Does backscattering from a solid prolate spheroid in water exhibit the axial focusing of glory scattering?

Many solid spheres in water illuminated by ultrasound display enhanced backscattering partially analogous to the optical glory of cloud droplets. The enhancement associated with glory scattering is axially focused. In the acoustical case, one of the mechanisms for producing the needed outgoing toroidal wavefront is the launching and guiding of a leaky Rayleigh-like wave around the surface of the sphere [K. L. Williams and P. L. Marston, J. Acoust. Soc. Am. 78, 722–728 (1985)]. A related backscattering enhancement was also demonstrated for spherical shells [S. G. Kargl and P. L. Marston, J. Acoust. Soc. Am. 85, 1014–1028 (1989)]. In the present research, we demonstrate that a similar backscattering enhancement is present for axisymmetric ultrasonic illumination of a solid prolate brass spheroid. By using tone burst illumination, the axially focused contribution to the scattering is distinguishable from the specular reflection contribution to the scattering. The evidence for the Rayleigh wave mechanism is seen in the timing of this contribution: experimental and model predictions are consistent and through bistatic measurements of the variation in its strength as scattering angle changes. This research is relevant to surface-guided ray theory extended to variable curvature objects in water. [Work supported by the Office of Naval Research.]