MAGNETIC RECONNECTION, HELICITY DYNAMICS, AND HYPER-DIFFUSION

We examine the influence of noise and Alfven wave turbulence on magnetic reconnection in a reduced magnetohydrodynamics model. We focus on the dynamics of magnetic helicity density. Helicity conservation is then used to calculate the global reconnection rate in terms of the helicity density flux. Two specific scenarios are explored-noisy reconnection and Alfven wave turbulent reconnection. For noisy reconnection, the current sheet is assumed to sit in a noisy state, marginal to plasmoid formation instability. The scaling of the reconnection rate in the presence of noise is proportional to (S-0(2)/ VAL2)(1/11), where S-0(2)/ VAL2 is the relative amplitude of the noise. We obtain this prediction using a symmetry analysis of the helicity density flux. For Alfven wave turbulent reconnection, a mean field closure scheme is applied. A reconnection rate proportional to ( <(B) over tilde2 > / < B >(2))(1/8) is obtained, where <(B) over tilde2 > / < B >(2) and < B > are the relative energy of Alfven wave turbulence and the reconnecting field. The constraint on reconnection rate enforced mean-square magnetic potential conservation is reexamined. A critical magnetic Reynolds number R-m,R- c is identified. For R-m >> R-m,R- c, the reconnection rate becomes independent of Spitzer resistivity and thus can be higher than the Sweet-Parker model prediction. Both cases exhibit a weak dependence of the reconnection rate on the amplitude of the turbulence. Therefore, even noise or weak turbulence can trigger fast reconnection if the system is marginally stable. The important distinction between turbulent reconnection and turbulent dissipation of magnetic energy is also discussed.