Dependence of fast magnetic reconnection on characteristic plasma quantities

The fundamental fast reconnection flows in a large-scale X-type field configuration are numerically studied for various plasma conditions. The fast reconnection becomes more effective as the plasma density ρ̃ in the antiparallel-field region becomes lower, and the reconnection rate approximately scales as ρ̃−0.2, whereas it is little influenced by the plasma β. The relative electron–ion drift velocity, enhanced by the buildup of fast reconnection, becomes much larger near the X point as the plasma density or the plasma β becomes lower, which is favorable to the occurrence of anomalous resistivity. An important plasma situation where the effective fast reconnection and the significant anomalous resistivity can simultaneously grow is that the plasma density and the plasma β are sufficiently low in the ambient antiparallel-field region outside the field-reversal region.