Evolution of magnetic field lines, electric currents, and velocity in the whole simulation process of CASE I. A: top view of magnetic field lines. The colored thick lines represent magnetic field lines and the colors denote the value of nonlinear force-free factor defined as α = J ⋅ B/B², which indicates the extent to which the field lines are of non-potential character. The background shows the magnetic flux distribution on the bottom boundary (i.e., plane of z = 0), and contours of B_z = ( − 48, −32, −16, 0, 16, 32, 48) G are shown. B: 3D prospective view of the same field lines shown in panel A. C: current density, J, normalized by magnetic field strength, B, in vertical cross section (i.e., the x = 0 slice). D: same as C, but with a large area. E: magnitudes of velocity. The largest velocity and Alfvénic Mach number are also denoted.