Initial configuration of the pre-SN ambient magnetic field for models MOD-B1 (upper panel) and MOD-B100 (middle panel). The green lines are sampled magnetic field lines; the scale of the magnetic field strength is log, shown by the bar on the left of each panel, in units of G. The unshocked equatorial ring material (with n ≥ 1000 cm⁻³) is in blue; the figure does not show the H II region around the ring. Lower panel: radial magnetic field strength in the equatorial plane for the two models; the dotted line indicates the radius of the BSG progenitor, the dashed line the inner edge of the H II region.