Wandering through a forest at night, you might be treated to a strange symphony of creepy cracking sounds. In some cases, the source of the sounds are the trees. Temperature changes, especially between day and night, can cause the wood in trees to expand or contract. And of course, trees may creak under the weight of moving animals or gusts of wind high in the canopy. Now though, science has revealed a different and surprising source of the sounds—the sky.
Aalto University acoustician Unto Laine had heard strange snapping noises during an auroral storm. While dismissed until recently as a hallucination, auroral sounds have long been reported in northern Scandinavia. For example, the Sámi—Indigenous people in the region—refer to the aurora borealis as guovssahas, which translates to the "light you can hear." But even if the auroras were causing the sound—and people could hear it somehow 50 miles down from where the auroral lights occur—Unto Laine recorded the sounds in a forest even when there was no visible auroral activity. Not only that, but triangulation data from his microphones revealed that the sounds were emanating from just 230 feet up from the ground.
From Nautilus:
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.
Eventually, Laine's experiments showed that the geomagnetic activity was strong enough to produce the sounds but not auroral light.
"Now that I have magnetic field measurements, it's impossible to confuse any auroral sounds with sounds from trees," Laine says. "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."
Previously:
• The amazing acoustics of Stonehenge
• Clapping at this Mayan temple echoes back as a quetzal bird call
• How scary is the sound of an Aztec death whistle?
