Highly structured electron anisotropy in collisionless reconnection exhausts

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.