Observations of energetic electrons up to 200 keV associated with a secondary island near the center of an ion diffusion region: A Cluster case study

An antiparallel reconnection event is recognized in the magnetotail, and a secondary island with a strong core magnetic field is identified near the center of the ion diffusion region. In the island, the electron density peaks in the outer region while the dip is in the core region with a strong core magnetic field. A strong electron beam parallel to the magnetic field, as well as an obvious current antiparallel to the magnetic field with density up to ~40 nA/m2, is observed in the outer region of the island. This suggests that the strong core magnetic field inside the island is generated by the electron beam and then the antiparallel current in the outer region of the island. The electron density dip in the core region is formed due to the existing strong core field, which expels electrons out of the core region. The electron flat-top distributions are detected in the ion diffusion region except the core region of the island, and the shoulder energy range of the flat-top distributions is from 100 eV to 4 keV. An enhancement of the energetic electron flux up to 200 keV is found in the ion diffusion region, and a further increase of energetic electron fluxes is observed inside the island. Waves at the lower hybrid frequency are intensified in the ion diffusion region, while the intensification is strong in the outer region of the island and becomes very weak in the core region.