The diffusion region in collisionless magnetic reconnection

Abstract

The structure of the dissipation region in collisionless magnetic reconnection is investigated by means of kinetic particle-in-cell simulations and analytical theory. Analyses of simulations of reconnecting current sheets without guide magnetic field, which keep all parameters fixed with the exception of the electron mass, exhibit very similar large scale evolutions and time scales. A detailed comparison of two runs with different electron masses reveals very similar large scale parameters, such as ion flow velocities and magnetic field structures. The electron-scale phenomena in the reconnection region proper, however, appear to be quite different. The scale lengths of these processes are best organized by the trapping length of bouncing electrons in a field reversal region. The dissipation is explained by the electric field generated by nongyrotropic electron pressure tensor effects. In the reconnection region, the relevant electron pressure tensor components exhibit gradients which are independent of the electron mass. The similarities 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 independence 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 locally 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 the hypothesis that the local physics in the dissipation region adjusts itself to the demands of the large-scale evolution. A further verification of this notion is provided by Hall-magnetohydrodynamic simulations which employ simple resistive dissipation models in otherwise similar large-scale models. These results also pertain to the inclusion of local reconnection physics in larger scale simulation models.