Top: Normalized critical field strength as a function of radius. We also show the maximum absolute value of the three components of the Prendergast field (see Sect. 2.2), which is scaled to B_cen = B_crit, cen. Furthermore, we show the radial component of an improvised magnetic field that has a plateau and scales with the critical field strength at the hydrogen-burning shell B_crit, H. The strength of the improvised field at the hydrogen-burning shell is set to B_H = B_crit, H. The hydrogen-burning shell is indicated by the vertical dotted line. Bottom: Trapped fraction as a function of the central field strength or the field strength at the hydrogen-burning shell for model 1A with ω = 10 ω_dyn and l = 1. The black symbols correspond to the case where all components of the Prendergast field are taken into account. They are the same as the black symbols in the top panel of Fig. B.1. The blue symbols indicate that only the radial component of the Prendergast field has been used and the other components were set to zero. The red symbols correspond to magnetic field featuring the plateau that is scaled by B_crit, H instead of B_crit, cen. The dotted line marks either B_crit, cen or B_crit, H. The line connecting the symbols is the fit to the trapped fraction (Eq. (13)).