Growth rate σ of tidal instability, predicted by formula (23) in equatorial plane (x₀ = 0.5, z₀ = 0), as a function of N₀/Ω_s and Ω₀. Colour bar shows the normalised ratio log₁₀(σ/β₀). White areas correspond to marginally stable areas. For neutral fluids, tidal instability is restricted to the allowable range −1 ≤ Ω₀ ≤ 3 when β₀ ≪ 1. When Ω₀ = 1 (horizontal white line), the basic state is synchronised (see Appendix D).