direct fallback. In principle, the materials that can fall back onto the surface should be trapped by the magnetosphere and be below the corotating radius $r_{\rm c}$. When no more outflow pressure exists, all the materials within the corotating ball will drop onto the surface if they are trapped by the magnetosphere. The condition of magnetic trapping requires the local magnetic energy density to be comparable of the kinetic energy density of the ejecta, so that the trapping radius is $r_{\rm t}=R B^{1/3} v^{-1/3} (4\pi\rho)^{-1/6}$, where $v$ is the typical velocity of the ejected materials, and $\rho$ is the mean density of the materials. We set $r_{\rm c}=
r_{\rm t}$ to roughly set the fallback condition, and get a rough estimate of $\rho \sim 7.0 \times 10^{-9}B_{12}^2 P^{-4}v_9^{-2} R_6^6 M_1^{-2}$ (about one day after the supernova explosion for $P=10$ms, see Fig.28 in[@woo88]). The total mass that deposits onto the surface (i.e. the mass of the crust) is then $\Delta M=(4/3)\pi r_c^3
\rho$, which reads $$\Delta M\sim 1.0\times 10^{17}B_{12}^2 P^{-2}v_9^{-2} R_6^6 M_1^{-1},$$ or $\Delta M\sim 5\times 10^{-15}M_\odot$ for typical values of $P =10$ms, $B_{12}=R_6=M_1=1$. Note that the typical velocity of the ejecta is adopted as $v=10^9 v_9$ cm/s, which is the 