Acceleration of Electrons in the Earth’s Magnetotail during Substorms Using Multi-Scale Simulations

Abstract:

Understanding particle acceleration in the magnetotail during substorms requires knowledge of changes in the global magnetospheric configuration and the local fields and micro-instabilities caused by processes associated with reconnection. To understand global structure of the magnetotail during substorms we use the UCLA global magnetohydrodynamic (MHD) simulation code and to understand local dynamics we couple the MHD simulation with the iPIC3D implicit particle-in-cell code. The MHD code provides realistic initial and boundary conditions for the iPIC3D code which models the reconnection and evolution of the substorm dipolarization front (DF) self-consistently with full kinetic physics. In the first case study we use a two dimensional version of iPIC3D to investigate the multi-scale nature of the electron energization during a substorm on February 15, 2008. In this multi-scale simulation the electric and magnetic fields show the quadrupolar signature of Hall-MHD, absent in the resistive MHD case. We note that during this event, just like in the case of the MHD, dipolarization fronts are formed mainly earthward of the neutral line. In the PIC simulation after a few seconds an active X-point forms and DF-like structures form about every two seconds and propagate earthward. The presence of the macroscopic scale magnetic field, featuring a significant dipolar component nearer the Earth affects the reconnection process causing the production of multiple repeating DFs. The electrons are preferentially accelerated in the separatrices and reach energies of 100 keV or more. This acceleration of the plasma associated with the DFs is greater than that occurring near the X-point for this substorm. We have extended this study to three dimensions since it is known that DFs develop 3D structures. Compared to 2D simulations conducted in a meridian plane (GSM XZ coordinates) important additional aspects are present when the GSM Y coordinate is included, e.g., the flapping and bending of the current layer, the development of instabilities such as ballooning, lower hybrid drift and interchange instabilities.