Direct observation of ion-electron decoupling in magnetic reconnection

Ion-electron decoupling at ion scale lead to Hall effect, the indicative process of collisionless magnetic reconnection. Past observations have revealed the features of Hall current system in L direction, Hall magnetic field in M direction and Hall electric field in N direction. However until now, there is no observations on the ion-electron decoupling（Hall current）in N direction, where Hall effect originates. In this study, using MMS observation, we find ion-electron decoupling（Hall current）in N direction in the separatrix region at the magnetospheric side of a reconnection current sheet. A comprehensive analysis provides the insight to the process of ion-electron decoupling, regarding electron motion, composition of Hall electron, and charge distribution. From micro mechanism, the results deep our understanding of solar wind-magnetosphere coupling triggered by reconnection, and support the upcoming study based the observation from launched SMILE mission.