Secondary reconnection sites in reconnection-generated flux ropes and reconnection fronts

The primary target of the Magnetospheric MultiScale (MMS) mission is the electron-scale diffusion layer around reconnection sites. Here we study where these regions are found in full three-dimensional simulations. In two dimensions the sites of electron diffusion, defined as the regions where magnetic topology changes and electrons move with respect to the magnetic field lines, are located near the reconnection site. But in three dimensions we find that the reconnection exhaust far from the primary reconnection site also becomes host to secondary reconnection sites. Four diagnostics are used to demonstrate the point: the direct observation of topology impossible without secondary reconnection, the direct measurement of topological field line breakage, the measurement of electron jets emerging from secondary reconnection regions, and the violation of the frozen-in condition. We conclude that secondary reconnection occurs in a large part of the exhaust, providing many more chances for MMS to find itself in the right region to hit its target. New three-dimensional simulations of magnetic reconnection suggest the existence of secondary reconnection sites that could be observed by the new NASA Magnetospheric MultiScale Mission.