In situ evidence for the structure of the magnetic null in a 3D reconnection event in the Earth's magnetotail


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 of 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.

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Figure 1: The 4-s resolution magnetic field (B ) and plasma velocity (V ) data of C1–C4 (no V2 data) during the interval 05:03–05:04UT on 15 September 2001.
Figure 2: Positions of spacecraft C1, C2 and C4 relative to C3 in GSM coordinates and illustration of the null point surrounded by Cluster satellites.
Figure 3: The high-resolution (0.04 s) magnetic field data of four Cluster spacecraft, and the calculated Poincaré index during 05:03–05:04UT on 15 September 2001.
Figure 4: Schematic diagrams of a positive-spiral-type null and positions of satellites crossing.


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This work is supported by the NSFC Programs (Grant Nos 40390150, 4050421, 10233050, 10575018, 40536030, 40425004, and 40228006) and the China Key Research Project (Grant No. G200000784), as well as the China Double Star-Cluster Science Team. The authors also thank D. S. Cai, E. R. Priest, and G. P. Zhou for helpful discussions and suggestions, as well as H. Schwarzl for producing the intercalibrated FGM data.

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Xiao, C., Wang, X., Pu, Z. et al. In situ evidence for the structure of the magnetic null in a 3D reconnection event in the Earth's magnetotail. Nature Phys 2, 478–483 (2006).

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