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Walking alone in the woods at night is one of the eeriest experiences. When it’s pitch black and the forest has fallen silent, it’s the inscrutable, mysterious sounds you can’t identify that are most frightful.
Of course, the strange noises filling the dark often have perfectly reasonable explanations. For people living in northern climates, certain sounds that emerge in the cold of winter, ringing out like muffled rifle shots, have long seemed to emanate from the trees themselves. The Lakota people, for example, refer to February as cannapopa wi: “moon when trees crack from the cold.”
Indeed, these loud cracking sounds are often attributed to large pressure splits in tree trunks, caused by sap freezing and expanding inside the tree’s interior. But while freezing sap in trees has been found to produce sounds at ultrasonic frequencies, outside of the range of human hearing, scientists have found no evidence this phenomenon might make sounds that are audible to the human ear.
Now an acoustics professor emeritus at Aalto University in Finland named Unto Laine has discovered an alternative and unexpected source for the sound. And it has absolutely nothing to do with trees.
Skeptics dismissed the noises as hallucinations—or “frost cracks” in the local trees.
After hearing inexplicable snaps of sound during an auroral storm one night back in 1990, he decided to apply his acoustician chops. The faint sounds seemed to correspond with waves of auroral light that were visible that night. Was he hallucinating?
Folklore regarding auroral sounds isn’t unusual in northern Scandinavia. Sami indigenous people have historically called the aurora borealis guovssahas, meaning the “light you can hear.” In the 1880s, explorers, scientists, and fur traders began recording their own reports of auroral sound in scientific and historical texts. At the time, little was known about the nature of the miraculous, enchanting northern lights or what caused them. Whether the lights were associated with sounds became the subject of lively debate and divided opinion.
Not much had changed by the year 2000, when Laine began what’s officially known as the Auroral Acoustics Project at Aalto University. He was able to make audio recordings as a first step toward proving their atmospheric origin. But skeptics dismissed the noises he recorded as hallucinations—or “frost cracks” in the local trees.
Geophysicists declared that he would have to overcome two major problems before the possibility of auroral sounds could even be considered. First, auroral curtains of light extend down only to about 50 miles above the Earth’s surface—well beyond earshot. So he’d have to explain how the sounds could travel from that height to Earth. Second, he’d have to explain how such sounds burst out of thin air.
Taking up the challenge, Laine set out with additional gear that enabled him to measure not just sound but auroral activity. What he found in his first observations was that the timing of the audio he recorded was strongly connected with increases in geomagnetic energy that manifest as moving curtains of auroral light.
Then, things got weird. On another night of recording, the sounds occurred without any visible aurora. At first, this finding appeared to support the theory that the sounds might actually be produced by frost cracks in the trees and not by the aurora. But with improved acoustic technology involving multiple microphones in different locations, Laine was able to triangulate the origins of the sounds from calculations based on the distance between the microphones and the speed of sound. The triangulation data revealed the origin of the sounds was indeed the sky. Inexplicably though, repeated tests suggested the sounds were being produced at a height of around 230 feet. What was going on?
“It was very surprising at the moment because there was nothing visible in the sky,” he says. “But I was sure that the sound source was at that altitude.”
After considerable investigation an answer came when Laine discovered that particular height—200 feet to 260 feet above the Earth’s surface—corresponds with a meteorological process called inversion. On calm, clear nights, warm air rises, carrying negatively charged ions from the Earth’s surface. As this warm air collides with cooler air from above, it forms an “inversion” layer of warmer air layered over cold air, which traps the ions. Meanwhile positively charged ions entering the atmosphere from solar wind reach this layer and get trapped there, too. The opposite charges interact, building energy to the point where it must discharge, like tiny lightning bolts that crackle and pop with sound. These discharges also release electric currents that produce magnetic field pulses.
To better understand what was happening when no aurora was present, and to get a final answer on the frost cracks, Laine decided to take another round of measurements. On a cold night last January, Laine recorded hundreds of sound events that resembled those often attributed to frost cracks and was able to localize them in the sky at around 246 feet above the Earth, under temperature inversion conditions. Again, no visible aurora was present, but the timing of the sounds was in sync with geomagnetic activity. He had all the evidence he needed. The sounds originated from the inversion layer, precisely in sync with the timing of increases in positive ions raining down into the layer, which he was able to measure using geomagnetic instruments—although this geomagnetic activity was too weak in this case to cause auroral light.
“Now that I have magnetic field measurements, it’s impossible to confuse any auroral sounds with sounds from trees,” says Laine. “Trees aren’t producing any magnetic pulses and you have a systematic correlation with these [pulses]. So 100 percent it’s true: The sounds are coming from the sky.” Laine presented his results at the Baltic-Nordic Acoustics Meeting in Finland in May.
Laine’s efforts in the cold uniquely bridge a gap between the seemingly distant research fields of atmospheric science and acoustics. Njål Gulbrandsen, a space and plasma physics researcher at the Tromsø geophysical observatory in Norway believes that Laine’s work is groundbreaking, but needs further, more rigorous testing by other scientists. “He might be on to something, but this research has yet to be evaluated by other scientists with expertise.”
It’s not to say that trees don’t crack—but rather that spooky noises long attributed to trees may emerge from the night sky itself. 
Lead image: Saskia B / Shutterstock
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