Unstable magnetic reconnection self-generates turbulence

Abstract

Magnetic reconnection and turbulence are deeply intertwined in magnetohydrodynamic flows, yet how reconnection self-generates turbulence remains unclear. Using an ensemble of high-resolution three-dimensional direct numerical simulations of an unstable magnetised jet with an initially weak mean field, we demonstrate a self-sustained transition from laminar reconnection to fully developed turbulence in the absence of external forcing. We show that a three-dimensional current-sheet instability triggers stochastic reconnection, leading to persistent turbulent energy injection. Energy-budget analysis reveals that the coupling between the turbulent electromotive force and the magnetic mean shear dominates turbulent production, with magnetic fluctuations subsequently transferring energy to the kinetic field through a nonlinear cascade.

Figures (6)

• Figure 1: Planar view of the transitioning jet at representative times. Mean-field magnetic field lines (black), the 99th percentile of the current-density magnitude (blue), and the second invariant of the velocity-gradient tensor (red) are displayed.
• Figure 2: Time evolution of the magnetic energy growth rate $\gamma$. The linear scaling $\gamma \propto t$ is shown for reference. Distinct dynamical regimes are identified: stochastic reconnection, nonlinear dynamo, and fully developed turbulent flow. Markers A–D denote the representative time instants discussed in the text.
• Figure 3: Turbulent energy budget across the transition and cascade. Energy fluxes entering the turbulent domain via production are treated as 'gain', while fluxes removed through dissipation are treated as 'loss'.
• Figure 4: Dominant energy fluxes in turbulent production and transfer: EMF–magnetic-mean-shear drives production, and the resulting energy cascades between domains; dissipation and minor transport are omitted for clarity.
• Figure 5: Magnetic field topology and turbulent energy production. Planar view of $\mathcal{E} \cdot \overline{\mathbf{J}}$ (shading) and mean magnetic field lines (contours) at $t=82$ and $t=88$. The figure illustrates the self-generation of turbulent energy within reconnection sites, driving the growth of the SGTR instability. Note the emergence of sink terms (blue) during the advanced stages of reconnection at $t=88$.