A two-dimensional hybrid simulation of the magnetotail reconnection layer

Two-dimensional (2-D) hybrid simulations are carried out to study the structure of the reconnection layer in the distant magnetotail. In the simulation an initial current sheet separates the two lobes with antiparallel magnetic field components in the x direction. The current sheet normal is along the z direction. It is found that a leading bulge-like magnetic configuration and a trailing, quasi-steady reconnection layer are formed in a magnetic reconnection. If the duration of the reconnection is sufficiently long, the trailing reconnection layer will dominate the plasma outflow region. For the symmetric lobes with B-y = 0, two pairs of slow shocks are present in the quasi-steady reconnection layer. The slow shocks are expected to be fully developed at a sufficient distance from the X line, where the separation between the two shocks is greater than a few tens of the lobe ion inertial length. The Rankine-Hugoniot jump conditions of the slow shock are found to be better satisfied as the distance from the X line along the x axis increases. For the cases with B-y not equal 0 in the two lobes, two rotational discontinuity-like structures appear to develop in the reconnection layer. On the other hand, in the leading bulge region of a magnetic reconnection, no steady MHD discontinuities are found. Across the plasma sheet boundary layer the increase of the how velocity appears to be much smaller than that predicted from the Rankine-Hugoniot jump conditions for a steady discontinuity, and the increase in the ion number density is much larger. In addition, a large increase in the parallel ion temperature is found in the plasma sheet boundary layer. The 2-D simulation results are also compared with the one-dimensional hybrid simulations for the Riemann problem associated with the magnetotail reconnection. It is found that the 2-D effects may lead to the presence of the non-switch-off slow shocks and thus the lack of coherent wave trains in slow shocks.