Spontaneous Evolution From Electron-Only to Standard Reconnection in a Force-Free Current Sheet

Electron-only reconnection is a novel reconnection regime observed recently in the turbulent magnetosheath and magnetotail, related to energy conversion and large-scale reconnection events. However, the relationship between this novel regime and standard kinetic reconnection is still not fully understood. In this paper, we investigate the spontaneous evolution from electron-only to standard reconnection in a force-free current sheet under guide fields using 2.5-dimensional particle-in-cell (PIC) simulations. Our results reveal that reconnection occurs within the electron-only regime initially, where ion outflow jets and ion heating are absent. This absence is attributed to the spatial limitations of the downstream region, which restricts the influence of electric forces, thereby preventing significant ion acceleration. Consequently, the negligible ion outflow velocities result in minimal work done by the pressure gradient term, and the contribution of ion enthalpy flux remains insignificant, thereby leading to the lack of ion heating during this phase. As reconnection proceeds, the available space for ion acceleration increases, allowing for both ion heating and the development of ion outflow jets, which ultimately results in a temporal evolution to the standard kinetic reconnection regime. We further examine the effect of ion temperature on this temporal evolution and discover that high ion temperature inhibits this temporal evolution from electron-only to standard reconnection, confining reconnection to the electron-only regime throughout the simulation even though the simulation domain is large. This work provides insights into the role of electron-only reconnection in the magnetosheath.