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Electron Acceleration by Dipolarization Fronts and Magnetic Reconnection: A Quantitative Comparison

Suprathermal electrons with energy from tens to hundreds of keV are frequently observed in the Earth's magnetotail. The generation of such electrons is typically attributed to magnetic reconnection, dipolarization fronts (DFs), or flux transport. However, which of these contributes more to this generation remains unclear. In this study, we quantitatively compare the electron acceleration by these processes, using the Cluster data. We analyze an event detected in the midtail and find that the suprathermal electrons there are first accelerated by magnetic reconnection and transported earthward, and then further accelerated locally at the DF. The acceleration process by reconnection and transport, resulting in an isotropic pitch angle distribution, contributes ∼70% to the total flux enhancement, while the acceleration process by the DF, resulting in a pancake distribution, contributes ∼30% to the flux enhancement. The electron acceleration at the DF is primarily attributed to a local betatron process that is successfully reproduced using an analytic model. In order to better understand this phenomenon, we examine an additional 16 similar events and find that the DFs and magnetic reconnection statistically contribute 11% ∼ 38% and 62% ∼ 89% to the total flux enhancement, respectively. This study greatly improves our understanding of the electron energization process in the magnetotail.