Cluster Observations of Extreme Energetic Electron Acceleration Event in the Earth's Magnetotail

Energetic electrons in the Earth's magnetotail play a crucial role in space weather. Although many studies have identified the basic processes of energetic electron acceleration, the conditions that lead to the most extreme acceleration events remain unclear. Here, we present a case study of one of the most extreme energetic electron acceleration events observed by the Cluster mission in the magnetotail over its entire lifetime (2001–2024). The event occurred on 3 October 2007, at around 13:59, during a substorm when a corotating interaction region in the solar wind was crossing the Earth. The event occurred in the transition region between the stretched tail and the dipole field and near the center of the current sheet. During the event, there are multiple bursty bulk flows (BBFs) and a large variation in plasma flows. Our analysis reveals that multiple reconnection sites occurred and contributed differently to the event. The initial reconnection site creates a large‐scale dipolarization and a hot plasma sheet. Subsequent reconnection sites impart Fermi and betatron accelerations on hot plasma sheet electrons. The observed peak in the total flux of energetic electrons results from the combination of these two acceleration mechanisms: Fermi‐accelerated electrons drifting eastward from shrunk flux tubes arrive first, followed by betatron acceleration due to the piling up of magnetic flux tubes in earthward BBF. The comparison between auroral images and in situ measurements shows that BBFs are in conjunction with the regions exhibiting auroral streamers that evolve from poleward boundary intensifications.