In situ detection of the electron diffusion region of collisionless magnetic reconnection at the high-latitude magnetopause

Magnetic reconnection is the most fundamental energy-transfer mechanism in the universe that converts magnetic energy into heat and kinetic energy of charged particles. For reconnection to occur, the frozen-in condition must break down in a localized region, commonly called the ‘diffusion region’. In Earth’s magnetosphere, ion diffusion regions have already been observed, while electron diffusion regions have not been detected due to their small scales (of the order of a few km) (Paschmann, 2008). In this paper we report, for the first time, in situ observations of an active electron diffusion region by the four Cluster spacecraft at the Earth’s high-latitude magnetopause. The electron diffusion region is characterized by nongyrotropic electron distribution, strong field-aligned currents carried by electrons and bi-directional super-Alfvénic electron jets. Also observed were multiple micro-scale flux ropes, with a scale size of about 5 c/ω_pe (12 km, with c/ω_pe the electron inertial length), that are crucial for electron acceleration in the guide-field reconnection process (Drake et al., 2006a). The data demonstrate the existence of the electron diffusion region in collisionless guide-field reconnection at the magnetopause.