Onset of collisionless magnetic reconnection in thin current sheets: Three-dimensional particle simulations

Abstract

Three-dimensional (3D) particle-in-cell simulations of collisionless magnetic reconnection are presented. The initial equilibrium is a double Harris-sheet equilibrium and periodic boundary conditions are assumed in all three directions. No magnetic seed island is imposed initially, and no flow conditions are imposed. The current sheet width is assumed to be one ion inertial length calculated with the density in the center of the current sheet. The ion to electron mass ratio is mi/me=150, which suppresses the growth of the drift kink instability. Two different runs have been performed: a simulation with exactly antiparallel magnetic field and a simulation with a constant guide field of the same magnitude as the antiparallel field superimposed. In the antiparallel case the inductive field of the waves excited by the lower hybrid drift instability (LHDI) leads to rapid acceleration of the electrons in the center of the current sheet and subsequently to a current sheet thinning. The current increase in the center is balanced by reverse currents in the gradient region. In the thin current sheet rapid reconnection sets in which self-organizes into a two-dimensional structure with a single X line. However, ∼15% of the total flux is reconnected while reconnection is still patchy and 3D. In the guide field case the growth rate of the LHDI is reduced, but leads nevertheless after a considerably longer time to electron acceleration in the current sheet center and to a thinning of the current layer, followed by single X line reconnection. It is suggested that electron acceleration due to LHDI in current sheets of the order of the ion scale results in rapid onset of reconnection.