The role of the Hall effect in collisionless magnetic reconnection

A short review of the role of the Hall effect in collisionless reconnection is given in relation to observations in the upper auroral ionosphere during substorm-reconnection events. We clarify the conditions when the Hall effect should be of importance and when the ion inertial length is the responsible scale which is the case for beta(perpendicular to) > 1. Moreover, we relate the auroral acceleration region to the reconnection region in the tail determining the relations between upward and downward currents and the different ranges of the Hall reconnection region in the tail. It is concluded that the downward-upward currents can be mapped to those domains in the near-Earth tail and suggest that these regions are in fact very narrow and that due to this narrowness and the abundance of upward-downward current regions reconnection in near-Earth space during substorms is probably highly patchy. (c) 2006 Published by Elsevier Ltd on behalf of COSPAR.