From Glossary of Meteorology
(Also called green segment.) The upper rim of the low sun usually displays a green rim.
The transient sighting of this rim, between its obscuration by the rest of the sun and its obstruction by the horizon, is sometimes credited with being the origin of the green flash. But, while the color of the flash is provided by the green rim, the size and transient nature has more to do with multiple-image mirages.