About 1 kilometer under the sea is a sound tunnel that carries the cries of whales and the clamor of submarines over great distances. Since scientists discovered this Sound Fixing and Ranging (SOFAR) channel in the 1940s, they have suspected that a similar conduit exists in the atmosphere. But few have bothered to look for it, other than a top-secret Cold War operation.

Now, by listening to distant rocket launches with solar-powered balloons, researchers say they’ve finally detected clues of an overhead sound channel, though it doesn’t appear to work as simply or reliably as oceanic SOFAR. If confirmed, atmospheric SOFAR could pave the way for a network of airborne receivers that could help researchers remotely detect explosions of volcanoes, bombs and other sources emitting infrasound, sub-zero acoustic waves. frequency of human hearing.

“It would help a lot to have these [detectors] up there,” says William Wilcock, a marine seismologist at the University of Washington, Seattle. Although seismic sensors in the ground pick up most of the biggest bangs on the planet, “some areas of the Earth are very well covered and some are not.”

In the ocean, the SOFAR channel is bounded by layers of warmer, lighter water above and cooler, denser water below. Sound waves, which travel the slowest at this depth, are trapped inside the channel, bouncing off the surrounding layers like a bowling ball guided by bumpers. Researchers rely on the SOFAR channel to monitor earthquakes and eruptions under the seabed, and even to measure ocean temperatures resulting from global warming.

After geophysicist Maurice Ewing discovered the SOFAR Channel in 1944, he set out to find a similar layer in the sky. At an altitude between 10 and 20 kilometers is the tropopause, the boundary between the troposphere, the lowest layer of the atmosphere (where weather occurs) and the stratosphere. Like the marine SOFAR, the tropopause represents a cold region, where sound waves should travel slower and farther. An acoustic waveguide in the atmosphere, Ewing explained, would allow the US Air Force to listen in on nuclear weapons tests initiated by the Soviet Union. He launched a top secret experiment, named Project Mogul, which sent hot air balloons equipped with infrasonic microphones.

Instruments often malfunctioned in high winds, and in 1947 debris from a balloon crashed just outside Roswell, New Mexico; this accident sparked one of the most famous UFO conspiracy theories in history. Soon after, the military disbanded the project. But the mission was not declassified for almost 50 years. By then, Cold War tensions had subsided and research in atmospheric acoustics had all but died out, says Stephen McNutt, a volcanic seismologist at the University of South Florida. “All of a sudden the rug was pulled and infrasound wasn’t funded for 30 or 40 years,” he says.

But Sarah Albert, a geophysicist at Sandia National Laboratories in New Mexico, never gave up on the idea. She was intrigued by the potential of new solar-powered balloons that can hover passively at stable altitudes and wireless telemetry that can stream data over long distances.

Equipped with modern technology, she and her colleagues flew to an airport near Albuquerque and, on April 14, 2021, launched a balloon at sunrise. They timed the flight to coincide with the launch of Blue Origin’s New Shepard rocket, which lifted off more than 400 kilometers away in Van Horn, Texas. Floating in the suspected sound channel, the balloon picked up three clear cues from the rocket – one as it launched, and two others as it ascended and descended through the tropopause, the researchers revealed at the meeting last week. annual of the Seismological Society of America. . The sighting marks the first verified detection of infrasound from a distant airborne source with an airborne receiver, Albert said.

But to their surprise, the scientists also picked up other sounds in the channel. “There are infrasound events of unknown nature that occur several times per hour,” explains Daniel Bowman, geophysicist at Sandia and collaborator on the project. “And there is no good explanation.”

To make matters even more confusing, the team repeated the experiment with another rocket launch from Vandenberg Air Force Base in California on September 27, 2021 and saw no signal. Although this new rocket launch is three times farther than the Texas one, they still expected the atmospheric SOFAR channel to carry the sound waves from the rocket to the balloon, Albert says.

“I believe there is an atmo-SOFAR channel,” says Albert. “But I’m not as convinced that he exists all the time and can channel sound as far as we previously thought.” That makes sense, she adds, because winds and temperature variations make the tropopause a much more dynamic place than the stable ocean channel.

In the future, the researchers plan to listen to launches with multiple staggered balloons at different altitudes to determine where the effects of the channel are strongest. They also plan to test the range of the signals and investigate the mysterious background noise.

Understanding how the channel works could help lay the groundwork for a future airborne infrasound network, which would continuously monitor Earth for major explosions and eruptions, Albert says. “In areas where there are volcanoes and limited infrasound arrays, having a balloon or an array of balloons in the air could fill that gap.”

When Hunga Tonga-Hunga Ha’apai erupted in Tonga in January, for example, the nearest ground detector was over 750 kilometers away, McNutt says. Setting up a network of balloons could help determine the location, time and size of these incidents from a distance.

“Earth is three-quarters water, and instruments don’t like being on water,” says Bowman. “Having an aerial perspective…allows us to ask questions that you simply cannot ask on the ground.”