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

Abstract

Electron 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.Plain 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. Key Points:• Magnetotail reconnection can generate a large number of suprathermal electrons • Small-scale flux ropes play an essential role in accelerating electron to suprathermal energy • Parallel electric field is the dominant mechanism that energizes electrons to suprathermal energy

Electron Acceleration in a Magnetotail Reconnection Outflow Region Using Magnetospheric MultiScale Data

“…This suggests that even though the sketch in Figure matches well many of the observations, there is a significant complexity in the spatial/temporal evolution of the CS that cannot be described by this sketch. This is consistent with the results found in the 3‐D numerical simulation by Zhou et al () showing the formation of a flux rope and how the relative importance of different acceleration mechanisms in the flux rope changes on a time scale of seconds. Thus, our observations support Zhou et al () suggestion that time‐dependent structures need to be taken into account to understand the electron acceleration in magnetic reconnection.…”

“…Our event (and also the full interval in Figure ) shows that the average power density due to Fermi acceleration is dominating all other processes and that the average of betatron acceleration is close to zero being slightly negative. These observations are consistent with numerical simulation results by Dahlin et al () but inconsistent with Zhou et al (). These discrepancies could possibly be due to the temporal variation of the different processes and/or due to the presence of flux tubes in the simulation by Zhou et al ().…”

“…These observations are consistent with numerical simulation results by Dahlin et al () but inconsistent with Zhou et al (). These discrepancies could possibly be due to the temporal variation of the different processes and/or due to the presence of flux tubes in the simulation by Zhou et al (). Similarly, for this event we could not assess detailed properties of due to the observational uncertainties in the E ‖ measurements.…”

“…However, we conclude that the E ‖ acceleration is much less important than the Fermi acceleration. This is consistent with what was reported for the low guide field case in Dahlin et al (), but inconsistent with Zhou et al (). Wang et al () analyze electron acceleration in the outflow region using test particles and conclude that E ‖ can be important but the contribution from each of the acceleration processes on a full distribution function locally are not estimated.…”

MMS Observations of Plasma Heating Associated With FTE Growth

“…It is further shown that Ions and electrons are, on average, net heated, > 0, inside and just outside FTEs. We find that, on average, parallel electric fields are the dominant heating mechanism (70–90% of ) for both ions and electrons, at the leading half of FTEs, in agreement with the kinetic simulation runs by Egedal et al () and Zhou et al (). At the leading part of the structure, ions and electrons gain further parallel energy via the Fermi process. As listed in the table in Figure d, electron heating due to Fermi acceleration is negligible.…”