(CNN) – Researchers sent giant solar balloons to an altitude of 21,336 meters to record sounds from Earth’s stratosphere, and microphones picked up some unexpected noises.
The stratosphere is the second layer of Earth’s atmosphere, and its lower level contains the ozone layer, which 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 layer of the atmosphere is rarely affected by turbulence.
Daniel Bowman, principal scientist at Sandia National Laboratories in New Mexico, was inspired to explore the acoustic landscape of the stratosphere after learning about the low-frequency sounds generated by volcanoes. This phenomenon, known as ultrasound, is inaudible to the human ear.
Bowman and his friends had already installed cameras on weather balloons “to photograph the black sky above and the earth below” and had succeeded in building their own sun balloon.
Bowman suggested installing ultrasonic recorders on the balloons to record the sounds of volcanoes. But then he and his advisor Jonathan Lees, of the University of North Carolina at Chapel Hill, realized that no one had tried to mistake stratospheric balloons in half a century, so we set out to explore what this new platform could do. Bowman explained. Liz is Professor of Earth, Marine, and Environmental Sciences, and researches seismology and volcanology.
The balloons can carry sensors twice the height of commercial aircraft.
“On our sun globes, we’ve recorded chemical explosions on the surface and burials, thunder, crashing ocean waves, helicopters, city sounds, suborbital rocket launches, earthquakes, and maybe even freight trains and jet planes,” Bowman explains via email. “We also recorded sounds whose source is not clear.”
The results were shared on Thursday 184th Meeting of the American Vocal Society in Chicago.
The recording Bowman shared from a NASA balloon that flew over Antarctica contains the ultrasound of crashing ocean waves, which sound like continuous sobs. But other squeaks and rustles have unknown origins.
In the stratosphere, “there are mysterious ultrasound signals going on a few times an hour on some flights, but their source is completely unknown,” says Bowman.
Build solar balloons
Bowman and his collaborators investigated balloons from NASA and other aerospace 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 balloons can be set up on a basketball court.
“Each balloon is made of painter’s plastic, masking tape, and charcoal dust,” Bowman explains over email. “It costs about $50 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’ll hold about 70,000 feet of payload.”
Coal dust is used inside the balloons to darken them, and when the sun shines on the darkening balloons, the air inside heats up and becomes buoyant. The cheap and easy-to-make design allows the researchers to drop several balloons to collect as much data as possible.
In fact, a group of high school students have access to the school gym I can build a sun balloonThere is also a cell phone app called RedVox that can record ultrasounds.”
Bowman estimates that he launched dozens of solar balloons to collect infrasound recordings between 2016 and April of this year. Micrometers, originally designed to monitor volcanoes, were attached to balloons to record low-frequency sounds.
The researchers tracked their balloons using the Global Positioning System (GPS), so they could travel hundreds of kilometers and land
The longest flight to date was 44 days aboard a NASA helium balloon, which recorded 19 days of data before the microphone’s batteries ran out. Meanwhile, solar balloon flights usually take about 14 hours during the summer, and they land once the sun goes down.
Detect mysterious sounds
The advantage of high-altitude airships is lower noise levels and greater detection range, making the entire Earth accessible. But the balloons also pose a challenge to the researchers. The stratosphere is a harsh environment, with sharp temperature swings between hot and cold.
“The sun balloons are a bit slow and we smashed some in the bushes trying to get them going,” Bowman explains. “We had to go into canyons and across mountains to haul our loads. Once, fellows from Oklahoma landed a balloon in a field, stayed up all night, and relaunched it for another full day.”
Lessons learned from multiple balloon flights have made the process somewhat easier, but now the biggest challenge for researchers is determining the origin of the signals recorded during the flights.
“There are a lot of trips that have signals that we don’t understand where they come from,” Bowman says. “It’s almost certainly mundane, maybe a spot of turbulence, or a severe storm in the distance, or some sort of human thing like a freight train, but sometimes it’s hard to tell what’s going on because there’s so little data there.”
Sarah Albert, a geophysicist at Sandia National Laboratories, investigated an “acoustic duct,” a channel that transmits sounds long distances through the atmosphere, located at the altitudes Bowman is studying. they recordings They detected rocket launches and other unidentified sounds.
“The sound may be trapped in the channel and reverberate until it is completely distorted,” Bowman explains. But it is not yet clear whether it is near and rather quiet (like a turbulent spot) or far and loud (like a distant storm).
Bowman and Albert will continue to investigate the airborne acoustic channel and try to determine where the stratospheric rumble comes from, and why some flights record it while others do not.
Bowman is eager to understand the stratosphere’s soundscape and uncover key features such as variability between seasons and locations.
Helium-filled versions of these balloons may be used one day Explore other planets Like Venus, it carries scientific instruments above or within the planet’s clouds for a few days as a test flight for larger and more complex missions.
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