Causal Relationship between Current Sheet Flapping, Electron-only Reconnection, and Ion-coupled Reconnection

Magnetic reconnection is a fundamental physical process operating throughout the Universe, yet its onset in planetary magnetotails remains elusive. Current-sheet flapping is a common dynamic phenomenon in planetary magnetotails, with its roles in driving reconnection largely underexplored. This paper reports the first observation of electron-dominant and ion-coupled reconnection accompanied by current sheet flapping. The Magnetospheric Multiscale mission (MMS) spacecraft was located in the premidnight sector current sheet, which was undergoing kink-mode flapping that propagated duskward, away from the midnight sector, with a frequency of 0.06–0.1 Hz and a wavelength of around 0.5 R E . The flapping led MMS to cross the current sheet repeatedly, from north to south and vice versa. During the initial crossing of the current sheet, distinct electron outflows were observed, while ion responses were weak. This indicates either electron-only reconnection, suggesting an early stage of reconnection (temporal effect) or MMS passing through the exhaust region close to an X-line (spatial effect). Signatures of the electron diffusion region’s limited length, yielding an unusually high aspect ratio, suggest that the former scenario is more likely. Regular ion-coupled reconnection that involves ion outflows became more prominent until MMS eventually crossed the X-line from tailward to earthward. This event indicates a potential causal relationship between electron-only reconnection, current sheet flapping, and subsequent ion reconnection. We discuss feasible mechanisms for current sheet flapping and the feedback between flapping and reconnection, providing new insights into the onset of magnetotail reconnection and cross-scale processes of importance to other heliospheric and astrophysical systems.