Time evolution of the phase spiral at R = 8 kpc. Columns represent azimuths equally spaced by 20°. Rows correspond to time, from t = 0.9 Gyr to 1.28 Gyr on the left block, and t = 1.29 Gyr to 1.67 Gyr on the right block. Each panel shows z/h_z−v_z/σ_z maps colour-coded by the density contrast, Δ ρ. Panels showing two-arm phase spirals are painted in blue tone. Contours indicate the normalised surface density, Σ(R = 8 kpc, ϕ, t)/Σ₀(R = 8 kpc, t). Arm regions (Σ/Σ₀>1) are shaded. One-arm spirals emerge first, followed by intermittent appearances of two-arm spirals around 200-250 Myr after the bending-to-breathing mode transition.