Asymptotic behaviour of the velocity profile, v(ϕ, t). The thick solid grey line represents the solar wind speed, w, and the asymptotic profile of v(ϕ, t). Two models for three instants in time are shown with red, green, and blue curves. The model with a higher initial speed than the solar wind speed is shown with three solid coloured curves above the grey asymptotic line, and the curves of the model with v₀ = 0.75 km s⁻¹ <  w are shown with dashed lines below the grey line.