In situ observation of magnetic reconnection in the front of bursty bulk flow

摘要

Using the Cluster observation in the magnetotail, we investigate the dynamic processes associated with a bursty bulk flow (BBF) event. The BBF is inferred to be caused by magnetic reconnection proceeding to the lobe region in its tail, called “primary reconnection.” On the BBF front, another reconnection was directly encountered by one of the four Cluster satellites, and no signatures of this reconnection were simultaneously measured by the satellite at the plasma sheet boundary. It indicates that this reconnection on the BBF front remained within the plasma sheet, called “secondary reconnection.” The secondary reconnection moved earthward and was followed by a magnetic island. A few earthwardmoving pulses of B_z were detected between the island and the primary reconnection site. These B_z pulses, propagating faster than the island ahead of it, would lead to a more compressed B_z magnetic field in the wake of the island. The observational scenario is in accordance to themodel proposed to explain the generation of dipolarization front in simulations. Furthermore, both electrons and ions were significantly accelerated in this process. The mechanism is discussed also.