Time-averaged phase speed of the fast mode at selected fixed geometrical heights (top panel) and selected optical depth levels τ₁ = 1, τ₂ = 0.025, τ₃ = 0.001, τ₄ = 0.0001, and τ₅ = 2.5 × 10^-5 (bottom panel). The bold lines indicate the regime where c_A/c_s > 1, while the thin lines represent the regime where c_A/c_s < 1. The asterisks mark the locations where the respective height levels cross the equipartition level.