Kinetic properties of magnetic merging in the coalescence process

The magnetic merging process associated with pairwise magnetic island coalescence is investigated using two-dimensional particle-in-cell simulations for the case where the initial island separation zeta is in the range of 3-12c/omega(pi), where c/omega(pi) is the ion inertia length. In this regime the coalescence process is driven by the electrons, the electron and ion bulk flows decouple on the global island scale (the electron flows are much larger than those for the ions), there is no magnetic flux pileup near the merging line, and the X-O line separation drops smoothly to zero on a time scale of the order of twice the linear e-folding time for the coalescence instability. For fixed island aspect ratio, the scaling of the merging electric field E-y as a function of zeta is rather weak; i.e., similar to zeta(-0.5). The magnitude of E-y, however, is strongly dependent on the magnitude of the current concentration at the initial O lines, suggesting that driven merging does not exhibit a universal rate. These kinetic results support the existence of a regime with merging rates faster than the linear Alfven scaling with island size, but we were not able to observe the transition between these two regimes. (C) 2007 American Institute of Physics.