|Green Flash at La Silla, Chile
Imaged by Yuri Beletsky from ESO La Silla observatory as the sun set over the distant Pacific Ocean. La Silla is 7,800 ft high in the Chilean Atacama Desert.
Green flashes come in several varieties. The two principal ones are the classical ‘Etruscan vase’ flash that occurs close to the horizon and this type, a ‘mock-mirage’ flash. Mock mirage flashes are at the top of the still visible solar disk and they are easier to photograph than to see directly.
A temperature inversion, unusually warmer air over colder, is necessary. Light is refracted as it passes between the layers of different temperature, density and thus refractive index.
In its purest form the resultant mock mirage forms three images of the sun. At sunset the upper one above the inversion descends and is the right way up. The topmost image inside the inversion is upside down and ascends. The lower image is again the right way up and descends.
the flash occurs when the mirage produces considerable vertical magnification of the sun's otherwise indiscernible green rim.
The mirage and flash are only visible when above the inversion.
Here the appearance is typical of a high altitude flash and the atmospheric temperature layering is complex.
Top image ©Yuri Beletsky
Lower image courtesy of European Southern Observatories.