Compressibility effect on coherent structures, energy transfer, and scaling in magnetohydrodynamic turbulence

Compressible magnetohydrodynamic (MHD) turbulence, a model often used to study space and astrophysical plasmas, differs from incompressible magnetohydrodynamic and hydrodynamic (HD) turbulence in many ways. Here direct numerical simulations of mechanically forced compressible MHD turbulence are used to study the degree to which some turbulence theories proposed in the incompressible case remain applicable in the compressible one. Several aspects of compressible turbulence are studied: (i) Intermittency in the compressible case is studied by addressing flows driven with varying forcing mechanisms; these display different features, such as compression and coherent structures. The more compressive simulation is characterized by sheet-like current density structures and shocks, which lead to saturated scaling exponents of high order structure functions of density and compressive velocity. (ii) Further investigations employing conditional averages of different energy transfer fluxes reveal that the influence of shocks (or compressions) makes differences in energy cascade and magnetic amplification. (iii) Cascade is also studied by examining the validity of Yaglom-type relations that verify the scale invariant property within the inertial range, where the transfer is relatively free of the effect of pressure dilation. Published by AIP Publishing.