Ion dynamics and resultant velocity space distributions in the course of magnetotail reconnection

We study a large‐scale magnetic reconnection using 2‐D hybrid simulation (ion particle, charge neutralizing massless electron fluid). After formation of fast plasma jets from the diffusion region, a quasi‐steady state is achieved. The outstanding difference in field configuration between the present result and the MHD is an appearance of global cross‐tail magnetic field component with a halflobe field strength. Inside the fast plasma jet, ion distribution functions show various features that can be classified into three types according to the distances from the reconnection line: (1) the modified Speiser distribution observed near the reconnection line, (2) the counterstreaming ion distribution observed around the jet front where the plasma density is compressed and the field lines are piled up due to the collision of the plasma jet with the preexisting plasma sheet, and (3) the partial‐shell distribution observed in the region between the above two. The ion dynamics leading to the formation of these characteristic distributions are fairly well understood with the aid of test particle studies. The results are expected to serve as a guide to identify the reconnection dynamics on the basis of the ion distribution functions.