Two distinct energetic electron populations of different origin in the Earth’s magnetotail: a Cluster case study

Abstract. Energetic electrons (E≥30 keV) travelling along and perpendicular to the magnetic field lines have been ob-served in the magnetotail at L∼17:00 and 22:00 MLT dur-ing the recovery phase of a storm-time substorm on 7 Oc-tober 2002. Three-dimensional electron distributions of the full unit sphere obtained from the IES/RAPID sensor sys-tem demonstrated a rather complicated and random behav-ior of the energetic electrons. Occasionally these electrons were appearing to travel parallel, perpendicular, or in both directions, relative to the magnetic field direction, forming in this way bi-directional, perpendicular-peaked, and mixed distributions. The electron enhancements occurred while the Cluster spacecraft were on closed field lines in the central plasma sheet approaching the neutral sheet from the northern tail lobe. Magnetic field and energetic particle measurement