Description
In turbulent plasma flows, magnetic field and flow fluctuations interact, resulting in complex multi-scale energy transfer and conversion processes. Here, I discuss an analytical method to decompose the energy flux across scales in magnetohydrodynamic (MHD) turbulence into terms that are physically interpretable and measurable from numerical and observational data. Applying this method to data from three-dimensional direct numerical simulations, we identify the physical MHD processes that drive the energy cascade, akin to vortex stretching and strain amplification in hydrodynamic turbulence. Independent of the level of anisotropy, magnetic energy is transferred from large to small scales mainly by current-sheet thinning, a process where current sheets are stretched by large-scale straining motion into regions of magnetic shear at smaller scales – a process with a natural relation to magnetic reconnection. Contributions from current-filament stretching – the analogue to vortex stretching – is negligible. The small-scale magnetic shear generated by current-sheet thinning in turn drives extensional flows at smaller scales, while hydrodynamic processes such as vortex stretching and strain self-amplification are strongly depleted. Hence, magnetic and kinetic energy are transferred from large to small scales almost exclusively by magnetic effects, connected with the generation of regions of small-scale intense strain induced by the Lorentz force. These results become exact in the limit of very strong background magnetic fields. In consequence, we show that all relevant physics driving the energy cascade are present in reduced MHD, as processes that enter at higher order are negligible, irrespective of the plasma beta. I discuss applications of the method to helicity fluxes and Hall physics and present an exact mathematical analogy between the Hall flux and the kinetic helicity flux, and show in the strong-field limit that magnetic energy transfer due to Hall physics is given by current-sheet thinning, now due shear currents, again a process with a natural relation to magnetic reconnection. Implications of these results for subgrid-scale modelling of magnetohydrodynamic turbulence are discussed.