Energetic electron acceleration in the downstream reconnection outflow region

Abstract

Energetic electrons in an earthward reconnection outflow region have been observed by Cluster/Research with Adaptive Particle Imaging Detectors. We found a good correlation between the energetic electron enhancement and a normal magnetic field (B_z) enhancement within a 0.25-s time resolution. The large normal magnetic field is thought to be associated with magnetic reconnection because the negative/positive B_z reversal observed during the fast proton tailward/earthward flow reversal is a good indicator of magnetic reconnection. Using the four-spacecraft Cluster, we can clearly see that this large positive B_z structure propagates in the earthward direction. Furthermore, we find that the energy spectrum of the energetic electrons becomes harder toward the downstream region. A negative B_z enhancement is also observed. The intensity of energetic electron enhancement associated with the negative B_z enhancement is weaker than that associated with the positive one. To discuss the temporal and spatial profile of energetic electron acceleration in the magnetic reconnection region, we determined the spacecraft position in the temporally evolving magnetic structures of reconnection. Our observation clearly indicates second-step acceleration, in addition to X line acceleration, of energetic electrons in the downstream reconnection outflow region.