Photometeors

7 Suns Illusion Appears In China!

That was the headline that caught me by surprise. As a sky-phenomenon enthusiast, if it is a weird thing that happens in the sky, I want to know about it and I want to take pictures. So, when I saw a plausible image of an atmospheric phenomenon that I was unaware of, I was taken aback. However, only a moment later the surprise was replaced by disappointment, it was an obvious fake. The shapes of the duplicate suns were too perfect and orderly to be caused by a large-scale optical illusion. It was probably a multilayered window that was causing an infinite mirror effect (the kind you see in trial rooms).

A quick Google search confirmed that the image was taken through a laminated hospital window.

The 7 Sun illusion that went viral.

The shock that I experienced from this deception has driven me to compile a list of real optical phenomena, to ensure that I am never bamboozled again.

Photometeors?

During the writing of this, I struggled to decide what these phenomena should be called as a collective. They are not all "illusions", calling something an illusion implies that it is deceptive in some manner and I don't think that a rainbow is deceiving you into believing anything unless you believe that there is a pot of gold involved. Optical phenomenon would have fit, but it doesn't convey that I am referring only to things that appear in the sky. Then, I found the word Photometeors, what a delightful word! Made up of photo, meaning light and meteor which has its roots in the Greek "meteōron", literally "thing high up,", hence the title.

Let's begin with the one optical phenomenon that gets the closest to showing you multiple images of the Sun.

Mirages

You are in the desert, the sun beating down on you, you are lost and all you want is water. You walk up a dune and you see in the distance, through the heat haze, water! The fabled desert Oasis has presented itself to you.

That is the classical image of a mirage, which inevitably ends in a mournful lament where you explain out loud "Of course! it was only an illusion caused by the sky being refracted through layers of increasingly hotter air near the ground. Making it appear as if there were a pool of water there and I believed it! Curses! What a fool I am." At this point you probably wake up from this fever dream.

The sky reflect from the ground appearing as a water body.

A superior image of a shipping vessel.

That classical depiction, where you see an object near the horizon reflected below it, is known as an inferior image. It is also possible to see a superior image when there is an inversion of air temperatures near the surface. Over the ocean or in really cold areas, Instead of the air getting colder as you move away from the ground it gets hotter because the surface is cooling the air close to it. This causes the refraction to occur in the opposite direction, making reflections of objects above them.

On exceedingly clear days as the disk of the Sun approaches the horizon a small mirage of it appears below it. That is the only way you can get a real "reflection" of the Sun in the sky.

Image of a mirage affecting the disk of the Sun.

Next time you come across a rainbow notice that it has formed diagonally away (at 42 degrees) from the point directly opposite to the Sun.

Rainbows

The most whimsical objects in the entire land. Everyone gets taught how a rainbow forms in school and in general everyone has the right idea. Light refracts and internally reflects inside a water droplet until it is able to re-emerge from the other end having been split into its constituent wavelengths. The distance from the centre of a drop at which a ray enters it is known as the impact parameter. The impact parameter of a ray is zero if it hits dead centre and 1 if it hits right on the edge. Each ray takes a different path through the drop based on its impact parameter and each one is split into a rainbow. Most of these rainbows are just scattered over one another and it all ends up making white nearly everywhere. That is everywhere other than 42 degrees from the point directly opposite the Sun. At this magic angle, a range of rays with an impact parameter of around 0.86, release their rainbows from the drop at the same angle. Each colour from this group of rainbows overlaps and stacks onto itself, forming a vibrant rainbow.

The overlapping rainbows released at 42 degrees are not unique, in theory, there are higher order rainbows that form at greater angles. It is not entirely uncommon to see a double rainbow form, the secondary rainbow forms at an angle of 52 degrees and is about 43% as bright as the main bow. It is also upside down, ROYGBIV as opposed to VIBGYOR (counting from the bottom). For every higher order rainbow, light has to undergo an additional reflection inside the drop and at each reflection point some of the light escapes the drop. This makes all rainbows higher than the second order nearly invisible to the naked eye, although, cameras can capture them in the right conditions.

Solar halos.

Ice crystal halos

Rainbows are hardly the only arcs in the sky, while rainbows are caused by water drops which are all near spherical, ice crystals are far more complex and as a result form a far more complex system of arcs. There is a chance that you have never seen one of these arcs. They require the sky to be filled with ice crystals like those found in high cirrus clouds.

Ice crystals come in many shapes and sizes, each of these shapes is made up of many unique facets joined together at various angles. This presents an array of unique paths that a ray of light can take through a crystal and re-emerge after refracting and reflecting within it. All these unique paths thus form unique arcs in the sky.

The most common among these is the 22 degree halo. Like a rainbow forms 42 degrees away from the opposite direction of the Sun, a 22 degree halo forms a perfect circle around the Sun (or Moon as pictured here) at a separation of 22 deg from it. Another famous feature of these arcs are Sun dogs, they form just outside the 22 deg halo and are also known as parahelia. Beyond these two features that are easily spotted there are many rarer arcs that require exceptional conditions for viewing. A full display of these arcs can criss cross the entire sky in geometric patterns made with circles and parabolas. (In the words of Finn Adventure Time) It's all very mathematical and the exact formation of each of these shapes would become an entire lecture series. Luckily someone has done that work and put it on the internet.

I would love to write an article explaining the mathematics and physics behind the ice crystals and how they translate atmospheric halos.

