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

## TL;DR

This paper demonstrates that inhomogeneities across the magnetic field enable a unidirectional cascade of MHD wave energy, challenging the traditional view that turbulence requires counter-propagating waves, with implications for solar plasma turbulence.

## Contribution

It reveals that perpendicular inhomogeneities induce a self-cascade of unidirectional waves in MHD turbulence, expanding understanding beyond the classical counter-propagating wave paradigm.

## Key findings

- Unidirectional wave solutions arise in inhomogeneous MHD.
- Nonlinear self-deformation leads to wave energy cascade.
- Implications for turbulence dissipation in solar plasma.

## Abstract

It is widely accepted in the MHD turbulence community that the nonlinear cascade of wave energy requires counter-propagating Alfv\'enic 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 Els\"asser fields, already in the incompressible case. One simple example of these wave solutions is the surface Alfv\'en 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 Alfv\'enic waves in structured plasma, as in the solar corona and solar wind.