Sketch of domain . The coordinate lines are shown in the plane θ = constant. The vertical curves are the magnetic field lines defined by the equation ψ = constant. The horizontal curves are defined by the equation ϕ = constant. The thick vertical curve is the boundary of the region where the wave propagation is considered. It is defined by the equation ψ = ψ_b; ϕ₁ = −H and ϕ₂ = −He⁻⁶. It is assumed that the waves are driven at the line ϕ = ϕ₁. The dotted line shows the level z = 0.