Magnetic reconnection in space

Models of magnetic reconnection in space plasmas generally consider only a segment of the magnetic field lines. The consideration of only a segment of the lines is shown to lead to paradoxical results in which reconnection can be impossible even in a magnetic field constrained to be curl free or can be at an Alfven rate even when the plasma is a perfect conductor. A model of reconnecting magnetic fields is developed which shows the smallness of the interdiffusion distance delta(d) of magnetic field lines does not limit the speed of reconnection but does provide a reconnection trigger. When the reconnection region has a natural length L-r, the spatial scale of the gradient of magnetic field across the magnetic field lines must reach L-g approximate to 0.3L(r)/ln(L-r/delta(d)) for fast reconnection to be triggered, which implies a current density j approximate to B/mu L-0(g) that is far lower than that usually thought required for fast reconnection. The relation between magnetic reconnection in space and in toroidal laboratory plasmas is also discussed. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754715]