Stable-mode-mediated nonlinear saturation, transport, and small-scale dissipation in MHD Kelvin-Helmholtz turbulence

Shear-flow plasmas often lead to instability and turbulence. Reduced models of this turbulence commonly employ unstable modes from linear stability analyses and allow nonlinear interaction amongst them to modify the mean flow (via turbulent stresses). This simplified approximation generally overestimates turbulent fluxes. To reliably model these fluxes and better under nonlinear interactions, here we simulate MHD Kelvin-Helmholtz turbulence using Dedalus. We consider a flow-aligned magnetic field, and the flow is forced to retain its shear profile, unlike decaying in the earlier work¹. We find the modes with frequencies complex-conjugate to the unstable ones (called stable modes herein) to be nonlinearly excited to an almost equal level as the unstable modes. These stable modes transfer energy to the mean flow from the large-scale fluctuations, thus reducing the energy cascade to smaller scales. We report that the stable and unstable eigenmode pairs reconstruct the turbulent flow largely and rectify the aforementioned overestimation of turbulent fluxes by capturing the counter-gradient momentum transport via stable modes. We detail the impacts of magnetic fields and forcing on energy transfer channels. ¹Fraser et al. 2021, PoP 28 (2), 022309