Minimum momentum p of particles that may reach the distance x_p downstream and return to the shock to continue acceleration. It is calculated from the equation $p=\overline{p}(x_{\text{p}})$ for Bohm diffusion. The colored lines correspond to a few time moments (see inset). The dashed lines show p calculated in a common approach, from x_p = D₂(p)/u₂ with uniform u₂ and uniform B₂ = 5 µG. The top plot is for model 1 and the bottom plot is for model 2.