I investigated the flash on youtube and came up with this excellent video. No need to watch the whole thing the flash occurs very briefly at 1.23. (permission has been granted to copy it).
Ira I think I have an idea about this but I going to ask my friends if they can explain the diffraction of light.
Amazingly when you see a sunset the Sun is actually quite a way below the horizon. What you are seeing is the light from the Sun refracted. The light from the Sun, during the day it is passing through five different atmosphere levels. At sunset it is just passing through one level – the lowest level and the ‘thickest’ level. This is usually linked to the red sunset …. diffraction by dust of the red part of the spectrum. This also, in very special atmospheric conditions causes the green part of the spectrum to appear at sunset. (will continue to look at this).