Magnetic reconnection is one of the most important processes in astrophysical, space and laboratory plasmas. Identifying the structure around the point at which the magnetic field lines break and subsequently reform, known as the magnetic null point, is crucial to improving our understanding reconnection. But owing to the inherently three-dimensional nature of this process, magnetic nulls are only detectable through measurements obtained simultaneously from at least four points in space. Using data collected by the four spacecraft of the Cluster constellation as they traversed a diffusion region in the Earth's magnetotail on 15 September, 2001, we report here the first in situ evidence for the structure of an isolated magnetic null. The results indicate that it has a positive-spiral structure whose spatial extent is of the same order as the local ion inertial length scale, suggesting that the Hall effect could play an important role in 3D reconnection dynamics.

Detection of a separator line that connects magnetic nulls and the determination of the dynamics and plasma environment of such a structure can improve our understanding of the three-dimensional (3D) magnetic reconnection process 1-9. However, this type of field and particle configuration has not been directly observed in space plasmas. Here we report the identification of a pair of nulls, the null-null line that connects them, and associated fans and spines in the magnetotail of Earth using data from the four Cluster spacecraft. With di and de designating the ion and electron inertial lengths, respectively, the separation between the nulls is found to be ～0.7±0.3 di and an associated oscillation is identified as a lower hybrid wave with wavelength ～de. This in situ evidence of the full 3D reconnection geometry and associated dynamics provides an important step toward to establishing an observational framework of 3D reconnection.

Magnetic reconnection is a main process converting the magnetic energy into thermal and kinetic energy in plasmas. It is one of the fundamental problems of crucial importance not only to space plasmas physics and space weather studies, such as the solar flare, coronal mass ejections and magnetospheric substorms, but also to the stability analysis in magnetically confined fusion. In general, except for cases with periodical boundary conditions, three-dimensional (3D) magnetic reconnection occurs on magnetic separatrices generated by magnetic nulls. Here we briefly introduce/review the theories and some recent satellite observations of 3D magnetic reconnection. Topics to be further studied are also discussed.

[1] Fast reconnection is crucial to magnetospheric substorms, solar and stellar flares and fusion plasmas. Ultimate confirmation of fast reconnection must be achieved by multi-spacecraft detections of the reconnection rate itself and associated dimensions of the diffusion region. Here we report a multi-spacecraft measurement of fast reconnection rate grec Vin y (0.07-0.15)VA based on directly measurements of the plasma flow into the diffusion region, where Vin is the speed of reconnecting flux and VA the characteristic Alfven speed. It falls in the range of (0.03-0.2)VA predicted in steady state reconnection simulation. The characteristic sizes for the diffusion region of the width Lz 0.9 di(=460km) and the length Lx (3.3-5.1)di (=1680-2597 km) are measured as well. The length of the diffusion region Lx is determined for the first time based on the in situ observations. Furthermore, other features detected during the event also match the previous observation and simulation results. Citation: Xiao, C. J., et al. (2007), A Cluster measurement of fast magnetic reconnection in the magnetotail, Geophys.