Hamiltonian magnetic reconnection

Magnetic reconnection in two dimensional (2D), collisionless, non-dissipati ve regimes is investigated analytically and numerically by means of a finit e difference code in the nonlinear regime where the island size becomes mac roscopic. The cross-shaped structure of the reconnection region, originally reported by Cafaro er nl (1998 Phs. Rev. Lett. 80 20) is analysed as a fun ction of the ratio between the ion sound Larmor radius and the inertial ski n depth. This cross shape structure is found to survive in the presence of weak dissipation. Further insight on the quasi-explosive behaviour of the m agnetic island width as a function of rime and on the spatial structure of the perturbed current density is provided. We confirm that the amount of re connected flux becomes of order unity on the time scale of the inverse line ar growth rare. Results in the collisionless limit are interpreted on the b asis of the Hamiltonian properties of the adopted collisionless, 2D, fluid model. Thus, collisionless reconnection is a fast, non-steady-state process , fundamentally different from 2D resistive magnetic reconnection, of which the Sweet-Parker model is the classic paradigm.