Description
3D direct numerical simulations have recently opened an important new window into the role of self-generated turbulence in magnetic reconnection at high Lundquist numbers. Interestingly, these simulations exhibit features associated with the Lazarian-Vishniac theory (turbulence and field line dispersion) and with 2D plasmoid mediated reconnection (flux ropes and reconnection rates). Motivated by the unexpected “duality” found in the simulations, we present a new model of magnetic reconnection in the presence of turbulence. Starting from the same equations as Lazarian and Vishniac, we show that the magnetic field line separation in turbulent plasma permits the existence of locally-coherent magnetic structures. Local coherence allows storage of magnetic helicity inside the reconnection layer, typically in the form of locally coherent twisted flux ropes that fray over longer distances. We then introduce the “Alfvén 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 turbulence is sufficiently anisotropic at the perpendicular scale matching the thickness of a marginally-stable current layer. Finally, we conjecture that turbulence generated within the reconnection layer may produce a critically-balanced state that maintains the flux-rope-mediated regime. The new model successfully accounts for the major features of 3D simulations of magnetic reconnection with self-generated turbulence, including reconnection rates of 0.01 in resistive MHD and 0.1 with collisionless physics.
