They record mysterious rumbles in the Earth’s stratosphere

() — Researchers sent giant solar balloons 21,336 meters up to record sounds from Earth’s stratosphere, and the microphones picked up some unexpected noises.

The stratosphere is the second layer of Earth’s atmosphere, and its lower level contains the ozone layer that absorbs and scatters ultraviolet radiation from the Sun, according to NASA. The thin, dry air in the stratosphere is where jet planes and weather balloons reach their highest altitude, and the relatively calm atmospheric layer is rarely disturbed by turbulence.

Daniel Bowman, principal scientist at Sandia National Laboratories in New Mexico, was inspired to explore the soundscape of the stratosphere after learning about the low-frequency sounds generated by volcanoes. This phenomenon, known as infrasound, is inaudible to the human ear.

A Sandia National Laboratories solar-powered hot air balloon takes flight carrying sensors, including a GPS tracker and a reusable infrasound sensor. Credit: Sandia National Laboratories

Bowman and his friends had already mounted cameras on weather balloons “to photograph the black sky above and the Earth below” and had successfully built their own sun balloon.

Bowman proposed installing infrasound recorders on the balloons to record the sounds of volcanoes. But then he and his adviser Jonathan Lees, of the University of North Carolina at Chapel Hill, “realized that no one had tried to bug stratospheric balloons in half a century, so we went on to explore what this new platform could do.” Bowman explained. Lees is Professor of Earth, Marine and Environmental Sciences and researches seismology and volcanology.

The balloons can carry sensors twice the height of commercial airplanes.

“On our sun globes we have recorded surface and buried chemical explosions, thunder, crashing ocean waves, propeller planes, city sounds, suborbital rocket launches, earthquakes, and maybe even freight trains and jet planes,” Bowman explains by email. . “We have also recorded sounds whose origin is not clear.”

The findings were shared Thursday at the 184th Meeting of the American Acoustic Society in Chicago.

A recording shared by Bowman from a NASA balloon that flew over Antarctica contains infrasounds of crashing ocean waves, which sound like continuous sighs. But other creaks and rustles have unknown origins.

In the stratosphere, “there are mysterious infrasound signals that occur a few times an hour on some flights, but their origin is completely unknown,” Bowman says.

build solar balloons

Bowman and his collaborators have investigated balloons from NASA and other flight providers, but decided to build their own balloons, each about 6 to 7 meters in diameter.

Supplies can be found at hardware and fireworks stores, and the balloons can be mounted on a basketball court.

“Each balloon is made from painter’s plastic, masking tape, and charcoal dust,” Bowman explains by email. “They cost about $50 to make and a team of two can build one in about 3.5 hours. Just take it to a field on a sunny day and fill it with air, and it will carry a pound of payload about 70,000 feet” .

Coal dust is used inside the balloons to darken them, and when the Sun shines on the dark balloons, the air inside them warms up and becomes buoyant. The cheap and easy-to-make design allows researchers to drop multiple balloons to collect as much data as possible.

This view from one of Sandia National Laboratories’ solar hot air balloons was taken at an altitude of about 21 kilometers above the Earth’s surface. Credit: Guide Star Engineering LLC/Sandia National Laboratories

“Actually, a group of high school students with access to the school gym I could build a sun balloonand there’s even a cell phone app called RedVox that can record infrasound,” Bowman said.

Bowman estimates that he launched several dozen solar balloons to collect infrasound recordings between 2016 and April of this year. Microbarometers, originally designed for monitoring volcanoes, were attached to the balloons to record low-frequency sounds.

The researchers tracked their balloons using GPS, as they can travel hundreds of kilometers and land in
inconvenient places.

The longest flight to date was 44 days aboard a NASA helium balloon, which recorded 19 days of data before the microphone’s batteries died. Meanwhile, solar balloon flights typically last about 14 hours during the summer, landing once the sun goes down.

unravel the mysterious sounds

The advantage of the high altitude balloons reach is that noise levels are lower and the range of detection is increased, making the entire Earth accessible. But the balloons also pose a challenge for researchers. The stratosphere is a harsh environment, with drastic temperature fluctuations between hot and cold.

“Sun balloons are a bit slow and we’ve smashed some into bushes trying to launch them,” Bowman explains. “We’ve had to go into canyons and across mountains to haul our loads. Once, our colleagues from Oklahoma state landed a balloon in a field, stayed overnight, and relaunched it to fly another full day.”

The lessons learned from multiple balloon flights have made the process somewhat easier, but now the biggest challenge for researchers is to identify the origin of the signals recorded during the flights.

“There are a lot of flights with signals whose origin we don’t understand,” Bowman says. “They are almost certainly mundane, perhaps a patch of turbulence, a distant severe storm, or some kind of human object like a freight train, but sometimes it’s hard to tell what’s going on due to the lack of data up there. “.

Sarah Albert, a geophysicist at Sandia National Laboratories, investigated a “sound channel,” a conduit that carries sounds long distances through the atmosphere, located at the altitudes Bowman is studying. Their recordings they have caught rocket launches and other unidentified noises.

Sandia National Laboratories geophysicists (left to right) Daniel Bowman and Sarah Albert show an infrasound sensor and the enclosure used to protect the sensors from extreme temperatures. Credit: Randy Montoya/Sandia National Laboratories

“It could be that the sound gets trapped in the channel and resonates until it’s completely distorted,” explains Bowman. “But it’s not yet clear if it’s close and fairly quiet (like a patch of turbulence) or distant and loud (like a distant storm).”

Bowman and Albert will continue to investigate the airborne sound channel and try to determine where the rumbles in the stratosphere originate, and why some flights record them and others don’t.

Bowman is eager to understand the soundscape of the stratosphere and unravel key features such as variability between seasons and locations.

Helium-filled versions of these balloons may one day be used for explore other planets like Venus, carrying scientific instruments above or within the planet’s clouds for a few days as a test flight for larger and more complex missions.