Fast Reconnection of Magnetic Fields in Turbulent Fluids

Reconnection is the process by which magnetic fields in a conducting fluid change their topology. This process is essential for understanding a wide variety of astrophysical processes, including stellar and galactic dynamos and astrophysical turbulence. To account for solar flares, solar cycles, and the structure of the galactic magnetic field, reconnection must be fast, propagating with a speed close to the Alfvén speed. Earlier attempts to deal with magnetic reconnection assumed that magnetized fluids are laminar and as a result obtained slow reconnection rates. We show that the presence of a random magnetic field component substantially enhances the reconnection rate and enables fast reconnection, i.e., reconnection that does not depend on fluid resistivity. The enhancement of the reconnection rate is achieved via a combination of two effects. First of all, only small segments of magnetic field lines are subject to direct Ohmic annihilation. Thus the fraction of magnetic energy that goes directly into fluid heating goes to zero as fluid resistivity vanishes. However, the most important enhancement comes from the fact that unlike the laminar fluid case where reconnection is constrained to proceed line by line, the presence of turbulence enables many magnetic field lines to enter the reconnection zone simultaneously. A significant fraction of magnetic energy goes into MHD turbulence and this enhances reconnection rates through an increase in the field stochasticity. In this way magnetic reconnection becomes fast when field stochasticity is accounted for. As a consequence solar and galactic dynamos are also fast, i.e., do not depend on fluid resistivity.