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Magnetic reconnection is a fundamental process known to play a crucial role in electron acceleration and heating, however, the mechanism of electron energization during reconnection is still not fully understood. This study introduces a novel electron acceleration mechanism in which electrons can be accelerated by secondary reconnection in the separatrix region. The secondary reconnection occurs in a thin current sheet resulted from the shear of the out‐of‐plane Hall magnetic fields of the primary magnetopause reconnection. It results in the intense electron energy fluxes toward the primary X‐line. This mechanism will likely be an important piece in the puzzle of particle acceleration by reconnection. Plain Language Summary: Magnetic reconnection is a fundamental process that plays a critical role in electron acceleration and heating. The separatrix region of magnetic reconnection, which distinguishes the inflow and outflow regions, is a crucial area in the particle acceleration of the reconnection process. However, previous studies mainly focused on the electron acceleration mechanism in the separatrix region under two‐dimensional reconnection, without considering the influence of the three‐dimensional structures. This paper reports, for the first time, the secondary reconnection occurring in the opposite Hall magnetic field in the three‐dimensional separatrix region of magnetopause primary reconnection. This secondary reconnection can accelerate and heat the inflow electrons of the primary reconnection, which provides important clues on how the particles are accelerated and heated by reconnection. Key Points: Secondary reconnection occurs between Hall magnetic fields in the separatrix region of the primary magnetopause reconnectionIntense electron enthalpy fluxes are injected toward the primary X‐line along the separatrixElectrons are accelerated by the secondary reconnection before being injected into the primary X‐line for further energization [ABSTRACT FROM AUTHOR]

This paper presents the magnetospheric multiscale (MMS) observations of three flux ropes (FRs) sequentially in a reconnection exhaust at Earth's magnetopause. Three active X‐lines, which are responsible for the formation of these FRs, were also detected by MMS, providing strong evidence that the FRs were generated by multiple X‐line reconnection. We find that the core field inside the FRs considerably affects the Hall current in the adjacent X‐lines. The intense core field of the FRs deflected the electron outflow jet, forming the electron‐scale Hall magnetic field on the magnetosheath side. Additionally, an active secondary reconnection was observed at the center of one FR. This secondary reconnection split the FR into two smaller FRs and produced sharp density gradients and intense fluctuations of electric field and current within the FR. These observations advance the understanding of the formation and evolution of FRs during multiple X‐line reconnection at magnetopause. Plain Language Summary: Flux transfer event (FTE) is generally explained as the flux rope (FR) generated by multiple X‐line reconnection at the dayside magnetopause. Although the theory and simulations have shown that the multiple active X‐lines and FRs can be produced simultaneously by the tearing instability in the magnetopause current sheet, there is no in‐situ evidence to directly support this scenario yet. Using high‐resolution measurements from the magnetospheric multiscale mission, we present a case study that all active X‐lines expected at the edges of three successive FRs were observed. These observations confirm that reconnection could simultaneously occur at multiple sites to produce the FRs at Earth's magnetopause. Interestingly, the electron‐scale Hall magnetic field was observed at the magnetosheath side in this multiple X‐line reconnection. This is due to the electron outflow jet deflected into the magnetosheath side by the strong core field of FR. Additionally, an ongoing secondary reconnection was observed at the center of one FR. Intense waves and current perturbations were observed in a smaller FR which was produced by this secondary reconnection. Key Points: Ongoing multiple X‐line reconnection generates a series of flux ropes at the magnetopauseElectron‐scale Hall magnetic fields are formed due to the high deflection of electron jets caused by the core field of the flux ropeSecondary reconnection within the flux rope split the flux rope into smaller flux ropes and produces intense electromagnetic fluctuations [ABSTRACT FROM AUTHOR]