Rapid loss of the plasma sheet energetic electrons associated with the growth of whistler mode waves inside the bursty bulk flows

Abstract

AbstractDuring the interval ~07:45:36–07:54:24 UT on 24 August 2005, Cluster satellites (C1 and C3) observed an obvious loss of energetic electrons (~3.2–95 keV) associated with the growth of whistler mode waves inside some bursty bulk flows (BBFs) in the midtail plasma sheet (XGSM ~ −17.25 RE). However, the fluxes of the higher‐energy electrons (≥128 keV) and energetic ions (10–160 keV) were relatively stable in the BBF‐impacted regions. The energy‐dependent electron loss inside the BBFs is mainly due to the energy‐selective pitch angle scatterings by whistler mode waves within the time scales from several seconds to several minutes, and the electron scatterings in different pitch angle distributions are different in the wave growth regions. The plasma sheet energetic electrons have mainly a quasi‐perpendicular pitch angle distribution (30°<α<150°) during the expansion‐to‐recovery development of a substorm (AE index decreases from 1677 nT to 1271 nT), and their loss can occur at almost all pitch angles in the wave growth regions inside the BBFs. Unlike the energetic electrons, the low‐energy electrons (~0.073–2.1 keV) have initially a field‐aligned pitch angle distribution (0°≤α≤30° and 150°≤α≤180°) in the absence of whistler mode waves, and their loss in field‐aligned directions is accompanied by their increase in quasi‐perpendicular directions in the wave growth regions, but the loss of the low‐energy electrons inside the BBFs is not obvious in the presence of their large background fluxes. These observations indicate that the resonant electrons in an anisotropic pitch angle distribution mainly undergo the rapid pitch angle scattering loss during the wave‐particle resonances.