The 3-dimentional structure of magnetic reconnection ion diffusion region has been studied in this paper. Steady magnetic null-pair structure is found among the Cluster tetrahedron within a thin current sheet when magnetic reconnection takes place in the near-Earth magnetotail. Two magnetic null points in the null-pair are well coupled, with an angle of about 3~7 ° between the spin line of one and the fan surface of the other. The magnetic null-pair detected in the ion diffusion region, is quasi-stable in spatial structure but fast evolved in time, consistent with the fast reconnection scenario. The spatially steady magnetic null-pair within the diffusion region of the collision less fast magnetic reconnection presents an advanced understanding of the magnetic reconnection process.
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