THE COALESCENCE INSTABILITY IN COLLISIONLESS PLASMAS

The process whereby magnetic island structures tend to merge into larger units is known as magnetic island coalescence. This coalescence process is investigated for the case of a collisionless plasma by means of two-dimensional particle simulations. The pairwise merging of a preformed island chain leads to a gentle redistribution of the density and current density profiles. The linear growth rates substantially exceed the magnetohydrodynamic (MHD) values for rho/lambda > 0.1, where rho is the asymptotic ion gyroradius and lambda is the current sheet half-thickness. The nonlinear evolution of the collisionless tearing instability in a long system is found to lead to a much more violent coalescence process resulting in the formation of a vacuum X region. This stage is characterized by enhanced growth rates for the long-wavelength modes that exceed the maximum linear tearing growth rate, super-Alfvenic flows away from the reconnection X line, substantial bulk heating in the parallel direction, and the formation of energetic particle tails in the direction of the inductive electric field.