The magnetic field reconnection site and dissipation region

Magnetic field reconnection is said to involve an ion diffusion region surrounding an electron diffusion region. Because of uncertainties in the meanings of these terms and on the physical parameters that characterize them, this paper defines the reconnection site as a region having an electron scale size and containing magnetic fields from four topologies, and the dissipation region, as having an ion scale size and surrounding the reconnection site. Two-dimensional, asymmetric, open, particle-in-cell simulations, with and without guide fields, examine these regions. It is found that significant values of (E+U-I X B) and/or (E+U-e X B) are not confined to either the reconnection site or the dissipation region. The reconnection site is uninteresting because, for asymmetric reconnection, it does not contain processes that serve to locate it, such as electron acceleration, parallel electric fields, super-Alfvenic electron flow, maximum electron beta, electron nongyrotopy, or demagnetized thermal electrons. However, the surrounding dissipation region exhibits these features. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3206947]