Suprathermal Electron Acceleration in a Reconnecting Magnetotail: Large-Scale Kinetic Simulation

摘要

abstract_textpElectron acceleration in the magnetotail has been investigated intensively. A major location for this process is the reconnection region. How electrons are accelerated to suprathermal energy in a realistic three-dimensional reconnection geometry, as opposed to an idealized configuration, is not fully understood. In this study, we employed a three-dimensional implicit particle-in-cell (iPIC3D) simulation and a large-scale kinetic simulation to address this problem. We simulated a near-Earth reconnection event observed by THEMIS spacecraft using an iPIC3D simulation with initial and boundary conditions set by a global magnetohydrodynamic simulation of Earth's magnetosphere. The simulated near-Earth reconnection was characterized by a primary X line and a nearby ion-scale flux rope. Using large-scale kinetic, we then followed millions of test electrons using the electromagnetic fields from the iPIC3D simulation. We found that magnetotail reconnection can substantially heat the electrons and generate a large number of suprathermal electrons. The small-scale flux rope played an essential role in energizing these suprathermal electrons, which were predominantly produced inside the flux rope. We identified several acceleration mechanisms that were responsible for energizing these electrons: parallel electric fields, betatron and Fermi acceleration, and nonadiabatic acceleration by the perpendicular electric field. It is found that the parallel electric field was the dominant cause of electron acceleration to suprathermal energy. We suggest that three-dimensional and time-dependent structures must be taken into account to understand electron acceleration during magnetic reconnection./ppPlain Language Summary It has long been suggested that magnetotail reconnection is a major source of energetic electrons in a disturbed magnetotail. How are the electrons accelerated by tail reconnection is still a frontier issue in magnetosphere physics. By using a new numerical approach that combines a large-scale three-dimensional particle-in-cell simulation and a test particle simulation, we explore this issue in a realistic magnetotail. The most intriguing finding is that small-scale flux rope, which is a coherent structure consisting of helical field lines, plays an important role in energizing electrons to suprathermal energy. Electrons are accelerated by multiple mechanisms, such as the parallel electric field, betatron acceleration, and Fermi acceleration, with the dominant contribution from the parallel electric field./p/abstract_text