The attenuation of the modes increases with wavenumber n. Therefore, essentially only the first two modes are involved in the wave propagation The first interference minimum between these two modes is at the same distance as that of the last interference minimum of ray theory (Eq. 3) indicating the equivalence of both theories As seen in Figure 3, the spacing between the mode interference minima is constant and about 1000 km in this example. The first mode becomes dominant at distances greater than about 1500 km, because the second mode is more strongly attenuated than the first mode.