Electron-Only Magnetic Reconnection: Lessons Learned From Magnetic Island Coalescence

Electron-only magnetic reconnection, in which ions do not couple, has recently been observed in turbulent magnetosheath and magnetotail, and in association with magnetic flux rope coalescence. This study reproduces the electron-only reconnection during magnetic island coalescence via 2.5-D particle-in-cell simulations. We find that the degree of ion coupling is not sensitive to the spatial scale of the merging current sheet but is determined by the persistent time of the coalescence, which suggests that the electron-only reconnection is the early stage of the coalescence. During the electron-only reconnection, ions are affected by the reconnection electric field though the ion bulk outflow is absent. It is revealed that the pre-existing inflows and Hall electric field E-z from the adjacent main reconnection impede the formation of ion outflow during coalescence. This study provides in-depth knowledge of electron-only reconnection through the perspective of ion kinetics.