Ingredients for a good Green Flash

A green or — more rarely — a blue flash can be seen associated with a setting or a rising sun under some atmospheric conditions when a clear view of a low horizon is available. Such conditions are more often achieved from a vantage point overlooking an ocean horizon. It is a long-established tradition for observational astronomers, if time permits, to watch the setting sun with one eye closed to maintain some dark adaptation in the hope of witnessing a naked-eye green flash.

To see what they can look like, see the photographs displayed on Les Cowley’s atmospheric optics website: www.atoptics.co.uk/atoptics/gf1.htm

The phenomenon is the result a fortuitous interplay between several processes operating in the Earth’s atmosphere when an object, in this case the Sun, is seen through a long, almost tangential atmospheric path.

The first is refraction of the sunlight within the atmosphere as the light travels through a gas whose density increases downwards. This causes the shorter wavelength (blue/green) light to appear at the eye from a direction slightly above the longer wavelength (orange/red) light, resulting in an image of the setting sun with a thin red rim at the bottom and a green/blue rim at the top. However, a stable atmosphere with a simple density gradient will not produce a green flash that can be seen with the naked eye (don’t look at the setting sun with binoculars or telescopes!).

To produce a magnified separation of the green and red images it is necessary to have a more complex atmospheric density structure that results in a mirage phenomenon (see Cowley’s explanation of this: www.atoptics.co.uk/atoptics/sunmir.htm ).

The third ingredient in the recipe is the mixture of processes that absorb and scatter certain colours of light as it travels through this long path of air. It is different combinations of these processes that result in the amazing range of colours to be seen in the daytime and twilight sky. The setting sun is orange/red and the clear daytime sky is blue largely as a result of these scattering processes that change the direction of some of the incoming radiation with a probability that is higher for blue than for red light. This scattering steals some of the blue light from the sun gives it to the sky.

The most intense blue results from the scattering by air molecules — this is called Rayleigh scattering after the 3rd Baron Rayleigh who worked out the theory — while the softer blue/white haze comes from droplets and particles floating in the air that, while still very small, are much larger than molecules: they are generally called aerosols. The stunningly beautiful palette of clear twilight colours is painted as bands of pink, orange, yellow, pale apple-green and blue across the sky by the interplay of these scattering processes.

The gaseous constituents of the air can also absorb light of certain wavelengths/colours. This absorption actually removes rather than redirects sunlight and converts it into heat. Much of this absorption happens beyond the range of our vision in the infrared and ultraviolet parts of the spectrum but there are some important absorptions that happen within our visual range. These include water vapour and oxygen but by far the most important of the gases for the twilight phenomena is ozone, most of which is present high above us at altitudes between about 15 and 40 km. It is produced from oxygen by ultraviolet photons from the sun and catalytic interactions with other atmospheric gases. Ozone is quite unstable and is continually created and destroyed in the atmosphere by the actions of sunlight. If you make it in the lab and can liquify it without blowing yourself up, it has a wonderful deep ultramarine blue colour ( www.bipm.org/en/bipm/chemistry/gas-metrology/ozone/ )

Ozone is responsible for the deep blue colour of the post-sunset twilight sky ( www.flickr.com/photos/bob_81667/39996325804/ ) but it is also an essential enabler for the green flash since it provides the spectral separation between the blue/green and the red refracted components of the setting sun without which the green flash would not be clearly apparent.

The spectra above illustrate how this works. They are computed form the known energy spectrum of the sun by using a simple atmospheric extinction model that includes Rayleigh and aerosol scattering and ozone absorption acting on light coming from the sun at different altitudes in the sky. I can vary the strength of aerosol scattering to represent either a very clear sky through to different amounts of aerosol haze. I can also vary the ozone content within the path taken by the sunlight as the sun sets. Although we know the amount of ozone in the ozone layer and can compute the amount expected in the sunset path, it is not easy to know the fraction of the refracted path that lies within the ozone layer in the presence of mirage-forming refraction. In these plots, I have chosen an amount that assumes that more of the light travels within this layer than travels at lower altitudes.

The upper plot shows the spectrum of the setting sun (blue line) in a very clear atmosphere with an aerosol content only 10% of a normal mid-latitude sky. The red dashed line is the setting sun as seen with no ozone in the atmosphere at all. This plot is on a logarithmic intensity scale to show the huge range in intensity across the spectrum of a factor of 100,000. The lower plot shows the same spectra on a linear scale to show the way in which the ozone absorption very clearly separates the blue/green from the red images of parts of the sun. The pale green line shows the same model with the aerosol content increased by a factor of four, showing that aerosols have a significant effect on the intensity of the green image (flash) but a minor effect on its colour.

The strongest conclusion from this is the enormous effect that the ozone has on the visibility of the green flash phenomenon. If there were no ozone, it would probably never be seen at all.


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