Why does steady-state magnetic reconnection have a maximum local rate of   order 0.1?
  Simulations suggest collisionless steady-state magnetic reconnection of Harris-type current sheets proceeds with a rate of order 0.1, independent of dissipation mechanism. We argue this long-standing puzzle is a result of constraints at the magnetohydrodynamic (MHD) scale. We perform a scaling analysis of the reconnection rate as a function of the opening angle made by the upstream magnetic fields, finding a maximum reconnection rate close to 0.2. The predictions compare favorably to particle-in-cell simulations of relativistic electron-positron and non-relativistic electron-proton reconnection. The fact that simulated reconnection rates are close to the predicted maximum suggests reconnection proceeds near the most efficient state allowed at the MHD-scale. The rate near the maximum is relatively insensitive to the opening angle, potentially explaining why reconnection has a similar fast rate in differing models. 
Yi-Hsin Liu, M. Hesse, F. Guo, W. Daughton, H. Li, P. A. Cassak and M.   A. Shay
physics.plasm-ph astro-ph.EP astro-ph.HE astro-ph.SR physics.space-ph