Leveling of MHD turbulence imbalance in shear flows

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.