Ion kinetic effects in magnetic reconnection: Hybrid simulations

Magnetic reconnection in a low beta plasma is studied by using a large-scale two-dimensional hybrid simulation code that treats the ions fully kinetically and the electrons as a massless fluid. Reconnection is realized by localizing an anomalous resistivity in the the center of a Harris-type current sheet. The results are applicable to reconnection in the geomagnetic tail. Within a distance of about 200 ion inertial lengths from the neutral line the fast reconnection Jet is characterized over most of the region by partial shell type ion distributions; i.e., the distribution in the central current sheet is not thermalized. Inward drifting cold lobe ions perform in the center of the reconnection wedge due to the small magnetic field curvature Speiser-type orbits and are subsequently ejected again onto lobe field lines. This stadium is similar to the collisionless reconnection scenario described by Hill [1975]. The cross-tail current of the thin current sheet is supported by the drifting ions. This current sheet warps and an instability develops, which leads further away from the neutral line to a filamentation of the cross tail current. These simulations suggested that after near-Earth reconnection proceeds to lobe magnetic field lines a postplasmoid plasma sheet with a thin current sheet builds up, which is of a boundary layer type. The thin current sheet is instable. At about 200 ion inertial lengths away from the neutral line the instability has reached a nonlinear state and the cross-tail current becomes patchy; the incoming and ejected cold lobe ions are isotropized in the ensuing magnetic field and thus constitute a hot plasma sheet distribution. This can then support the occurrence of slow mode shocks at the boundary of the reconnection layer with backstreaming ions upstream of the shocks.