Solutions s_e of Eq. (23) as a function of s₀/L for ρ_t/ρ₀ = 200 (orange lines) and ρ_t/ρ₀ = 400 (black lines). The expression L = 10 H and l_t = 0.2 H. The curves correspond to and (solid lines) and (dashed line). The bifurcation points around s₀/L = 0.3 and s₀/L = 0.7 (intersections between vertical and horizontal orange dashed lines) have been calculated by solving Eqs. (23) and (24) simultaneously.