Leveling of MHD turbulence imbalance in shear flows

We investigate magnetohydrodynamic (MHD) turbulence in plane shear flows with a streamwise background magnetic field in the super-Alfvénic regime, i.e., when the flow velocity is larger than the local Alfvén speed. In this regime, turbulence is energetically supplied by large-scale shear of the flow via transient or nonmodal amplification of Alfvén waves, which are the building blocks of MHD turbulence, and streamwise-uniform, mostly nonmagnetic modes. We show that shear reduces turbulence imbalance, driving the system toward a balanced state-the energies of counter-propagating Alfvén waves become essentially equal, even for an initially perfectly imbalanced Alfvénic turbulence. This balancing occurs due to the shear-induced linear nonmodal dynamics of Alfvén waves-their nonmodal growth, resulting in over-reflection, and coupling between counterpropagating wave branches. Because of its linear origin, this process of balancing of MHD turbulence by shear is fundamentally different from the nonlinear processes operating in standard MHD turbulence in the absence of a mean shear flow. It can have direct implications for understanding the balanced/imbalanced nature of MHD turbulence in shear regions of the solar wind.