Propagation of a 500 kms^-1 perturbation of period 36 ms initiated at a r₀ = 100 km. The period corresponds to a wavelength of λ₀ = 0.25R_WD at the optimal radius: r₀ = 350 km. Initially geometry dampens the wave but then the gradient steepens and it is amplified. We compare the theoretical formula (9) for a dissipationless case to the simulation.