Acceleration of migration at crossing MMRs. The upper panel shows the evolution of a_U, while the lower panel displays the period ratio between Neptune and Uranus. Different α are represented by different colors. The M_disk are not explicitly distinguished, but their relative positions are fixed, with four lines of the same color representing M_disk of 30 M_⊕, 40 M_⊕, 50 M_⊕, and 60 M_⊕ from bottom to top. In the lower panel, two dashed black lines indicate positions where the period ratio between Neptune and Uranus is 2 and 1.5.