Abstract:
There are several types of coherent structures in magnetohydrodynamic (MHD) turbulence, including current and vortex structures. The article reviews some of the recent works on the study of coherent structures and the related energy transfer in MHD turbulence. Different from the tubular structures in hydrodynamic (HD) turbulence, the coherent structures in MHD are mostly sheet-like structures, with strong current sheets accompanied by sheet-like vortex structures. Unlike the energy cascade in HD turbulence, where kinetic energy is transferred from large scales to small scales locally, the energy cascade in MHD is more complicated. In addition to the local transfer of kinetic energy, there is also a local transfer of magnetic energies across scales, as well as a non-local conversion between kinetic and magnetic energies. Studies have shown that energy transfer and dissipation in MHD turbulence are strongly intermittent, concentrated in a small area, and associated with coherent structures. In the presence of a strong background magnetic field, the coherent structures in MHD turbulence are elongated in the direction of the background magnetic field, while energy transfer in the same direction is suppressed. In the case of compressible MHD turbulence, the generation of shocks significantly enhances the energy transfer, and at the same time brings a strong intermittency. The scaling of density structure functions is found far from a linear scale, and scaling exponent saturates with the increase of the order of structure functions.