For the first time, scientists have systematized all the halos

… discovered over thousands of years of observations.

The secrets of the origin of some of them have not been solved so far.

For the first time in the history of observations, scientists from the Helsinki and Ural Federal Universities Jarmo Moilanen and Maria Gritsevich have systematized information about all forms of atmospheric halos recorded by mankind at the end of 2021. From numerous sources of data on observations, the history of which includes 4-5 millennia, 119 different forms of atmospheric halo are known today. There are also extra-atmospheric halos and halos that can be seen on various surfaces. An article describing the work carried out and the conclusions reached was published in the Journal of Quantitative Spectroscopy and Radiative Transfer.

In the study, scientists summarized not only the general information regarding the phenomena, but also the conditions necessary for seeing the optical illusions in the atmosphere (the formation of crystals of ice or other minerals), such as temperature and humidity. In addition, the authors of the article identified knowledge gaps in the study of atmospheric halos and outlined promising methods for observing and processing the data. Among them are new digital photography techniques that are being used to improve the quality of detection of atmospheric halos and that lead to the discovery of their new forms.

Scientists have grouped atmospheric halos into commonly observed and rarely seen categories. It is believed that if a certain form of halo is observed at least once a year, it belongs to the first category; halos with a rare light source configuration, orientation, or unusual crystal shape make up only about 1% of all observed halos.

The frequently observed halos are usually formed by the scattering, refraction and focusing of light by hexagonal ice crystals in a disordered, horizontal or vertical orientation. The main sources of light that cause halo are the Sun and the Moon, as well as artificial sources. Rare halos, in turn, are divided into those whose origin has been established, and exotic, those that have not yet been sufficiently studied and are not yet amenable to modeling.

“As a rule, halos are formed as a result of the interaction of light with hexagonal crystals of water ice,” says Jarmo Moilanen, author of the article, researcher at the Finnish Geospatial Research Institute, PhD student at the University of Helsinki. “However, some of the documented exotic halos cannot be explained in this way. For example, the mysteries of the origin of elliptical halos and Bottlinger rings have not been solved since their discovery at the beginning of the 20th century. Among the mysterious ones is the so-called Moilanen arc, which I first discovered in 1995.”

In the case of exotic halos, science is presumably dealing with anomalous crystal shapes (such as cubic water ice crystals), with their heterogeneity, or with crystals of some other minerals scattered in the air. However, what exactly should be the ice crystals and the paths of the rays that pass through these crystals in order to form a halo of one or another exotic form is still unclear.

“The researchers suggest that the unusual shape of exotic halos is due to anthropogenic factors, such as emissions into the atmosphere or the influence of the strong electromagnetic field of high-voltage power lines, which can disrupt the orientation of ice crystals in the air. To unravel such mysteries, samples of ice crystals, forming exotic halos, were specially collected in the atmosphere, but this experience also gave more questions than answers,” says Maria Gritsevich, adjunct professor at the University of Helsinki, senior researcher at the Finnish Geospatial Research Institute and the Ural Federal University.

Finally, the halos formed in the atmospheres of Venus, Mars, Jupiter and their satellites are of interest. Halos have been already extensively documented in the atmosphere of Mars.

“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. Monte Carlo modeling of the factors that can lead to the formation of a halo will provide valuable information about the state of the Martian atmosphere.”

Note

Atmospheric halos are light scattering phenomena caused by the accumulation of water ice crystals smaller than 10 micrometers in the atmosphere. The combination of the shape of ice crystals (or minerals), their orientation, and the path of light rays through the crystals determines the configuration of the halo, whether it is colored and exhibits white rings, spots, or arcs. In other words, the shape of the halo indicates what types of ice crystals are present in the atmosphere.

The oldest mentions of the halo are from 4 to 5 thousand years old: information about the halo is recorded on cuneiform tablets of the Sumero-Babylonian culture. At the time of Aristotle, at least three halo forms were known. One of the oldest sightings of a halo in Jerusalem dates back to the beginning of the 12th century. By 1820, the number of registered halos was about 20, by 1990 – 60.

Significant progress in documenting the halo has occurred in connection with the proliferation of mobile phones with photo and video cameras. The authors of the article call on the public to record the observed halos and thus contribute to the study of these phenomena and the discovery of new ones.

Journal: Journal of Quantitative Spectroscopy and Radiative Transfer
DOI: 10.1016/j.jqsrt.2022.108313
Article Title: Light scattering by airborne ice crystals – An inventory of atmospheric halos
Article Publication Date: 2-Jul-2022

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