(Image credit: Adam Paulsen / The Royal Society Journal of the History of Science / CC By 4.0)
It’s a question that has puzzled observers for centuries: Do the fantastic displays of crimson and green light from the northern lights produce any perceptible sound?
Conjured by the interaction of solar particles with gas molecules in Earth’s atmosphere, the aurora generally occurs near the Earth’s poles, where the magnetic field is strongest. However, reports of the aurora making noise are rare, and have historically been discounted by scientists.
But a Finnish study in 2016 claimed to have finally confirmed that the Northern Lights actually produce sound audible to the human ear. A recording made by one of the researchers involved in the study even claimed to have captured the sound of the captivating lights 70 meters above ground level.
Related: Photos: Recording Mysterious Northern Lights Sounds
Still, the mechanism behind the sound remains somewhat mysterious, as are the conditions that must be met for the sound to be heard. My recent research takes a look at historical reports of auroral sound to understand the methods of investigating this elusive phenomenon and the process of establishing whether the reported sounds were objective, illusory, or imaginary.
(Image credit: The Royal Society Journal of the History of Science / CC By 4.0)
The noise of the auroras was the subject of particularly lively debate in the early decades of the 20th century, when accounts of settlements in the northern latitudes reported that the sound sometimes accompanied the fascinating displays of light in their skies.
Witnesses spoke of a quiet, almost imperceptible crack, hiss or buzzing during the particularly violent Northern Lights displays. In the early 1930s, for example, personal testimonies began to flood in The Shetland News, the weekly newspaper of the sub-arctic islands of Shetland, comparing the sound of the northern lights to the “rustle of silk” or “two planks. that meet in flat paths. “
These accounts were corroborated by similar testimonies from northern Canada and Norway. However, the scientific community was less than convinced, especially considering that very few Western explorers claimed to have heard the elusive noises.
The credibility of the aurora noise reports from this time was closely tied to the altitude measurements of the northern lights. It was considered that only those screens that descended low in the Earth’s atmosphere could transmit sound that could be heard by the human ear.
The problem here was that the results recorded during the Second International Polar Year 1932-3 found that auroras occur most frequently 100 km above Earth and very rarely below 80 km. This suggested that it would be impossible for the discernible sound of the lights to be transmitted to the surface of the Earth.
(Image credit: David Clapp / Getty Images)
Given these findings, eminent physicists and meteorologists remained skeptical, dismissing accounts of auroral sound and very low auroras as folk tales or auditory illusions.
Sir Oliver Lodge, the British physicist involved in the development of radio technology, commented that the sound of the auroras could be a psychological phenomenon due to the vivid appearance of the aurora, just as meteors sometimes evoke a hiss. in the brain. Similarly, meteorologist George Clark Simpson argued that the appearance of low auroras was likely an optical illusion caused by interference from low-lying clouds.
However, the leading auroral scientist of the 20th century, Carl Størmer, published accounts written by two of his assistants who claimed to have heard the aurora, adding some legitimacy to the large volume of personal reports.
Størmer’s assistant, Hans Jelstrup, said he had heard a “very curious low whistle, clearly undulating, which seemed to follow exactly the vibrations of the dawn”, while Mr. Tjönn experienced a sound like “burning grass or dew”. However convincing these last two testimonies may have been, they have not yet proposed a mechanism by which the sound of auroras could operate.
Light and sound
The answer to this enduring mystery that has subsequently garnered the most support was first tentatively suggested in 1923 by Clarence Chant, a well-known Canadian astronomer. He argued that the movement of the northern lights alters the Earth’s magnetic field, inducing changes in the electrification of the atmosphere, even at a significant distance.
This electrification produces a crunch much closer to the Earth’s surface when it encounters objects on the ground, much like the sound of static. This could take place on the observer’s clothing or glasses, or possibly on surrounding objects, including fir trees or the cladding of buildings.
Chant’s theory correlates well with many accounts of auroral sound, and is also supported by occasional reports of the odor of ozone, which reportedly carries a metallic odor similar to an electrical spark, during displays of the Northern Lights.
However, Chant’s paper went largely unnoticed in the 1920s, only receiving recognition in the 1970s when two auroral physicists reviewed the historical evidence. Chant’s theory is widely accepted by scientists today, although there is still a debate about how exactly the mechanism to produce sound works.
What is clear is that, on rare occasions, the aurora produces sounds audible to the human ear. The eerie reports of crackles, buzzes, and hums that accompany the lights describe an objective audible experience, not something illusory or imagined.
Sample the sound
If you want to hear the Northern Lights for yourself, you may need to spend a considerable amount of time in the polar regions, considering that the auditory phenomenon only occurs in 5% of violent aurora displays. It is also heard most often at the top of the mountains, surrounded by only a few buildings, so it is not a particularly accessible experience.
However, in recent years, the sound of the aurora has been explored for its aesthetic value, inspiring musical compositions and laying the groundwork for new ways of interacting with its electromagnetic signals.
The Latvian composer Ēriks Ešenvalds has used magazine excerpts from the American explorer Charles Hall and the Norwegian statesman Fridjtof Nansen, who claimed to have heard the Northern Lights in their music. Its composition, Northern Lights, intertwines these reports with the only known Latvian folk song chronicling the phenomenon of the auroral sound, sung by a tenor solo.
Or you can also listen to the radio signals of the Northern Lights at home. In 2020, a BBC 3 radio program reassigned very low-frequency radio recordings of the aurora in the audible spectrum. Although not the same as hearing audible noises produced by the northern lights in person on top of a snowy mountain, these radio frequencies give an amazing sense of the transient, fleeting and dynamic nature of the aurora.
This article has been republished from The Conversation under a Creative Commons license. Read the original article.[Get the best of The Conversation, every weekend. Sign up for our weekly newsletter.]
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