Double-crossing mode conversion in nonuniform media

A new type of mode conversion between two linear waves in a nonuniform medium is investigated. Single-crossing conversion occurs when a ray of one wave crosses transversely the dispersion manifold of another wave. Double-crossing mode conversion theory describes when the ray punctures the dispersion manifold twice due to ray curvature. To study this new process, a one dimensionally nonuniform background medium is considered, which gives rise to four scenarios. These scenarios are distinguished on the basis of whether the two waves have equal or opposite energy signs, and whether they are copropagating or counterpropagating. Using modular-eikonal theory (suitable for multiple crossings), each scenario is first studied analytically by constructing an S-matrix relation between the outgoing and incoming asymptotic wave amplitudes. The analytical results are then compared with numerical results and excellent agreement is found. (C) 1998 American Institute of Physics.