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tokens: the spacecraft to the magnetic null (see Fu et al. 2015 for details). The PI method significantly depends upon the size of SC tetrahedron. Usually, a large tetrahedron (large SC separation) is favorable for null detection, while a small tetrahedron (small SC separation) is not. In contrast to the PI method, the FOTE method can detect a null both inside and outside of the tetrahedron, and can avoid limitations of the PI method such as data resolution, instrument uncertainty ( offset), and tetrahedron size (Fu et al. 2015 ). Taking the advantages and disadvantages of these two methods into account, in this study we will use the FOTE method to find magnetic nulls. In addition to null detection, the FOTE method can also be used to identify magnetic-null types based on the three eigenvalues () of the matrix and reconstruct magnetic-null topology by tracing magnetic field lines around the null (Fu et al. 2015 , 2016 ). In theory, the sum of the three eigenvalues is zero , meaning that either all of the three eigenvalues are real, or one is real while the other two are conjugate complex. If all of the eigenvalues are real, the null is of radial type, which includes A- and B-type. If one eigenvalue is real while the two others are conjugate complex, the null is of spiral type, which includes - and -type. A, B, and , are all null types in the 3D regime (Lau Finn 1990 ). They can, however, degenerate into 2D structures in certain conditions. Following Fu et al. ( 2015 ), we simplify the A- and B-type nulls to 2D X-line when , and simplify the - and -type nulls to 2D O-line when . If the relationship among the three eigenvalues belongs to none of the above six categories, we classify the null type as “unknown.” Such identification of magnetic-null types is summarized in Table 1 . Table 1 Null Type Identification Using the Three Eigenvalues () of Matrix δ , which is Derived from Four-point Measurements 3. Observations and Analyses 3.1. Event Overview The Earth's magnetosheath is a region downstream of the bow shock where the magnetosphere interacts with the solar-wind flow. Due to this interaction, usually strong turbulence of magnetic field and its resultant thin current sheet, where magnetic reconnection quite possibly occurs, can be observed (Burgess et al. 2005 ). Therefore, the magnetosheath is an ideal place to investigate magnetic reconnection driven by turbulence or reconnection in turbulent plasmas. In this study, the reconnection event we consider was measured by Cluster (Escoubet et al. 2001 ) on 2002 March 27. This event has been reported by Retinò et al. ( 2007 ) and further investigated by others (e.g., Sundkvist et al. 2007 ; Chasapis et al. 2015 ). In this event, Cluster was located in the magnetosheath at [9.6, −8.0, −8.1] RE in Geocentric Solar Ecliptic (GSE) coordinates; it forms a regular tetrahedron, with inter-spacecraft separation of ∼100 km. Such separation is quite small, so that the measurements of magnetic field by four Cluster