Norbert Magyar (KU Leuven): Uniturbulence, a new type of MHD turbulence
Abstract: The current understanding of the MHD turbulence generation mechanism originated in the works of Iroshnikov (1963) and Kraichnan (1965) (IK for short), and is based on the Alfvén effect: the nonlinear cascade of energy, essential in turbulence, is the result of successive collisions between counter-propagating Alfvén waves along the mean magnetic field. This theory is still at the basis of current models in MHD turbulence. However, this approach is strictly valid only in incompressible and homogeneous (and infinite) plasma, under which conditions the only existing wave type is that of purely magnetically-driven Alfvén waves. Allowing for compressibility introduces a much richer spectrum of MHD waves, especially if the plasma inhomogeneity is also accounted for. Recently, using full 3D MHD numerical modelling of the propagation of Alfvénic waves in a plasma inhomogeneous in the perpendicular direction, Magyar et al. (2017) showed that due to the modified nature of the MHD waves in inhomogeneous plasmas, characterised by linear coupling of incompressive and compressive perturbations, the Elsässer variables usually representing the up and down-propagating pure Alfvén waves along a mean magnetic field are both nonzero even for unidirectionally propagating waves.
This leads to the same nonlinear advection term in the MHD equations being nonzero as in the IK phenomenology, but now for the waves propagating only in one direction. We name this new turbulence generation mechanism in inhomogeneous plasmas uniturbulence, stemming from the unidirectional wave cascade. Plasmas which are permeated mostly by waves propagating in one direction include coronal holes and the solar wind, thus these are expected to be the most influenced by the uniturbulence. This finding constitutes a paradigm shift in our understanding of the MHD turbulence as a whole, opening up a new alley in turbulence studies. Most excitingly, it might be relevant to the decades-long unsolved problem of coronal heating, especially in open magnetic field regions.