Electron crossover and diffusion region width in symmetric, collisionless magnetic reconnection

We investigate electron-scale dynamics near the X-line during steady-state collisionless magnetic reconnection without a guide field, using fully kinetic particle-in-cell simulations. By labeling electrons from opposite inflow regions, we examine the evolution of their bulk flows, force balance, and contributions to the electron current layer. The interpenetration of directed labeled flows from both sides gives rise to off diagonal pressure tensor elements, such that a significant contribution to the reconnection electric field arises from the crossover of electron flows originating from the two different inflow regions. We further show that the bulk acceleration of each electron population determines the size of the electron diffusion region near the X-line. This work demonstrates how flow labeling provides new insights into reconnection structure, revealing behaviors that are otherwise inaccessible with conventional computational diagnostics.