From AMS Glossary
A flash of green light seen on or (seemingly) adjacent to the upper rim of the low sun (at either sunrise or sunset).
The green flash is a mirage, but the image formed in this case is of a portion of the sun rather than of an earthbound object. In addition to the displacement and distortion that is characteristic of mirages, there is also significant dispersion. The upper edge of the low sun normally has a thin green rim (occasionally blue) that is too narrow to be seen by the naked eye unless the rest of the sun is obstructed, say, by the horizon. It is often asserted that the green flash is seen in this way: a mere transient view of the green rim between obscuration by the rest of the sun and obstruction by the horizon. Yet such a sequence produces a singularly poor flash. Rather, the remarkable flashes always seem to involve multiple and magnified images of the green rim. Indeed, the presence of such multiple images of a small portion of the sun is a good indicator of a forthcoming flash. The optical signature of multiple images is a serrated edge to the sun. The refraction that displaces the image of the low sun up from the position it would occupy in the absence of an atmosphere does so more strongly for shorter wavelengths. This leads to a red rim on the bottom of the sun and a blue or green rim on the top.