Pitch angle distribution of suprathermal electrons behind dipolarization fronts: A statistical overview

摘要

We examine the pitch angle distribution (PAD) of suprathermal electrons (>40keV) inside the flux pileup regions (FPRs) that are located behind the dipolarization fronts (DFs), in order to better understand the particle energization mechanisms operating therein. The 303 earthward-propagating DFs observed during 9 years (2001-2009) by Cluster 1 have been analyzed and divided into two groups according to the differential fluxes of the>40keV electrons inside the FPR. One group, characterized by the low flux (F 40 keV electrons as the low-flux situation may lead to large uncertainties in computing the anisotropy factor that is defined as A = FF_(-1) for F > F_t, and A =-F _t/F_t + 1 for F_t 0.5), and 44 events have field-aligned distribution inside the FPR (A<-0.5). The perpendicular distribution appears mainly inside the growing FPR, where the flow velocity is increasing and the local flux tube is compressed. The field-aligned distribution occurs mainly inside the decaying FPR, where the flow velocity is decreasing and the local flux tube is expanding. Inside the steady FPR, we observed primarily the isotropic distribution of suprathermal electrons. This statistical result confirms the previous case study and gives an overview of the PAD of suprathermal electrons behind DFs.