Distributions of 3D X and O-type grids within a 2.5D Harris-sheet reconnection. The 3D magnetic field has z-symmetry and is constructed from a 2.5D simulation with the same configurations as the 3D Harris sheet in Sect. 5. This figure shows the result at t = 4. In panel a, the blue dots have the same definition as in Figs. 3 and 4. The black “x” and red “o” markers depict the positions of 2D X and O points on the x–y plane as determine d by the 2D method, while the black and red lines are the corresponding X and O lines in 3D. The principal reconnection site with strongest |J_‖| is marked by a magenta “x” marker. On the plane z = 0.2, the profile of u_x is depicted while the magnetic field lines are shown by gray curves. Panels b and c are similar to panel a but exhibit results with different thresholds of |J_‖| and θ_eig. Panel d has the same thresholds as c but also plots 3D O-type grids with orange dots.