For the first time in nearly 5,000 years of observations, researchers have fully cataloged the optical illusions created in the sky when light shines through ice crystals known as atmospheric halos.
The Atmospheric Halos “Inventory” details commonly seen atmospheric optical illusions from known sources and sheds light on rarer halos, including those whose origins are currently a mystery.
Halos are caused by the accumulation of water ice crystals smaller than 10 micrometers in the atmosphere. The qualities of these atmospheric illusions such as their colors or whether they possess white arcs, spots or rings, are determined by the shape and direction of the ice scattering and the path the light takes to these crystals. Often the type of crystal behind the scattering can be identified by the shape of the halo it creates.
Related: What makes Earth’s atmosphere so special?
These atmospheric illusions have been documented by mankind since at least the Babylonian era – which began around 1895 BC – when the phenomena were detailed on cuneiform tablets. However, thanks to the availability of cameras following the proliferation of cell phones, scientists have never had so much data on these phenomena at their fingertips.
In addition to an intensive catalog of halos, this new research, which is based on observations collected until the end of 2021, highlights gaps in the study of the phenomena. The authors of the inventory also outline promising methods for their subsequent observation and processing of the data collected, urging the public to get involved in recording these phenomena using their phones and digital photography.
The comprehensive catalog divides the phenomenon into 119 different types and also details the conditions such as temperature and humidity required to create them.
Halos observed at least once a year have been defined as “commonly observed” and are usually formed by the scattering, refraction and focusing of sunlight or reflected from the moon by hexagonal ice crystals in a disordered orientation , horizontal or vertical.
Another category – “rare halos” – which includes atmospheric illusions accounting for only about one percent of all sightings, is further divided into halos of known origin and those of unknown and exotic source.
“Typically, halos form as a result of the interaction of light with hexagonal crystals of water ice,” said the scientist and Ph.D. from Finland’s Institute for Geospatial Research. University of Helsinki student Jarmo Moilanen said in a statement (opens in a new tab). “However, some of the documented alien halos cannot be explained in this way.”
Moilanen, who developed the inventory of atmospheric halos with University of Helsinki professor Maria Gritsevich, cited some examples of these exotic halos. “The Mysteries of the Origin of Elliptical Halos and Bottlinger Rings (opens in a new tab) have not been solved since their discovery in the early 20th century,” the researcher said. “Among the mysterious is the so-called Moilanen Arch (opens in a new tab)which I first discovered in 1995.”
The researchers suggest that the key to producing mysterious atmospheric halos could be abnormally shaped ice crystals or crystals of other minerals scattered through the air.
Gritsevich said research suggests the unusual shape of alien halos is due to man-made factors, such as atmospheric emissions or strong electromagnetic fields generated by high-voltage power lines. She added that these factors can disrupt the orientation of ice crystals in the air, possibly causing exotic halos.
“To unlock such mysteries, samples of ice crystals forming exotic halos were specially collected from the atmosphere, but this experiment also gave more questions than answers,” Gritsevich said in the statement, adding that the Atmospheric halos spotted around other planets could be used to unravel this mystery.
This includes observed and documented halos in Mars’ atmosphere, which could also be used to better understand the red planet’s thin diffuse atmosphere.
“This observation proves that clouds of hexagonal crystals of water ice or other minerals exist in the atmosphere of Mars,” says Maria Gritsevich. “There are suggestions that the halos could be formed by carbon dioxide crystals.”
She added that mathematical modeling of the factors that can lead to the formation of a halo could provide valuable information about the state of the Martian atmosphere.
The duo’s research has been published in the Journal of Quantitative Spectroscopy and Radiative Transfer. (opens in a new tab)
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