The diffusion region in collisionless magnetic reconnection

The structure of the dissipation region in collisionless magnetic reconnect ion is investigated by means of kinetic particle-in-cell simulations and an alytical theory. Analyses of simulations of reconnecting current sheets wit hout guide magnetic field, which keep all parameters fixed with the excepti on of the electron mass, exhibit very similar large scale evolutions and ti me scales. A detailed comparison of two runs with different electron masses reveals very similar large scale parameters, such as ion flow velocities a nd magnetic field structures. The electron-scale phenomena in the reconnect ion region proper, however, appear to be quite different. The scale lengths of these processes are best organized by the trapping length of bouncing e lectrons in a field reversal region. The dissipation is explained by the el ectric field generated by nongyrotropic electron pressure tensor effects. I n the reconnection region, the relevant electron pressure tensor components exhibit gradients which are independent of the electron mass. The similari ties of the gradients as well as the behavior of the electron flow velocity can be derived from the electron trapping scale and the electron mass inde pendence of the reconnection electric field. A further model which includes a significant guide magnetic field exhibits almost identical behavior. The explanation of this result lies in a Hall-type electric field which locall y eliminates the magnetizing effect on the electrons of the guide magnetic field. The resulting electron dynamics is nearly identical to the one found in the model without guide magnetic field. This result strongly supports t he hypothesis that the local physics in the dissipation region adjusts itse lf to the demands of the large-scale evolution. A further verification of t his notion is provided by Hall-magnetohydrodynamic simulations which employ simple resistive dissipation models in otherwise similar large-scale model s. These results also pertain to the inclusion of local reconnection physic s in larger scale simulation models. (C) 1999 American Institute of Physics . [S1070-664X(99)97405-0].