Distributions of mass density p a, temperature T b, parallel current density |J_‖| c, and 3D X and O-type grids d at t = 9. For p and T, we only depict their distributions on the current-sheet middle plane (y = 0), the front and back planes (z = ±0.2), and the left and right planes (x = ±1). Panel c shows the 3D profile of |J_‖| satisfying |J_‖| > 80. In panel d, we plot the grids that (1) satisfy |J_‖| > 50, (2) have local magnetic structures of 3D X and O types, and (3) are 2D projected extreme points of J_‖. The X-type (type 1) grids are depicted by blue dots, while the O-type grids (types 2 and 3) are plotted by orange ones. The magenta and black curves in panels c and d are examples of sheared and twisted field lines, which are traced from the initial sampling positions near 3D X-type and O-type grids, respectively. The green spheres mark the regions for sampling start positions of field-line tracing. An animation of this figure showing the entire evolution from t = 0–9 is available.