In situ detection of collisionless reconnection in the Earth's magnetotail

Magnetic reconnection is the process by which magnetic field lines of oppos ite polarity reconfigure to a lower-energy state, with the release of magne tic energy to the surroundings. Reconnection at the Earth's dayside magneto pause and in the magnetotail allows the solar wind into the magnetosphere(1 ,2). It begins in a small `diffusion region', where a kink in the newly rec onnected lines produces jets of plasma away from the region. Although plasm a jets from reconnection have previously been reported(3-7), the physical p rocesses that underlie jet formation have remained poorly understood becaus e of the scarcity of in situ observations of the minuscule diffusion region . Theoretically, both resistive and collisionless processes can initiate re connection(8-14), but which process dominates in the magnetosphere is still debated. Here we report the serendipitous encounter of the Wind spacecraft with an active reconnection diffusion region, in which are detected key pr ocesses predicted by models(8-13) of collisionless reconnection. The data t herefore demonstrate that collisionless reconnection occurs in the magnetot ail.