Mass and internal-energy transports in strongly compressible magnetohydrodynamic turbulence

Abstract

Turbulent mass and internal-energy transports in strongly compressible magnetohydrodynamic (MHD) turbulence are investigated in the framework of the multiple-scale direct-interaction approximation, an analytical closure scheme for inhomogeneous turbulence at very high Reynolds numbers. Utilising the analytical representations for the turbulent mass and internal-energy fluxes and their transport coefficients, which are expressed in terms of the correlation and response functions, turbulence models for these fluxes are proposed. In addition to the usual gradient-diffusion transports, cross-diffusion transports mediated by the density variance and the transports along the mean magnetic field mediated by the compressional or dilatational turbulent cross-helicity (velocity–magnetic-field correlation coupled with compressive motions) are shown to arise. These compressibility effects are of fundamental importance since they provide deviations from the usual gradient-diffusion transports. Analogies of the dilatational cross-helicity effects to the magnetoacoustic waves are also argued.