Abstract

Superior mirages of great complexity are frequently seen under arctic temperature inversions. A mirage group observed at Resolute Bay, Canada, over a horizontal range crossing ~70 km of sea ice shows clearly identifiable sequences of five simultaneous images, alternating between erect and inverted. The marked similarity between these images and the solar Novaya Zemlya effect¹ suggests that optical ducting is the underlying phenomenon. Two ducting models are compared. The first assumes that the inversion forms a short duct along the line of sight. If duct length and atmosphere beyond the duct are specified, the temperature profile within the duct can be computed directly, using a ray steering algorithm, and the observed image can be simulated arbitrarily closely. The duct of the second model is based on a shear-free three-layer atmosphere,² in which the middle layer supports gravity waves propagating along the line of sight. The waves are represented as isothermal surfaces of sinusoidal form, whose amplitude decreases with vertical distance from the inversion. The waves permit some rays to escape and transmit image information across the inversion. An analysis of imaging properties as a function of wave parameters indicates whether this model is capable of generating the observed fivefold images.

© 1987 Optical Society of America