Cluster observations of bidirectional beams caused by electron trapping during antiparallel reconnection

So-called bidirectional electron beams have been observed by a number of spacecraft missions mainly in the inflow of reconnection regions. Here we show that these beam-like features in the electron distribution function are explained by electron trapping. The trapping is mainly controlled by a positive acceleration potential, Phi(parallel to), which is related to the structure of the parallel electric fields in the vicinity of the reconnection region. Guided by the results of a kinetic simulation, we extend a recent analytical model for the electron distribution function applicable to the inflow region in antiparallel reconnection. The model is successfully compared to data observed by the four Cluster spacecraft inside an active reconnection region. The anisotropy recorded in the electron distributions is consistent with mainly electric trapping of electrons by Phi(parallel to). In the analysis we determine the profiles of Phi(parallel to) along the paths of the Cluster spacecraft during their encounter with a reconnection region. Typical values of e Phi(parallel to) are in excess of 1 keV (much higher than the electron temperature in the ambient lobe plasma) and Phi(parallel to) traps all thermal electrons. This is important for the internal structure of the Hall current system associated with the ion diffusion region because extended trapping significantly alters the electrical and kinematic properties of the electron fluid. Finally, at the boundary between the inflow and exhaust regions the values of e Phi(parallel to) in the inflow region smoothly approach the shoulder energies (up to 15 keV) of the so-called flat-top distribution observed in the reconnection exhaust. This suggests that Phi(parallel to) may be important also to the formation of the flat-top distributions.