Phase portraits (level curves of resonant Hamiltonian) of apsidal resonance with critical argument of σ₁ = h^* + sign(H^*)g^* at different levels of motion integral specified by i_*. Here i_* represents the magnitude of motion integral Σ₂ (see text for details). i₀ shown on the y-axis corresponds to the inclination when the angle g is equal to zero. Dynamical separatrices are marked as red lines. The resonant width, denoted by Δi₀, measures the maximum size of the island of libration.