Force ratio F_Ph∕F_D under the assumption that I = L∕(4πr²). The solid lines correspond to the force ratio calculated from the normal model (f_d = 10⁻¹ and α = 10⁻³), and the dashed lines are the dust-poor model (f_d = 10⁻⁶ and α = 10⁻²). The vertical lines represent the location of the light barrier. The line colors represent the mass accretion rate (Ṁ = 10⁻⁷M_J yr⁻¹ for blue, Ṁ = 10⁻⁸M_J yr⁻¹ for pink, andṀ = 10⁻⁹M_J yr⁻¹ for green). As a reference, the current orbit of Io is r = 5.9 R_J.