Number of emergences (N) as a function of the normalized underlying azimuthal flux density (). The different curves correspond to different choices of the free parameters. Continuous black line: u₀ = 3 m s^-1 and η₀ = 100 km² s^-1. Dashed blue line: u₀ = 5 m s^-1 and η₀ = 400 km² s^-1. Dashed red line: u₀ = 1 m s^-1 and η₀ = 25 km² s^-1. Continuous green line: u₀ = 6 m s^-1 and η₀ = 600 km² s^-1.