Flux-Rope Mediated Turbulent Magnetic Reconnection

Abstract: We present a new model of magnetic reconnection in the presence of turbulence, applicable when the magnetic helicity is non-zero. The new model differs from the Lazarian-Vishniac turbulent reconnection theory by emphasizing the role of locally coherent magnetic structures, whose existence is shown to be permitted by the properties of magnetic field line separation in turbulent plasma. Local coherence allows storage of magnetic helicity inside the reconnection layer and we argue that helicity conservation produces locally coherent twisted flux ropes. We then introduce the "Alfv\'en horizon" to explain why the global reconnection rate can be governed by locally coherent magnetic field structure instead of by field line wandering, formally extending to 3D the principle that reconnection can be made fast by fragmentation of the global current layer. Coherence is shown to dominate over field line dispersion if the anisotropy of the turbulence at the perpendicular scale matching the thickness of a marginally stable current layer exceeds the aspect ratio of the current layer. Finally, we conjecture that turbulence generated within the reconnection layer may produce a balance of anisotropies that maintains the system in the flux-rope mediated regime. The new model successfully accounts for the major features of 3D numerical simulations of self-generated turbulent reconnection, including reconnection rates of 0.01 in resistive MHD and 0.1 with collisionless physics.