Leveling of MHD turbulence imbalance in shear flows

TL;DR

This paper demonstrates that large-scale velocity shear in MHD turbulence drives the system toward a balanced state by suppressing imbalance through linear non-modal dynamics, offering new insights into solar wind turbulence.

Contribution

It introduces a linear mechanism for balancing MHD turbulence in shear flows, distinct from nonlinear processes, with implications for solar wind modeling.

Findings

Shear suppresses turbulence imbalance, leading to balanced Alfvén wave energies.

Linear non-modal dynamics cause the transition to balanced turbulence.

Implications for understanding solar wind turbulence in shear flow conditions.

Abstract

We investigate magnetohydrodynamic (MHD) turbulence in plane shear flows with a streamwise background magnetic field in the super-Alfv\'enic regime. We show that the large-scale velocity shear suppresses turbulence imbalance, driving the system toward a balanced state -- the energies of counter-propagating Alfv\'en waves become essentially equal, even at initially perfectly imbalanced Alfv\'enic turbulence. This balancing is due to the shear-induced linear non-modal dynamics of Alfv\'en waves, including their transient growth and over-reflection. This linear route to balancing turbulence is new -- fundamentally different from nonlinear ones operative in shearless MHD turbulence -- and have direct implications for understanding balanced/imbalanced MHD turbulence in the solar wind, which is modeled as a shear flow in a thermodynamically complex plasma.