Space Sciences Laboratory, University of California, Berkeley, California

Magnetic reconnection between two oppositely directed magnetic field lines takes place in a narrow "diffusion region" but its consequences are large scale and leads to the entry of solar wind plasma into the magnetosphere at the dayside magnetopause as well as in the magnetotail. The processes in the diffusion region determines how fast reconnection can proceed. However, the identity of the diffusion process in the magnetosphere is not known since spacecraft rarely encounter this small region. Previous reports of in-situ evidence of the reconnection process consist mainly of observations of plasma jets observed outside the diffusion region after reconnection has already taken place. Here we report a fortuitous event where the Wind spacecraft encountered an active reconnection diffusion region in the Earth's magnetotail where it detected bipolar "Hall" magnetic field signatures and electron beams associated with the separation between ions and electrons in the diffusion region - an effect postulated by theories of collisionless reconnection. These observations provide evidence for the existence of a magnetotail reconnection regime where collisionless rather than resistive mechanisms dominate; a regime which, according to recent theoretical studies, produces much higher reconnection rates than the often assumed resistive regime.

submitted, Nature, 2001