Multiscale study of electron energization during unsteady reconnection events

Understanding particle acceleration in the magnetotail during substorms requires knowledge of changes in the global magnetospheric configuration and the local regions of intense fields and microinstabilities caused by processes associated with reconnection. We simulated a substorm on 15 February 2008 by coupling the University of California, Los Angeles global magnetohydrodynamic simulation code and a two‐dimensional version of the iPIC3D implicit particle‐in‐cell code. The MHD code provides realistic initial and boundary conditions for the particle‐in‐cell (PIC) code, while the PIC code models the reconnection and evolution of the dipolarization front (DF) self‐consistently with full kinetic physics. In the PIC simulation, after a few seconds, an active X point forms and DF‐like structures form about every 2 s and propagate earthward. In the near‐Earth tail, the earthward moving fronts combine to form thicker structures. The presence of the macroscopic‐scale magnetic field, featuring a significant dipolar component nearer the Earth, affects the reconnection process, chocking the flow that cannot freely propagate earthward, causing the production of multiple repeating DFs. In the region away from the equator and equatorward of the separatrices, the DFs become associated with a series of spatial stripes that are visible in the electron temperature and the magnetic and electric fields and are also caused by the unsteady nature of the process of reconnection. The electrons are preferentially accelerated in the dipolarization region in the extension of the dipolarization field lines to high latitudes and reach energies of 100 keV or more. A streaming instability may be responsible for the parallel acceleration. This acceleration of the plasma associated with the DFs is greater than that occurring near the X point for this substorm. Key Points Acceleration of plasma near DFs is greater than that occurring near the X point Multiple dipolarization fronts are caused by unsteady reconnection The electrons are preferentially accelerated in the separatrices