Double layer electric fields aiding the production of energetic flat-top distributions and superthermal electrons within the exhausts from magnetic reconnection

Using a kinetic simulation of magnetic reconnection it was recently shown that magnetic-field-aligned electric fields (E||) can be present over large spatial scales in reconnection exhausts. The largest values of E|| are observed within double layers. The existence of double layers in the Earth's magnetosphere is well documented. In our simulation their formation is triggered by large parallel streaming of electrons into the reconnection region. These parallel electron fluxes are required for maintaining quasi-neutrality of the reconnection region and increase with decreasing values of the normalized electron pressure upstream of the reconnection region. A threshold normalized pressure is derived for strong double layers to develop. We also document how the electron confinement, provided in part by the structure in E||, allows sustained energization by perpendicular electric fields. The energization is a consequence of the confined electrons' chaotic orbital motion that includes drifts aligned with the reconnection electric field. The level of energization is proportional to the initial particle energy and therefore is enhanced by the initial energy boost of the acceleration potential, acquired by electrons entering the region. The mechanism is effective in an extended region of the reconnection exhaust allowing for the generation of superthermal electrons in reconnection scenarios, including those with only a single x-line. An expression for the phase-space distribution of the superthermal electrons is derived, providing an accurate match to the kinetic simulation results. The numerical and analytical results agree with detailed spacecraft observations recorded during reconnection events in the Earth's magnetotail.