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Session 80 - Education, Green Flashes and Future Missions.
Oral session, Thursday, June 11

[80.02] Understanding Green Flashes

A. T. Young (SDSU)

Most astronomers learn about green flashes from either Minnaert's old book (Dover, 1954) or O'Connell's ``The Green Flash....'' Both have defects. Minnaert's account mostly represents what was known in the 1920s; it repeats Mulder's 3-fold classification, which omits Joule's second type of flash --- the one most commonly seen from mountain observatories.

O'Connell searched only the astronomical literature, missing Dietze's crucially important paper (Z.f.Met. 9, 169 (1955)) showing that the ``textbook'' mechanism cannot produce flashes visible to the naked eye. He also erred in thinking that distortions of the setting Sun arise in the upper atmosphere (they are due to the marine boundary layer), and copied an error from Feenstra Kuiper's thesis that misidentified a common mirage-like phenomenon as Wegener's ``blank strip'' (Young et al., Appl. Opt. 36, 2689 (1997).) Most phenomena shown in O'Connell's book are caused by inversion layers below eye level, not above as in Wegener's phenomenon.

The two commonest forms of green flash are associated with the inferior mirage and the mock mirage, corresponding to Fisher's Type A and Type B sunsets, respectively. Superrefraction, advocated by Wood and by Rayleigh as the cause of large flashes, actually suppress them: the airmass is proportional to the refraction (by Laplace's extinction theorem), so no green is transmitted when refraction is much larger than average.

Although there is a physical green flash that can be photographed, the colors seen at sunset are strongly modified by bleaching of the L cones. Most ``green'' sunset flashes are actually yellow.

Writers should stop representing Jules Verne's ``ancient legend'' as fact, as it was invented by Verne as a plot device for his novel ``Le Rayon Vert.''

Green-flash photos and simulations will be shown.

This material is based upon work supported by the NSF under Award No. ATM-9714357.

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