Roots of the mode equation for propagation in an elevated duct

The significance of the different classes of modes in a mode theory solution of the problem of propagation in an elevated duct is studied. The roots of the mode equation are interpreted in terms of the more familiar ray theory picture, and a one-to-one correspondence between classes of zeros in the mode picture and ray trajectories in the ray picture is demonstrated. A previously published paper is shown to have misinterpreted the correspondence between the ray and mode theories for elevated duct propagation.