Multi-point observations of the Hall electromagnetic field and secondary island formation during magnetic reconnection

A key feature of collisionless magnetic reconnection is the formation of Hall magnetic and electric field structure in the vicinity of the diffusion region. Here we present multi-point Cluster observations of a reconnection event in the near-Earth magnetotail where the diffusion region was nested by the Cluster spacecraft; we compare observations made simultaneously by different spacecraft on opposite sides of the magnetotail current sheet. This allows the spatial structure of both the electric and magnetic field to be probed. It is found that, close to the diffusion region, the magnetic field displays a symmetric quadrupole structure. The Hall electric field is symmetric, observed to be inwardly directed on both sides of the current sheet. It is large (similar to 40 mV m(-1)) on the earthward side of the diffusion region, but substantially weaker on the tailward side, suggesting a reduced reconnection rate reflected by a similar reduction in E-y. A small-scale magnetic flux rope was observed in conjunction with these observations. This flux rope, observed very close to the reconnection site and entrained in the plasma flow, may correspond to what have been termed secondary islands in computer simulations. The core magnetic field inside the flux rope is enhanced by a factor of 3, even though the lobe guide field is negligible. Observations of the electric field inside the magnetic island show extremely strong (similar to 100 mV m(-1)) fields which may play a significant role in the particle dynamics during reconnection.