Average properties of the magnetic reconnection ion diffusion region in the Earth's magnetotail: The 2001–2005 Cluster observations and comparison with simulations

Magnetic reconnection plays a key role in the circulation of plasma through the Earth's magnetosphere. As such, the Earth's magnetotail is an excellent natural laboratory for the study of reconnection and in particular the diffusion region. To address important questions concerning observational occurrence rates and average properties, the Cluster data set from 2001–2005 has been systematically examined for encounters with reconnection X lines and ion diffusion regions in the Earth's magnetotail. This survey of 175 magnetotail passes resulted in a sample of 33 correlated field and flow reversals. Eighteen events exhibited electric and magnetic field perturbations qualitatively consistent with the predictions of antiparallel Hall reconnection and could be identified as diffusion region encounters. The magnitudes of both the Hall magnetic and electric field were found to vary from event to event. When normalized against the inflow magnetic field and the current sheet number density the average peak Hall magnetic field was found to be 0.39 ± 0.16, the average peak Hall electric field was found to be 0.33 ± 0.18, and the average out of plane (reconnection) electric field was found to be ∼0.04. Good quantitative agreement was found between these results and a large, appropriately renormalized particle‐in‐cell simulation of reconnection. In future missions, the magnitude of the total DC electric field may be a useful tool for automatically identifying ion diffusion region encounters.