Electron Acceleration in the Separatrix Region during Magnetic Reconnection

One long outstanding issue in the study of magnetic reconnection is exactly where and how electrons are energized, although the observations have demonstrated that a substantial part of magnetic energy is converted into energetic electrons in reconnection. Here, we present the first evidence of electron acceleration by parallel electric field (up to -20 mV/m) carried by the Double Layer (DL) in the separatrix region during reconnection.Based on theory and auroral observations, the DL is inferred to propagate away from X-line at a velocity about ion acoustic speed and thereby accelerates electrons effectively on its way. The identification of multiple similar DLs within mere 16s interval indicates that they are persistently produced and play an important role in energy conversion during reconnection. The observations suggest electrons are energized during reconnection not only in certain fixed regions as expected previously but in any region where as long as the DL reaches.