Unsteady, “bursty reconnection” : Tearing mode instability
fragments magnetic field into magnetic islands, which form
magnetically neutral O and X-points.
   - Tearing mode growth rate: t_G,tear = [t_diff * t_A]^1/2
   - diffusive time scale: t_diff= L^2/eta
     (2L=width of current sheet, eta=magnetic diffusivity)