Magnetic null is the fundamental element of magnetic reconnection in plasma, which together with the ambient 3D topology is crucial for developing the magnetic reconnection theory. Scientists have been pursuing the magnetic null in space plasma for over six decades since the birth of magnetic null concept, but encountered with the difficulty due to the zero-measure of dot-set in space geometry.

A new diagnosis procedure based on the Poincaré method has been developed, and was successfully applied to find the existence of magnetic null in the geo-tail magnetic reconnection. The achievement was published on Nature Physics in July 2006, and featured as the Highlight of that issue. As the second step, a pair of magnetic nulls together with separator, separatrix and lower-hybrid waves was identified, which establishes the 3D scenario of magnetic reconnection and was published on Nature Physics in September 2007. Moreover, the research team proposed a novel method to reconstruct the real-time 3D reconnection geometric configuration based on the simultaneous 4-point measurements from Cluster. Electrons are found to be trapped around the magnetic nulls and accelerated probably along the separator.

The above research achievements are presented as the press release on the official website of ESA under the title of "Cluster hits the magnetic bull's eye", and featured three times successively as the "Top Story" of Cluster project, stating that "For the first time, a reconnection line has been observed in-situ in 3-D. This result is another major scientific achievement of the Cluster mission obtained thanks to fruitful scientific collaborations between Chinese, American and European scientists". It was selected by the National Ministry of Education as the "Top Ten Advancements in Science and Technology by Chinese Universities in 2006", and also listed as one of the five highlighted main achievements of the Cluster & Double-Star projects in 2009.

Figure-3. The real-time 3D magnetic geometry of a reconnection event as reconstructed based on the Cluster 4-satellite in-situ measurements

References：

(1) Xiao, C.-J., Wang, X.-G., Pu, Z.-Y., et al., In situ evidence for the structure of the magnetic null in a 3D reconnection event in the Earth's magnetotail, Nature Physics, 2, 478-483, doi:10.1038/nphys342, 2006.

(2) Xiao, C.-J., Wang, X.-G., Pu, Z.-Y., et al., Satellite observations of separator-line geometry of three-dimensional magnetic reconnection, Nature Physics, 3, 609-613, doi:10.1038/nphys650, 2007.

(3) He, J.-S., Zong, Q.-G., Deng, X.-H., Tu, C.-Y., et al., Electron trapping around a magnetic null, GRL, 35, L14104, doi:10.1029/2008GL034085, 2008