Top panel: color-coded values of plasma β in a plane of x constant located at the center of the simulation box. The values of β are saturated at 15 for a better visualization. Bottom panel: isosurface of β = 1 layer in a subvolume (black box) close to the center of the simulation box (view is along the positive X-axis). Also shown are the fan and spine field lines, together with the Z component of the magnetic field at the bottom boundary of the simulation box.