The aurora really does make sounds — and they appear to be generated in the air about 230 feet above the ground, according to a Finnish study that used an array of microphones to record and pinpoint the source of a weird surging hiss during a magnetic storm last September.
The results provide what may be the first hard evidence to rebut the notion that auroral whispers belong in the realm of Far North myth — or, at least, are a sign that winter cabin fever has finally pushed the listener to the bloodshot brink.
"Our research proved that, during the occurrence of the northern lights, people can hear natural auroral sounds related to what they see," said engineer and acoustic researcher Unto K. Laine in this story posted by Aalto University.
An eight-second clip of the sounds captured by Laine can be heard in this YouTube video.
"A recording produced Sept. 9, 2011 during a geomagnetic storm by using three microphones and a VLF antenna picked up 20 similar clap sounds," Laine reported last week on his website. "Some of them were close enough in order to be detected by all three microphones. The collected data allowed the estimation of the location of the sound source. The sound source was the open sky."
While the results identified the physical location in the air where the sounds came from, they don't explain how the aurora triggered the hissing. But Laine speculated that the same solar waves of charged particles that generate the display are likely culprits.
"These particles or the geomagnetic disturbance produced by them seem to create sound much closer to the ground," he said here.
The idea that the aurora somehow produces audible noise has long been a source of fierce (and often fluid-quenched) debate among Alaskans and other denizens of the Far North. Scientists have generally dismissed the possibility that an ionospheric light show 60 to 200 miles up could produce sound waves that propogate through the air to the ground. But they didn't rule out other possibilities.
Infrasound researchers have previously reported that auroras can heat the upper atmosphere and sometimes create a distant noise below the threshold of human hearing. And the aurora does produce detectable radio signals. But sounds that can be discerned by the human ear? How could that be?
"It is easy to say that the aurora makes no audible sound," explained physicist and aurora expert Dirk Lummerzheim, a research professor emeritus at the University of Alaska Fairbanks, in this primer about the aurora and its audible mysteries.
"The upper atmosphere is too thin to carry sound waves, and the aurora is so far away that it would take a sound wave 5 minutes to travel from an overhead aurora to the ground. But many people claim that they hear something at the same time there is aurora in the sky."
Laine explained further here:
The sounds do not occur regularly when the northern lights are seen. The recorded, unamplified sounds can be similar to crackles or muffled bangs, which last for only a short period of time. Other people who have heard the auroral sounds have described them as distant noise and sputter. Because of these different descriptions, researchers suspect there are several mechanisms behind the formation of these auroral sounds. These sounds are so soft that one has to listen very carefully to hear them and to distinguish them from the ambient noise.
"The voice of the aurora is still a great mystery," added Ned Rozell, in an Alaska Science Forum discussion of auroral muttering. "Tom Hallinan, a professor of geophysics at the Geophysical Institute, has studied the aurora for decades. He said he's heard the aurora and has talked to many others who have.
" 'There's something going on,' Hallinan said of the aurora's whisper. 'It's scientifically unreasonable, yet people do hear it.'"
Laine's new study — reported this week during the 19th International Congress on Sound and Vibration in Vilnius, Lithuania — is part of his ongoing investigation into the mystery of auroral sounds that he began in 2000. His goal was to reach out with sophisticated audio equipment and snatch them from the air.
"The literature showed that even though many observations of these strange sounds had been performed, there was very little — if any — instrumentally collected evidence," he wrote in a 2004 paper about his early efforts to make the recordings. "Studying the question further also revealed that very few researchers had a background in acoustics, psychoacoustics, electroacoustics, and digital-signal processing."
Last fall, Laine succeeded.
His findings prove that the sounds aren't illusions or hallucinations, nor created directly in the human brain by electromagnetic radiation, he wrote. They aren't coming from the tips of the trees, emerging from frost or ice, or rumbling up out of the Earth itself.
"The result shows that the sound source — at this particular case — was real and on the sky, not far away from the ground," he concluded.
Contact Doug O'Harra at doug(at)alaskadispatch.com