June 1, 2023

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Unidentified mysterious sounds recorded in the stratosphere

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Giant solar balloons were sent 70,000 feet into the air to record the sounds of the Earth’s stratosphere — and microphones picked up some unexpected sounds.

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 of the stratosphere is where jet planes and weather balloons reach their maximum 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, got inspired in graduate school to explore the acoustic landscape of the stratosphere after being introduced to the low-frequency sounds generated by volcanoes. This phenomenon is known as ultrasound, and it is inaudible to the human ear.

Bowman and his friends had previously mounted cameras on weather balloons “to take pictures of the black sky above and the earth below” and had successfully built their own solar balloon.

He suggested attaching infrasound recorders to the balloons to record the sounds of volcanoes. But then he and his advisor Jonathan Lees of the University of North Carolina, Chapel Hill, realized, “No one had tried to put microphones on stratospheric balloons for half a century, so we focused on exploring what this new platform could do,” Bowman said. Liz is a professor of Earth, Marine and Environmental Sciences who researches seismology and volcanology.

Sensors can take balloons twice as high as commercial aircraft.

“On our solar balloons, we’ve recorded surface chemical explosions and burials, thunder, ocean waves, helicopter crashes, city sounds, suborbital rocket launches, earthquakes, and maybe even freight trains and jet planes,” Bowman said via email. “We also recorded sounds of unclear origin.”

The results were shared on Thursday 184th Meeting of the American Vocal Society in Chicago.

The recording Bowman shared from a NASA balloon around Antarctica contains ultrasounds of the crashing of ocean waves, which sound like a continuous sigh. But other rattles and rustles have unknown origins.

Listen to the sounds of the stratosphere

Solar balloons have picked up on a slew of sounds in Earth’s second layer of atmosphere, including the crashing of ocean waves — as well as sounds of unknown origin.

Source: Daniel Bowman / Sandia National Laboratories

In the stratosphere, “there are mysterious ultrasound signals going on a few times an hour on some flights, but their source is completely unknown,” Bowman said.

Bowman and his collaborators did research using balloons from NASA and other aerospace providers, but decided to build their own balloons, each with a span of about 19.7 to 23 feet (6 to 7 meters).

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

“Each balloon is made of painter’s plastic, shipping tape, and charcoal dust,” Bowman said via email. They cost about $50 to make and a team of two can build one in about 3.5 hours. One simply takes it out into a field on a sunny day and fills it with air, and it will carry a pound of payload to about 70,000 feet.”

Charcoal 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 inexpensive and easy DIY design means that researchers can release several balloons to collect as much data as possible.

“Really, a group of high school students have access to the school gym You could build a solar balloonAnd there’s also a mobile app called RedVox that can record ultrasounds,” Bowman said.

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), as they could travel hundreds of miles and land in inconvenient locations.

The longest flight to date was 44 days aboard a NASA helium balloon, which recorded 19 days’ worth of data before the batteries in the microphone ran out. Meanwhile, solar balloon flights tend to last about 14 hours during the summer and land once the sun goes down.

The advantage of the higher altitude the balloons reach means that noise levels are lower and the detection range is increased – and the entire ground can be reached. But balloons also present challenges for researchers. The stratosphere is an extreme environment with wild temperature swings between hot and cold.

“The solar balloons are a little slow, and we destroyed a few in the bushes when we tried to launch them,” Bowman said. “We had to hike in canyons and through mountains to get our payloads. Once upon a time, our fellow Oklahoma State folks had a balloon that would land in a field, spend the night, and then get off again into the air to fly another whole day!”

Lessons from multiple balloon flights have made the process somewhat easier, but now the biggest challenge for researchers is identifying the signals recorded during the flights.

“There are so many flights that carry signals that we don’t understand where they come from,” Bowman said. “It’s almost certainly mundane, maybe a spot of turbulence, a severe storm in the distance, or some sort of human thing like a freight train — but it’s hard to know what’s going on sometimes because there’s so little data there.”

Sarah Albert, a geophysicist at Sandia National Laboratories, investigated an “acoustic duct” — a channel that transmits sounds over great distances through the atmosphere — located at the altitudes of Bowman’s studies. Ha The recordings recorded missile launches And other unidentified grumbles.

Sandia National Laboratories geophysicists (from left) Daniel Bowman and Sarah Albert show an infrasound sensor and the box used to protect the sensors from temperature extremes.

“This sound may be trapped in the channel and reverberated until it is completely jammed,” Bowman said. “But whether it is close and fairly quiet (like a turbulent spot) or far and loud (like a distant storm) is not yet clear.”

Bowman and Albert will continue to investigate the atmospheric acoustic channel and try to determine where the stratospheric rumble comes from — and why some flights record it while others don’t.

Bowman is eager to understand the sonic landscape of the stratosphere and unlock key features, such as variability across seasons and locations.

Helium-filled versions of these balloons may one day be used Explore other planets like Venuscarrying 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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