Inhomogeneous Two-Stream Instability and Electron Holes near the Magnetic-Reconnection Separatrix

The bipolar electric-field signatures of electron holes have been observed by satellites in the vicinity of magnetospheric reconnection sites and are a common feature of many 2D and 3D reconnection simulations. One class of holes observed in 2D simulations are found near the magnetic separatrix branches, which divide the inflow plasma from the reconnection exhaust. Such holes have been seen in both guide-field and anti-parallel simulations, with mass ratios up to that of hydrogen. In guide-field simulations, a strong electron flow (fluid velocity moment) toward the x-point along one pair of separatrix arms has been thought to drive holes through an e-i (Buneman) instability. Here, we present results of an analysis of electron phase space and particle orbits from a series of simulations with different guide fields. These simulations indicate that the holes are localized to field lines on the exhaust side of the separatrix, where the electron fluid velocity moment is textit{not} strong. Electron phase space reveals that these regions contain electron beams counter-streaming parallel to B, suggesting that the holes here can be driven by an inhomogeneous e-e two-stream instability on these field lines. The magnetic mirror force is found to play a key role in the spatial dependence of the parallel electron distribution. One consequence of this spatially inhomogeneous distribution function is the existence of a "stagnation point" with holes on one side moving toward the x-point and holes on the other side moving away from the x-point.