Reconnection Rate and Transition from Ion-coupled to Electron-only Reconnection

Standard collisionless magnetic reconnection couples with both electron and ion dynamics. Recently, a new type of magnetic reconnection, electron-only magnetic reconnection without ion outflow, has been observed, and its reconnection rate has been found to be much higher than that in ion-coupled reconnection. In this paper, using 2D particle-in-cell simulations, we find that when the ion gyroradius is much smaller than the size of the simulation domain, magnetic reconnection is standard with ion outflows. As the ion gyroradius increases, the ion response gradually weakens, and the reconnection rate becomes higher. Electron-only reconnection occurs when the ion gyroradius is comparable to the size of the simulation domain. This trend applies to both strong and weak guide field situations. Therefore, the key factor that controls the transition from ion-coupled reconnection to electron-only reconnection is the ratio between the ion gyroradius and the size of the simulation domain. We further show that, in electron-only reconnection, when the initial electron current sheet is thinner, the reconnection rate and the electron outflow speed are higher.