“Ash light” or AL is a faint mysterious glow or hue seen in the night hemisphere of Venus. It is often compared to Earthshine, the reflected light that illuminates the far side of the Moon.
First described by Italian astronomer Giovanni Riccioli in 1643, AL has been observed many times since then, but its faint, ephemeral, and elusive nature has prevented serious research. It’s here.
Even more problematic, AL has so far only been detected by the human eye, and no scientific instruments, either earth-based or space-based, have recorded this phenomenon.
Some authorities have declared this phenomenon to be an illusion, perhaps an eye contrast effect or even an “expectation bias.” Some have suggested that a defect in the equipment could explain the phenomenon. Light scattering, optical aberrations, background sky brightness, weather, etc.
But there are enough reliable reports about AL that some scientists can offer an explanation. These include reflected light from Earth, auroras, “airglow” radiation, lightning, and infrared (thermal) radiation from Venus’ atmosphere.
Most of these explanations are ignored for some reason. However, there is ample evidence that not only ultraviolet light from the sun, but also high-energy solar wind particles can excite oxygen atoms in Venus’ atmosphere.
This creates a pale green glow similar to that seen in the aurora borealis on Earth. However, the process is somewhat different because auroras on Earth are caused by Earth’s magnetic field interacting with solar particles, whereas Venus has no appreciable magnetic field.
It remains to be seen whether this explanation can explain all or some of the AL observations. Therefore, the long-standing mystery of AL may still turn out to be an illusion.
This article is an answer to the question (asked by Herman Townsend of Liverpool): “What is Ashen Light?”
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