Intermittent Electron Acceleration by Magnetotail Reconnection

Electron acceleration is pervasive in astrophysical and space plasmas. It is suggested that turbulence[1], shock[2] and magnetic reconnection[3] are efficient engines for energizing electrons. Magnetic reconnection can rapidly convert magnetic energy into plasma energy during the macroscopic topology changes of the magnetic field. It is commonly recognized that part of the converted energy goes to heat and accelerate electrons[4]. However, despite decades of intensive study, where and how are electrons being accelerated in reconnection remains an open question. This Letter presents a comprehensive study of the electron acceleration from different mechanisms in the near-Earth magnetotail using the unique measurement capabilities of the Magnetospheric Multiscale (MMS) mission. We find that the average acceleration rates and occurrence rates of large acceleration tend to be higher in outflows with larger speeds (Figure 2). Betatron and Fermi accelerations are intensified near the neutral sheet, while the acceleration from E|| is not only intensified in the neutral sheet, but also significant far from the neutral sheet, likely in the separatrix region. In contrast to previous studies suggesting that the acceleration and energy conversion predominantly occurs in the outflow region, we find that the acceleration rate near the X line is comparable to that in the outflow. One interesting finding is that electrons are intermittently accelerated in association with reconnection (Figure 2), suggesting that the near-Earth magnetotail activated by reconnection is in a turbulent state. This work is supported by the National Natural Science Foundation of China (NSFC) under grants No. 41774154, 42074197, and the project funded by China Postdoctoral Science Foundation grant 2021M691395.