Equilibrium with divergent magnetic field lines. The vertical curves are the magnetic field lines defined by the equation ψ = const. The horizontal curves are defined by the equation φ = const. The thick vertical curves are the boundaries of the region where the wave propagation is considered. They are defined by the equation ψ = ± ψ_b. The thick horizontal curve is defined by the equation φ = φ₁. It is assumed that the wave is driven at this line. The dotted line shows the level z = 0.