Alfvenic collisionless magnetic reconnection and the Hall term

The Geospace Environment Modeling (GEM) Challenge Harris current sheet prob lem is simulated in 2 1/2 dimensions using full particle, hybrid, and Hall MHD simulations. The same gross reconnection rater is found in all of the s imulations independent of the type of code used, as long as the Hall term i s included. In addition, the reconnection rate is independent of the mechan ism which breaks the frozen-in flux condition, whether it is electron inert ia or grid scale diffusion. The insensitivity to the mechanism which breaks the frozen-in condition is a consequence of whistler waves, which control the plasma dynamics at the small scales where the ions become unmagnetized. The dispersive character of whistlers, in which the phase velocity increas es with decreasing scale size, allows the flux of electrons flowing away fr om the dissipation region to remain finite even as the strength of the diss ipation approaches zero. As a consequence, the throttling of the reconnecti on process as a result of the small scale size of the dissipation region, w hich occurs in the magnetohydrodynamic model, no longer takes place. The im portant consequence is that the minimum physical model necessary to produce physically correct reconnection rates is a Hall MHD description which incl udes the Hall term in Ohm's law. A density depletion layer, which lies just downstream from the magnetic separatrix, is identified and linked to the s trong in-plane Hall currents which characterize kinetic models of magnetic reconnection.