J. R. Shuster

Based on particle‐in‐cell simulations of collisionless magnetic reconnection, the spatiotemporal evolution of electron velocity distributions in the electron diffusion region (EDR) is reported to illustrate how electrons are accelerated and heated. Approximately when the reconnection rate maximizes, electron distributions in the vicinity of the X line exhibit triangular structures with discrete striations and a temperature (T_e) twice that of the inflow region. T_e increases as the meandering EDR populations mix with inflowing electrons. As the distance from the X line increases within the electron outflow jet, the discrete populations swirl into arcs and gyrotropize by the end of the jet with T_e about 3 times that of the X line. Two dominant processes increase T_e and produce the spatially and temporally evolving EDR distributions: (1) electric field acceleration preferential to electrons which meander in the EDR for longer times and (2) cyclotron turning by the magnetic field normal to the reconnection layer.

Results from two‐dimensional particle‐in‐cell simulations of collisionless magnetic reconnection with zero guide field discussed in this paper reveal that around the time when the reconnection rate peaks, electron velocity distributions become highly structured in magnetic islands and open exhausts. Rings, arcs, and counterstreaming beams are generic and lasting components of the exhaust electron distributions. The temporal dependence of electron distributions provides a perspective to explain an outstanding discrepancy concerning the degree of electron anisotropy in reconnection exhausts and enables inference of the reconnection phase based on observed anisotropic electron distributions. Some of the structures predicted by our simulations are confirmed by measurements from the Cluster spacecraft during its encounter with reconnection exhausts in the magnetotail.