Mercury transit sub-duct slab green flash

Last updated on September 16, 2023

Mercury Transit Mirage: Exploring the Phenomenon of the Sub-Duct Slab Green Flash

The mesmerizing world of atmospheric optics never fails to surprise us with its stunning displays. One such rare phenomenon is the "Mercury transit sub-duct slab green flash." On November 11th, 2019, Amir Hossein Abolfath captured a breathtaking image in Tehran, Iran, showcasing distant mountains silhouetted against the setting sun, with Mercury appearing just above the mountains. However, what made this image truly extraordinary was the faint green band above and separated from the sun's upper limb. This elusive green band is not a camera artifact but a very rare atmospheric effect, rarer even than witnessing a Mercury transit itself.

Amir's green rim was revealed by isolating the green channel from his original camera RAW file. The luminous patch within the sun's disk indicates scattering by hazy air. Tamás Ladanyi's white light image further illustrates how the green limb typically clings narrowly to the sun's disk. To understand this phenomenon better, we delve into the concept of optical ducting.

When sun rays slant downwards across an inversion layer in the atmosphere, they undergo refraction or deflection. The rays from warm air to cool air deflect towards the cooler layer. In cases where the inversion is strong enough, these rays become trapped in a "duct" just below the inversion layer, allowing them to travel for large distances around the Earth's curvature.

The observer's distance below the duct and inversion layer plays a crucial role in experiencing the sub-duct slab green flash. Factors such as an ideal sun height, hazy aerosol-filled air, and Amir's fortunate position just below the duct all contributed to this highly unusual sighting. The camera clicked at the exact right moment to capture the fleeting beauty of the green flash.

To understand the formation of the green rim at the sun's upper limb, we start with ordinary atmospheric refraction, which raises the sun from its true position. Green rays refract more than red rays, resulting in a green upper rim. The sun's apparent height of 0.8° in this case was perfect for producing a bright and wide green rim. However, the rim alone would be hardly visible without the aid of a long lens and image processing. It is important to note that rims are often mistakenly attributed to green flashes, even though they are typically too narrow.

The presence of aerosols in the atmosphere played a significant role in this phenomenon. These aerosol particles scatter the green rim rays strongly forwards, effectively blurring and broadening the rim. As the scattered rays encountered a temperature inversion layer with warm air above cooler air, an optical "duct" formed. Ducts act as wave-guides, trapping the sun rays and causing them to bounce up and down along the duct. This process leads to the formation of sunset mirages, which magnify color separations and give rise to certain types of green flashes.

Amir's fortuitous position just below the duct allowed for the creation of a highly unusual "sub-duct flash" or "green slab." These green slabs change rapidly with the distance below the duct and the sun's changing altitude. In Amir's case, the green slab lasted no more than a few seconds, explaining its absence in adjacent images. This sighting was truly exceptional, as reported by experts in mirage and green flash phenomena.

Andrew Young, a renowned expert in mirages and green flashes, confirms the rarity of such sightings. Visual observers have occasionally reported seeing these green slabs in the atmosphere. Some have described how "everything lit up green along the horizon" during a sunset green flash. In one instance, a photographer who had given up on capturing the setting sun witnessed the landscape around him turning green. He managed to turn around just in time to capture the vanishing green flash at the astronomical horizon. Unfortunately, the photograph was not given the recognition it deserved by an astronomy magazine.

These phenomena are undoubtedly real but extremely rare. The intricacies of atmospheric optics continue to astound us, reminding us of the hidden wonders that surround us every day. It is through the keen observations of individuals like Amir Hossein Abolfath and the expertise of scientists like Andrew Young that we can uncover the secrets of these captivating displays.

Mercury Transit Mirage

Amir Hossein Abolfath (Photography, TWAN) took this at Tehran, Iran on 11th November 2019. Distant mountains are silhouetted against the setting sun. At left, just above the mountains, another world is silhouetted - Mercury. There is more...

Above, and separated from the sun's upper limb is a faint green band. Amir took many images but the green band appears only on this one. Dismiss it as a camera artefact? No. It is a very rare atmospheric effect. Rarer than a Mercury transit!

CAmir Hossein Abolfath

Amir's green rim is revealed by showing only the green channel of his original camera RAW file. The luminous patch within the sun's disk indicates scattering by hazy air.

A white light image by Tamás Ladanyi showing how the green limb is usually so narrow and clings to the sun's disk.

Optical ducting

Sun rays slanting downwards across the inversion layer are refracted or deflected. Rays from warm air to cool deflect towards the cool layer. When the inversion is strong enough the rays

are trapped in a 'duct' just below the inversion. They can travel for large distances around the Earth's curvature.

The small deflections of mirages are difficult to draw. This diagram is NOT to scale!

Sub-duct green slab flashes computed by Andrew Young.

The observer's distance below the duct and inversion layer is critical. Here, there is only 1m

height difference between each simulation.

Several factors combined to create this highly unusual sighting. The sun at an ideal height, hazy aerosol filled air, a temperature inversion with ducting (see below), Amir's position just below the duct, the camera clicked at the exact right moment.

We start with a green rim at the sun's upper limb. Ordinary atmospheric refraction raises the sun (0.43° in this case) from its true position. Green rays refract more than red producing a green upper rim. The sun's apparent height of 0.8° was ideal for producing a bright and wide green rim. Nonetheless, the rim alone would be unexceptional and hardly visible without a long lens and image processing.Rims are all too often wrongly used to explain a green flash,

they are far too narrow.

The aerosol intervened, scattering the green rim rays strongly forwards. Large aerosol particles scatter light forwards with only a small change of direction effectively blurring and

broadening the 'rim'.

The scattered rays intercepted a temperature inversion layer, warm air layered above cooler air.

This inversion was strong enough to form an optical 'duct'. Ducts trap rays like a microwave wave-guide. Sun rays effectively bounce up and down along the duct to produce sunset mirages. The mirages magnify colour separations and produce some types of green flash.

Amir was in a fortuitous position not far below the duct. The

result, a highly unusual 'sub-duct flash' or 'green slab'.

Green slabs change quickly with distance below the duct and

with the suns changing altitude. Amir's would have lasted

no more than a few seconds. That explains it's non-

appearance on adjacent images.

An exceptional sighting.

Mirage and green flash expert Andrew Young comments: "A few visual observers have reported seeing these green slabs in the atmosphere. One who saw a sunset green flash reported to me that "everything lit up green along the horizon." A photographer who had given up on a setting sun and was facing away from the sunset saw the landscape around him turn green, and had enough time to turn around and catch the vanishing green flash at the astronomical horizon that is typical of sub-duct flashes; unfortunately, he sent the picture to an astronomy magazine, but they weren't impressed enough to print it, and managed to lose the original film.

These phenomena are real, but very rare."

A thank you to mirage and green flash expert Andrew Young for his analysis of this sighting. Errors of telling the story are mine.

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CAmir Hossein Abolfath

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