Moon halos.

GLory

I did once experience a real sense of awe, that the fake 7 Sun illusion could never have given me. It was early morning and we had woken up to a sea of clouds below us, the sun was still hidden beyond a ridge. As dawn grew and Sun rose above the ridge, we could see our shadows projected onto the clouds below. Wrapped tightly around the head of my shadow was a colourful ephemeral halo. Everyone could only see the halo around their own shadow and they were easier to see with the naked eye than with a camera. Later I found that the halos are called glories and are more commonly seen from airplane windows when their shadow falls on the clouds below.

A number of physicists have taken a stab at attempting to prove this and had come up with various theories. When I first saw a glory I assumed that it must be caused by a mechanism similar to a rainbow. I could not be more mistaken, the currently agreed upon mechanism involves quantum wave tunneling that allows rays of light that would have otherwise missed a drop of water to instead leak some energy into it. This energy "collects" in the drop and begins leaking out causing a diffraction pattern to be seen. That's the short of it, for a slightly more in depth review of the theory read the paragraph below or skip to the next section.

Initial theories suggested that light would be able to scatter from the back of the water drop by reflecting just once, however, the refractive index of water is too low to produce scattering angle of 180 degrees in this manner. In 1957, Van de Hulst suggested that light waves could "move" along the surface of the water drop increasing the scattering angle up to 180 degrees. Further investigation of this theory revealed that too much of the wave's energy would be lost in this process to form a glory. In 1977, Vijay Khare and H. Moysés Nussenzveig introduced wave tunneling to the explanation, they suggested that light waves that graze the surface of the drop without touching it could jump the gap and leak some of the wave energy into the droplet. As if one insane quantum phenomenon was not enough, once inside, the light repeatedly reflects inside the droplet, setting up what is known as a Mie resonance. The researcher who came up with this theory likened it to whispering rooms like those found in some forts in Jaipur (Their article gives the example of the whispering gallery in the dome of St. Paul’s Cathedral, London). Similar to sound waves in a whispering chamber, the waves that tunneled into the drop begin reflecting repeatedly along the edges of the drop and get reinforced further by the wave tunneling. Some of this amplified light begins escaping the drop and creates a diffraction pattern that we see as glory!

It cannot be understated how simplified even the above explanation is, in the original 1977 paper the authors list 8 separate mechanism that contribute to the overall effect. You can read the Scientific American article and the 1977 paper "Theory of Glory" for more complete explanations.

cloud iridescence

Cloud iridescence is another diffraction phenomenon but with fewer steps. While a glory has to redirect the light by 180 degrees, cloud iridescence is formed by light coming through the cloud. Most clouds are made up of a wide range of water droplet sizes, however, clouds with uniformly sized water drops or ice crystals can form in certain conditions. They are often lenticular in shape and form above large cumulus columns. When a thin cloud forms in this manner with the sun (or the moon) behind it, the rays of light passing through the thin layer of uniformly sized particles create a coherent diffraction pattern. The colourful display, also known as a Corona when it appears around the Sun or the Moon, appears as if there was a giant soap film or oil slick in the sky.

Auroras

As I am sure you saw in the news in May this year, Aurorae are a brilliant display of pastel green, blue and pink streaks arranged in abstract patterns like rivers of light in the sky. They are only visible at night (they are obscured by the Sun during the day) and only at very high latitudes like in Iceland, Norway or Antarctica.

A number of phenomena from across the solar system have to weave together to create an Aurora Borealis (in the North) or an Aurora Australis (in the South), making it the first photometeor on this list that produces its own light. It starts from the Sun which is constantly bombarding the solar system with high energy particles, known as the solar wind. Solar wind is made up of high energy charged particles that get ejected from the surface of the Sun during a solar flare. When they reach a planet like Earth with a magnetic field they get deflected. For most of these particles that is the end of the story, which is great for life on Earth, because organic material, electronics and even the entire electric grids can be damaged by these high energy particles. However, Some of these particles don't get fully deflected and instead follow the magnetic field lines towards Earth's poles. Their path towards the poles intersects with the atmosphere, where they slam into the air molecules at speeds of up to 73 million Kmph. The collisions strip the air molecules of a few electrons, creating colourful streaks of ionised gas in the sky. The specific colours of an Aurora are decided by the type of ionised molecule that is produced and the height at which it happens.

This extremely exclusive phenomenon was made accessible to millions of people in Europe, North America and Southern ends of South America, Africa and Australia earlier this year. This rare opportunity was made possible due to extreme solar flares and an intensified solar wind caused by the strongest solar storm since 2003. The Sun goes through cyclic periods of high and low activity, during periods of Solar maxima indicated by a higher number of Sun spots the solar wind is stronger and is interspersed by highly energetic coronal mass ejections (CMEs). The stronger the solar wind the more of these particles get channeled into the atmosphere creating bigger and stronger Aurorae.

Imagine what a vacation in the Bahamas that would have been, to potentially see a rainbow, a sea mirage and the Aurora on the same day.

This is an incomplete list and will receive updates to add more photometeors to it or to explore quirks of the ones already mentioned. You may notice that unlike some previous articles this one contains many more pictures taken from the internet. This list also serves as a personal bucket list, to picture all the phenomenon listed here so I can replace all the images with personal credits. In writing the article I have come to the conclusion that I need to take a long and well timed trip to Iceland, that should knock off a number of items from the list.

References and Further reading

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