Electron-Only Reconnection as a Transition From Quiet Current Sheet to Standard Reconnection in Earth's Magnetotail: Particle-In-Cell Simulation and Application to MMS Data

Standard magnetic reconnection couples with both electron- and ion dynamics. Recently, a new type of magnetic reconnection, electron-only reconnection without the coupling of ion dynamics, has been observed in space. Using a two-dimensional particle-in-cell simulation, we show that in the externally-driven magnetotail, electron-only reconnection is a transition from quiet current sheet to standard reconnection. We find that (a) energy conversion j center dot E' is around zero in quiet current sheet but nonzero in electron-only reconnection, and (b) ion temperature does not change across electron-only reconnection but peaks at the center of standard reconnection. These two differences can be used as criteria to distinguish magnetotail electron-only reconnection from quiet current sheet and standard reconnection, respectively. Based on the two criteria, we justify that the MMS 17 June 2017 event is a magnetotail electron-only reconnection event. Plain Language Summary The Earth has a long stretched magnetic tail, that is, magnetotail, formed by the interaction between the geomagnetic field and the plasma (ions and electrons) wind originated from the Sun. At the center of the magnetotail, there is a current sheet flanked by two different topologies of magnetic field lines. Magnetic reconnection, a fundamental plasma process during which magnetic field line topologies change, can occur in this magnetotail current sheet. Based on computer simulations in the past 20 years, a standard reconnection model with both ion and electron dynamics has been established, which can well describe magnetic reconnection observed in the magnetotail. Recently, a new type of reconnection with solely electron dynamics has been observed to occur in the magnetotail. Here using computer simulations, we show that electron-only reconnection in the magnetotail is a transition from magnetotail current sheet to standard reconnection. Based on the simulation results, we give two criteria for identification of electron-only reconnection in spacecraft observations in the magnetotail, using which we justify that the MMS 17 June 2017 event is a magnetotail electron-only reconnection event.