Potential field and the open field with the same magnetic flux distribution on the bottom surface, and the distribution of CS in the open field. A: 3D prospective view of the potential field lines. The colored thick lines represent magnetic field lines and the colors denote the height. The background shows the magnetic flux distribution on the bottom boundary. B: 3D prospective view of the fully opened field lines, with same footpoints shown in panel A. The red iso-surface represents the CS, whose current density is J = 0.34 × 10⁻³ A m⁻². C: current distribution and magnetic field lines on the central cross section (i.e., the x = 0 plane). D: profile of current density along z axis. We note that here with finite grid resolution, the CS has a finite thickness of 360 km. Thus the current density is not infinite.