The Green Flash

The green flash refers to any of several types of atmospheric phenomena involving the color of the rising or setting Sun. They are brought about by a combination of refraction, absorption, and the dispersive action of the atmosphere. It is related to the phenomenon of wedge refraction, in which telescopic images of stars near the horizon are seen to be tiny spectra (see §3.1.3). The atmospheric refraction bends starlight upward toward the zenith, but the index of refraction for bluer wavelengths is slightly greater than that for the red so that the blue starlight is raised slightly higher. A similar situation holds for the Sun: The most violet spectral component of sunlight is scattered away by atmospheric molecules ("Rayleigh scattering," see §3.1); but of the remaining light, the blue-green images of the Sun are lifted higher than are the yellow-red images. Minnaert (1954, p. 63) reports that the Moon, Venus, and Jupiter have exhibited the same phenomenon. Some mythological references suggest the possibility that the green flash was observed and noted by ancient groups.

The frequency of the green flash phenomenon depends on the viewing location. The green flash is a relatively rare naked eye phenomenon, but telescopically it is more frequent. DHK saw it only a few times during a year at the Blue Hill Meteorological Observatory, Massachusetts, using only the naked eye. It is easiest to see over a sea horizon, and some optical aid (projected or carefully filtered to avoid temporary insensitivity or blindness) is a prudent precaution. EFM has had the most success at spotting the green flash telescopically from Mt. Lemmon, an 8000-foot mountain northwest of Tucson, Arizona, where the setting sun is seen at a fairly large dip angle. O'Connell (1958) suggested that visibility depends on (spatially resolved) patches of air of higher density momentarily enhancing the refraction of the bluer portion of the spectral image of the Sun; with this explanation, because telescopes increase spatial resolution, even weak effects can be observed. The transient density enhancement theory is disputed, however, by Young and Kattawar (1998), who provide a more detailed explanation. See also §3.1.3 and Young et al. (1997) for a discussion of the green flash in the context of what these authors call a "mock mirage," an inverted overlying image, commonly seen by the naked eye in the setting Sun.

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