Theoretical studies of collisionless reconnection in the framework of
two-fluid theory are presented. In the high-beta case (beta greater th
an or similar to 1) reconnection is controlled by the whistler mode, l
eading to decoupling of ions from electrons on scales <c/omega(pi). Th
ough reconnection requires electron inertia, the reconnection rate is
independent thereof, controlled only by ion inertia. Reconnection is h
ence much faster than in the absence of the Hall term. In the opposite
limit of small beta the strong axial field suppresses the whistler mo
de. Hence ions have to follow the electrons in the narrow reconnection
layer delta similar to c/omega(pe), forming a macroscopic current she
et which strongly reduces the reconnection rate. Theoretical scaling l
aws are confirmed by numerical simulations. (C) 1997 American Institut
e of Physics.