The Green Flash Phenomenon

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"Ever gazed upon the green flash, Master Gibbs?"
"I reckon I've seen my fair share. Happens on rare occasion; the last glimpse of sunset, a green flash shoots up into the sky. Some go their whole lives without ever seeing it. Some claim to have seen it who ain't. And some say... "
"It signals when a soul comes back to this world, from the dead!
"

― Hector Barbossa, Joshamee Gibbs and Pintеl to Will Turner. Pirates of the CaribbeanAt Worlds End

 

The Green Flash Phenomenon

 

Yovelina Zinkova, Radostina Velevska and Ivana Yankova

Astronomical observatory by Youth center, Haskovo

(Summer Astronomical School on 01-10 August 2011, NAO Rozhen)

 

 

     In many cultures the Green Flash has been part of different myths and legends. It has been connected with supernatural events which interest the people around the world. In nowadays science the ideas about the phenomenon are much more realistic. Its existence has been proved scientifically, but it still brings excitement to the observer

 

Fig. 1. In 1869 James Prescott Joule received an approval for his explanation about the Green Flash event. The English physicist introduced his results by a letter to the Manchester Literary and Philosophical Society.

 

    The astronomical object that we connect with the phenomenon is the Sun. Just a glimpse at the night sky reveals to us amazing number of stars in the Universe. The Sun has physical characteristics like many of the other stars, but the fact that it’s the closest one to us makes it more valuable than all of them together. This gives us the opportunity to explore its structure in details, observe active regions on its surface and follow its evolution.

    Electromagnetic spectrum – the range of all the electromagnetic radiation, which an astronomical object (for example the Sun) absorbs, reflects and omits.The Sun emits in all ranges of the electromagnetic spectrum, but only a small part of it, with wavelength from 380 to 750 nm, could be accepted by the human eye. The strongest radiations are in the green range of the spectrum, which is the reason for its perception by our ophthalmological organ. The so called visible spectrum consists of the 6 familiar main colors – violet, blue, green, yellow, orange and red – arranged in descending order by their frequency (fig. 2).

Fig. 2. Electromagnetic spectrum. (more)

     These colors are called monochromatic. The emitted waves spread across the space with the speed of light in vacuum ~ 300 000 km/s. But not in all materials the speed of the electromagnetic waves is a constant, for example when they enter the Earth’s atmosphere they are influenced by the change in its density and they refract, reflect, absorb and scatter.

    The optical phenomenon called Green Flash could be observed shortly before sunset and sunrise of the Sun, Moon and some of the brighter planets. Each one of ushave seen the changes in the bigger sky objects (Sun and Moon) when they are close to the horizon, no matter if they are rising or setting. They are: flatness of their discs, their reddening and change in the visible position in their limbs with an angle r=35 arcminutes, which leads to a change in the duration of the day (fig. 3). The event is called refraction.

Fig. 3. Visible change in the position of the sky objects

 

    The reddening of the Sun and the Moon (fig. 4) are due to the scattering of the electromagnetic waves by the molecules in the atmosphere, called Rayleigh scattering. These molecules are too small in size compared to the wavelength that is why the violet light (400 nm) scatters 7 times more than the red light (650 nm).

Fig.4. Reddening of the Sun and the Moon. (more)

    The visible disc of the Sun consists of all the colors from the visible range of the spectrum. Let us imagine the Earth’s atmosphere is a big prism. When the light enters from material with one coefficient of refraction into material with different coefficient we notice a reduction in the speed with which the waves spread and a change in their trajectory (fig. 5).

Fig. 5. Passing of a beam through materials with different densities.

On fig. 5. n1 and n2  are coefficients of refraction of the two materials, θ1 and θ2 are the angles of falling and refraction.

    The waves of the visible spectrum have different frequencies and are refracted to a different grade and they create an image of the Sun, which consists of several smaller discs in variant colors which are not exactly placed on each other. The best time to observe dispersion of light is the last moment of a sunset, when we should be able to see a violet or a blue flash. These two colors are strongly scattered by the Earth’s atmosphere unlike the green, which remains uninfluenced and leads to the occurring of a Green Flash. Another reason is the fact that our eyes are most sensitive to the green part of the visible spectrum. Basing on the examined events the Green Flash phenomenon occurs (fig. 6 & 7). In rare occasions a Blue Flash could be observed (fig. 8).

Fig. 6. Green Flash photographed from NAO Rozhen, Bulgaria. CanonEOS 1DsMarkII, ISO 50, exp. 1/5000 with Lenses 300mm f/2.8 + 2x Canon EF Extender II.

Fig. 7. Green Flash from the Moon(Gerhard Hudepohl, Cerro Paranal, the 2600metre-high mountain in Chile’s Atacama Desert). (more)

Fig. 8. Blue Flash (Mario Cogo, Canary islands). (more)

    To observe a Green Flash the horizon must be clear and the meteorological conditions should be very good. The most appropriate places for picturing the event are deserts and places close to sea level (fig. 9), where the thin ground layer of the atmosphere is defining for the high light dispersion.

 

Fig. 9. Green Flash,Tel Aviv, Canon 40D, 500 mm f/11 lenses, exp. 1/200, ISO 200. (more)

 

There are 4 kinds of Green Flash. They are:
 
Inferior-mirage flash (fig. 10) – this kind of mirage could be observed from areas close to the sea level. We can see this event when just above the water surface there is a layer of hot air and the temperature rapidly changes with the increase of the height; the hot layer should be bellow the observer’s level. When the real Sun goes downwards, its mirage goes upwards and they merge into an image called an Etruscan vase or an Omega. In the moment when the two suns are under the horizon, the distance between the red and green beams is the largest and the Green Flash becomes visible.
 
Fig. 10. Sub-duct flash, California. (more)
 

Mock-mirage flash (fig. 11 & 12) – the best places to picture a mock-mirage flash are with high altitude. To observe such a mirage you must be above the layer of cold air and there should be a temperature inversion – with increase in the height the temperature keeps rising. Unlike the inferior- mirage, the mock-mirage can create multiple images which change unpredictably in time. The most common shapes are: triangles, quadrangles and pancakes. The Green Flash could be seen when a part of the Sun’s disc separates from it

Fig. 11. Mock-mirage flash, Mila Zinkova, San Francisco. (more)

 

 

Fig. 12.  Mock-mirage flash, NAO Rozhen, Bulgaria. Canon EOS 1Ds Mark II, ISO-50, exp. 1/1600, f/18, Canon 300 f/2.8 + 2x extender lenses.

 

Sub-duct flash (fig. 13) – the phenomenon could be seen at any latitude in condition the observer is bellow a layer with strong temperature inversion. The Sun’s disc has the shape of an hourglass and its upper part becomes a Green Flash for about 15 seconds
Fig. 13.Sub-duct flash, California. (more)
 
Green ray – this kind of Green Flash is better to be observed from close to the sea level place; the air must be hazy and you need an unobstructed horizon. It occurs in the exact moment when the Sun’s disc is just under the horizon, usually it is a few degrees long and last several seconds (Fig. 14).

 

Fig. 14. Green ray flash (more)

 

 

More about Green Flash:

http://en.wikipedia.org/wiki/Green_flash

http://mintaka.sdsu.edu/GF/