θ = 0°: the blue solid line, orange dashed line (zero in this case) and green dotted–dashed line correspond to v_∥, v_⊥and ρ/ρ₀, respectively. The shock front is formed and travels through the a = c layer unaltered. The solid vertical line is where a = c, which moves in response to the incident wave. The pink horizontal line is the equilibrium sound speed. The blue dot indicates the steepest point in the acoustic shock. The top-right panel indicates that the shock reaches the conversion layer with almost-maximal density contrast (X = 4).