Have you ever witnessed a stunning green flash during a sunset? This captivating optical phenomenon, known as a "Mock Mirage" or "M-Mir" green flash, occurs due to the miraging action of a temperature inversion layer in the atmosphere. In this article, we will delve deeper into the science behind M-Mir green flashes and explore their mesmerizing characteristics.
To understand the formation of M-Mir green flashes, let's focus on the interface between a warmer temperature inversion layer and the cooler, denser air beneath it. As the sun begins to set, its rays pass through this inversion layer. The inversion layer causes the downward rays to curve slightly towards the Earth, resulting in a fascinating miraging effect.
When the sun's rays pass through the inversion layer, they follow different paths towards an observer above the layer. As a result, up to three solar images can be seen simultaneously. The uppermost image is erect but is not shown here for clarity. The upper parallel ray, which glances through the inversion layer, is strongly curved and appears to originate from a higher point in the sky. This contributes to an upper (and inverted) image of the sun. On the other hand, the lower parallel ray is less deflected and contributes to a lower solar image.
Additionally, in the tangential direction to the base of the inversion layer, small vertical differences in ray directions are highly magnified. This magnification effect stretches the normally insignificant green rim of the sun vertically, creating the distinct green flash phenomenon.
One intriguing aspect of M-Mir green flashes is their ability to magnify small angular differences in ray directions. This magnification enhances the separation between red and green solar images, resulting in a more pronounced green flash. The green flash, which is typically brief and elusive, becomes more visible and captivating due to this magnification effect.
In some cases, inversion layers follow vertical atmospheric waves, causing variations in the height of mirage effects and green flashes. While the example discussed here showcases the mirage and green flash below the astronomical horizon, these phenomena can appear higher in the sky when inversion layers follow vertical atmospheric waves.
Under certain conditions, the inversion layer can be so strong that the curvature of rays equals or exceeds that of the Earth's surface. This phenomenon, known as "ducting," guides the rays along the inversion layer, making objects located at a considerable distance below the horizon visible. Ducting allows for unique visual experiences, enabling us to see objects that would typically be hidden from view.
To witness the captivating beauty of M-Mir green flashes, a clear horizon is essential. These flashes always occur below the astronomical horizon, which is defined by a horizontal plane passing through your eye. Even when you are slightly above sea level, the real horizon is always below the astronomical one. When observing from an aircraft, the real horizon appears even lower. Therefore, for optimal viewing conditions, a clear horizon, preferably with a sea horizon, enhances the chances of experiencing this awe-inspiring atmospheric optics phenomenon.
If you're intrigued by M-Mir green flashes and want to explore more about atmospheric optics phenomena, we recommend visiting Andrew Young's website. Andrew Young provides further explanations and even offers simulations to deepen your understanding of these captivating phenomena.
In conclusion, M-Mir green flashes are a remarkable display of atmospheric optics. By understanding the role of temperature inversions and the magnification of small angular differences in ray directions, we can appreciate the beauty and rarity of these phenomena. So, the next time you find yourself witnessing a sunset, keep your eyes peeled for the elusive green flash that might just take your breath away.
Miraging action of a temperature inversion layer. The layer need only be a few feet above the surface (the vertical scale here is exaggerated). Parallel rays from the setting sun follow two paths to the observer above the layer and up to three solar images are seen (the uppermost is erect and for clarity is not shown here). The mirage can greatly magnify small angular differences in ray directions. The separation between red and green images is enhanced and can give a green flash.
As the sun sinks its rays start to pass through the temperature inversion. The action of the inversion is most easily understood by concentrating on the interface between the warmer layer and the cool more dense air beneath. Rays passing downwards through it are refracted so that they curve. slightly towards the earth.
The upper parallel sun ray glances through the inversion and is most strongly curved. To the eye, the ray appears to be coming from a point higher in the sky and it contributes to an upper (and inverted) image of the sun. The lower parallel ray is less deflected and contributes to a lower solar image.
In a direction tangential to the base of the inversion layer small vertical differences in ray directions are highly magnified. The normally insignificant green rim of the sun is vertically stretched to produce a green flash.
When the inversion layer, as shown here, is at constant height the mirage effects are all below the astronomical.. horizon. Sometimes inversion layers follow vertical atmospheric waves, the mirage and green flash can then appear rather higher in the sky as in the example here.
See Andrew Young's pages for further explanation and a simulation.
Ray curvature. Sometimes the inversion can be sufficiently strong that the ray curvature equals or is stronger than that of the earth. The rays are then guided along the inversion layer - a duct. Ducting can make objects at a considerable distance below the horizon visible.
The astronomical horizon is defined by a horizontal plane passing through your eye. When you are even a small distance above sea level the real horizon is always below the astronomical one. From an aircraft the real horizon is obviously lower. Pure I-Mir and M-Mir flashes always occur below the astronomical horizon which is why a clear horizon, and preferably a sea horizon, is needed.
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"Mock Mirage "M-Mir" green flashes". Atmospheric Optics. Accessed on December 10, 2023. https://atoptics.co.uk/blog/mock-mirage-m-mir-green-flashes/.
"Mock Mirage "M-Mir" green flashes". Atmospheric Optics, https://atoptics.co.uk/blog/mock-mirage-m-mir-green-flashes/. Accessed 10 December, 2023
Mock Mirage "M-Mir" green flashes. Atmospheric Optics. Retrieved from https://atoptics.co.uk/blog/mock-mirage-m-mir-green-flashes/.