Published in Appl. Opt. 50, F80-F86, doi: 10.1364/AO.50.000F80 in 2011
Gunther P. Können and Siebren Y. van der Werf
From an aircraft, a short
distinct vertical structure is sometimes seen above the setting sun. Such a
feature can be understood as a halo which is the counterpart of the well-known
subsun. Whereas the latter arises from reflections off basal faces of
plate-oriented ice crystals when illuminated from above, what we call the
supersun emerges when these crystals are illuminated from below. The supersun
occurs when the sun is below the true horizon and is only visible from elevated
positions. The curvature of the Earth causes the ensemble of reflecting crystal
faces to act as a hollow mirror and the supersun appears as a vertical band of
uniform width, extending from the sun upwards to its supersolar point. We
discuss the geometrical properties of the phenomenon and simulate its shape and
radiance distribution with an extended version of an atmospheric ray-tracing
program.