The glory ray and the rainbow are analyzed by considering the scattering of light from inhomogeneous particles. It is shown that melting ice crystals may be strong contributors to the glory ray. Geometrical optics is used to investigate and catalog a wide variety of particle inhomogeneities which support rainbow and glory rays.
C. L. Brockman, Jr. and N. G. Alexopoulos, "Geometrical optics of inhomogeneous particles: glory ray and the rainbow revisited," Appl. Opt. 16, 166-174 (1977)