The growth of whistler-mode waves and the loss of anisotropic distribution electrons inside the bursty bulk flows

Abstract:

During the interval ~07:45:36-07:54:24 UT on 24 August 2005, Cluster satellites (C1 and C3) observed the growth of whistler-mode waves and the loss of anisotropic distribution electrons (~3-95keV) inside some bursty bulk flows (BBFs) in the midtail plasma sheet (X_GSM ~ -17.25R_E). However, the fluxes of the higher-energy electrons (>120 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 one minute, and the electron scatterings in different pitch angle distributions are different in the wave growth regions. The energetic electrons have mainly a quasi-perpendicular pitch angle distribution 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 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. By modeling the electron pitch angle scattering process, we find that the loss of the initial field-aligned distribution electrons is obviously rapider than that of the initial quasi-perpendicular distribution electrons.