractile and one extensional directions. Using simulation data we find that for MHD turbulence the two types of deformation are approximately equally probable, while in HD turbulence disk-like flattening is known to be the dominant feature, associated with strain self-amplification and vortex stretching. This suggests that the small-scale structure of MHD turbulence is fundamentally different from HD turbulence. We will discuss a theoretical ansatz to explain the impact of the Lorentz force on the compressive and contractile flow directions.