Electron-scale physics in magnetic reconnection

Magnetic reconnection is one of the important physical mechanisms for the conversion of magnetic energy into plasma energy, which is closely related to many explosive phenomena in space. Magnetic reconnection is a cross-scale physical process, in which the physical process at the electron-scale plays key roles in triggering reconnection and facilitating reconnection fast, hence it has attracted much attention. After the launch of the magnetospheric multiscale (MMS) satellite in 2015, the unprecedented high-resolution data provided by MMS enable us to have a new understanding of the electron-scale physics. This paper briefly summarizes the recent research progresses of electron-scale physics in magnetic reconnection based on the observational results of MMS. We divided the reconnection region into the electron diffusion region, ion diffusion region and reconnection exhaust, and summarized the electron scale physical processes from many aspects. The main contents are as follows: The electron diffusion region can form a multiple structure in the inflow or outflow direction, and its formation process may involve a variety of instabilities; The crescent distribution in the electron diffusion region is affected by many factors, such as Hall electric field and electron meandering motion, so that the electron distribution function in this region no longer presents a crescent distribution in some cases; Plasma waves in the electron diffusion region can effectively heat electrons and provide anomalous resistance to break the frozen-in conditions, which plays a key role in reconnection; The non-ideal electric field in the electron diffusion region can be balanced by the electron pressure tensor term, but the anomalous resistance term may dominate in some events; The formation and evolution of electron scale coherent structures in the ion diffusion region are very important for energy conversion and dissipation, including the turbulence driven by the merging of magnetic islands, the secondary reconnection triggered by Kelvin-Helmholtz instability and the formation of kinetics scale magnetic holes; The coalescence of magnetic flux ropes and the magnetic reconnection between the magnetic flux ropes and the geomagnetic field in the reconnection exhaust can release the magnetic energy through multi-scale coupling, and the generation of high-energy electrons can be excited in the magnetic flux ropes through a variety of acceleration mechanisms; Near the reconnection front, there are often various plasma instabilities and strong current structures, which play an important role in the release of magnetic energy. In addition, we briefly discuss the triggering mechanism of magnetic reconnection at the electron-scale, such as the electron-only reconnection in turbulence and reconnection driven by electron kinetics in the magnetotail. Finally, we discuss the importance of multi-scale coupling in magnetic reconnection and some key open questions.