Understanding uniturbulence: self-cascade of MHD waves in the presence of inhomogeneities

Abstract

It is widely accepted in the MHD turbulence community that the nonlinear cascade of wave energy requires counter-propagating Alfvénic wave-packets, along some mean magnetic field. This fact is an obvious outcome of the MHD equations under the assumptions of incompressibility and homogeneity. Despite attempts to relax these assumptions in the context of MHD turbulence, the central idea of turbulence generation persists. However, once the assumptions of incompressiblity and homogeneity break down, the generally accepted picture of turbulent cascade generation is not universal. In this paper, we show that perpendicular inhomogeneities (across the mean magnetic field) lead to propagating wave solutions which are necessarily described by co-propagating Elsässer fields, already in the incompressible case. One simple example of these wave solutions is the surface Alfvén wave on a planar discontinuity across the magnetic field. We show through numerical simulations how the nonlinear self-deformation of these unidirectionally propagating waves leads to a cascade of wave energy across the magnetic field. The existence of this type of unidirectional cascade might have an additional strong effect on the turbulent dissipation rate of dominantly outward propagating Alfvénic waves in structured plasma, as in the solar corona and solar wind.

arXiv commentApJ manuscript, accepted 24th of July 2019

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• A statistical study of propagating MHD kink waves in the quiescent corona
• Propagating Kink Waves in an Open Coronal Magnetic Flux Tube with Gravitational Stratification: Magnetohydrodynamic Simulation and Forward Modelling
• Unveiling the True Nature of Plasma Dynamics from the Reference Frame of a Super-penumbral Fibril
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• On collective nature of nonlinear torsional Alfvén waves
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