Average properties of magnetic reconnection ion diffusion regions in the Earth’s magnetotail: 2001 - 2005 Cluster observations and comparison with simulations

Collisionless magnetic reconnection plays a key role in the dynamics of the Earth’s magnetosphere. Of particular importance is the diffusion region, since this is where the magnetic field ultimately reconnects. Although considerable progress has been made in recent years, many questions remain, in part because it is difficult to make in-situ observations of this region, and so relatively few individual events have been reported in the literature. To address this problem, the Cluster magnetotail dataset from 2001 - 2005 (constituting 175 magnetotail passes) has been comprehensively surveyed for encounters with reconnection sites in the plasma sheet by searching for correlated reversals in the normal magnetic field and tailward/earthward plasma flow. This survey resulted in 33 events. Of these events, 23 (70%) were in qualitative agreement with the expected pattern of Hall fields, indicating a diffusion region encounter. Within the set of diffusion region encounters corresponding to anti-parallel reconnection, the absolute size of both the Hall electric and magnetic field were both found to vary from event to event. However, with appropriate normalization to the observed boundary conditions, consistent magnitudes of both the average peak Hall magnetic and the average peak Hall electric field are found. To better understand this experimental data, large particle-in-cell simulations were performed, normalized in the same way as the data. A comparison of the observed data and the predictions of simulations will be shown. These results help to establish the occurrence rate of diffusion regions relative to macroscopic signatures and the average properties of the diffusion region in the Earth's magnetotail. We also discuss the most useful signatures for establishing experimentally the in-situ measurement of diffusion regions.