Fiona Amery has delved into the accounts of some of the earliest polar explorers for traces of the once-mysterious crackling sounds of the northern lights. So when she visits northern Norway later this month, she’ll be listening.
“I expect it’s not very likely — but if I manage to see the northern lights, it would be rather wonderful to hear them,” she said.
Amery, a historian at the University of Cambridge in the United Kingdom, is the author of a recent study detailing the scientific search for the sounds of the aurora borealis — the northern lights. The almost phantasmagorical noises have been a mystery to scientists for over a hundred years. But now it seems that a lone researcher has figured out how auroras really do make sounds.
“It’s pretty much been corroborated all over the world,” she said. “In Canada, Norway, the Shetland Islands, Russia … they are all hearing very much the same sounds.”
Listeners have described them as a faint rustling, clapping or popping. An observer in the 1930s said the northern lights made “a noise as if two planks had met flat ways — not a sharp crack but a dull sound, loud enough for anyone to hear.”
Scientific expeditions investigated auroral sounds during the first International Polar Year in 1882-83. Little was then known about the northern lights, and figuring out what caused them was a big deal, Amery said. Despite efforts by several scientists, research into the noises was inconclusive.
That auroras can occur hundreds of miles above the ground — a discovery made during the second International Polar Year in 1932-33 — strengthened suggestions that their sounds were just an illusion, she said. Many scientists argued that auroras were too far away to hear even the most thunderous noises and that any sounds would take several minutes to reach the ground — so it was impossible for them to seem to change in time with the auroras. The British physicist Sir Oliver Lodge suggested that the sounds might be a psychological phenomenon caused by the aurora’s appearance, just as meteors sometimes conjure up an illusory whooshing sound, Amery said.
But other scientists were convinced that the northern lights really did make noises. The Canadian astronomer Clarence Chant first suggested in the 1920s a mechanism by which they could occur: The motion of the aurora, he argued, induced changes in the electrification of the atmosphere that created crackling sounds close to the ground, like the sounds of sparks of static electricity.
It’s now known that auroras are caused by charged particles from solar flares’ interacting with the Earth’s magnetic field; the particles rain into the upper atmosphere, exciting nitrogen and oxygen atoms in the air to create dramatic light shows. And almost 100 years later, Chant’s suggestion seems close to what could be the true reason for the sounds.
In 2012, the acoustician Unto Laine of Aalto University in Finland released a recording of auroral sounds — the culmination of years of monitoring auroras from his home village, Fiskars. Then, in 2016, he announced the mechanism that makes the sounds — an inversion layer of cold air in the atmosphere that can form below an aurora and a short distance above the ground in calm weather. Meteorological reports showed that such an inversion layer had formed above Fiskars in 2013 during a vivid aurora when Laine recorded hundreds of its sounds.
Laine argues that visible changes in the aurora induce changes in the inversion layer, causing accumulated electricity to discharge as sparks that create audible sounds. That explains how the sounds correspond with the aurora’s visible movements — they originate in the inversion layer only about 250 feet above the observer.
“There is no doubt that the so-called auroral sounds have been recorded now many times and that they are made by electric discharging processes in the temperature inversion layer,” Laine said in an email. “The solar wind variations are behind both phenomena, visual and auditory.”
Amery said Laine seems to have solved the mystery of auroral sounds. “I am pretty convinced by his work,” she said. “I would love there to be perhaps somebody on a different continent that was running the same experiments. … The sounds are certainly not unique to Finland.”
Laine continues to study auroral sounds. His research has revealed accounts dating to ancient Mesopotamia.
“Aurora borealis is probably even behind the myths of angels and ancient gods,” he said. “The heavens have whispered for thousands of years.”