m = 1 mode perturbations σ₁/σ₀ for different parameters of equilibrium states. It generates one arm structure appearing as a crest (with colors from green to yellow) on an axisymmetric disk. The bright arm is undergoing reverberation with response to variations of ionizing continuum. The dark arm shows a density valley without or weakly response to the continuum variations; Ω_p = ω/m is the rotation speed of the arm pattern.