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Giant solar balloons were sent 70,000 feet into the air to record the sounds of Earth’s stratosphere — and microphones picked up some unexpected sounds.
The stratosphere is the second layer of Earth’s atmosphere, and at its lower level is the ozone layer, which absorbs and scatters the sun’s ultraviolet radiation, according to NASA. The thin, dry air of the stratosphere, where jet aircraft and weather balloons reach their maximum altitude, is a relatively quiet atmospheric layer rarely disturbed by turbulence.
Daniel Bowman, principal scientist at Sandia National Laboratory in New Mexico, was drawn to exploring the soundscape of the stratosphere after being introduced to the low-frequency sounds produced by volcanoes in graduate school. This phenomenon, called infrasound, is inaudible to the human ear.
Bowman and his friends had previously installed cameras on weather balloons to “take pictures of the black sky above and the earth far below” and successfully built their own solar balloon.
He proposed attaching infrared recorders to 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 put microphones in stratospheric balloons for half a century, so we moved forward to explore what this new platform could do,” Bowman said. Lees is a professor of earth, ocean and environmental sciences who conducts research in seismology and volcanology.
Balloons can pick up sensors twice as fast as commercial jets.
“In our solar balloons, we have recorded chemical explosions, thunder, crashing ocean waves, propeller aircraft, city sounds, auxiliary rocket launches, earthquakes, and freight trains and jet planes,” Bowman said by email. “We have recorded sounds, and its origin is unclear.”
The findings were shared on Thursday 184th Meeting of the Acoustical Society of America, Chicago.
In a recording shared by Bowman from a NASA balloon orbiting Antarctica, the infrasound of crashing ocean waves sounds like a continuous sigh. But other explosions and rumblings are of unknown origin.
Check out this interactive content on CNN.com
Bowman said that in the stratosphere, “some aircraft have mysterious infrared signals a few times an hour, but the source of these is completely unknown.”
Bowman and his collaborators conducted the research using NASA balloons and other aviation providers, but they decided to build their own balloons, each about 19.7 to 23 feet (6 to 7 meters) across.
Materials can be found at hardware and pyrotechnic supply stores, and balloons can be assembled on the 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 two-man crew can build one in 3.5 hours. If you bring it out into a field on a sunny day and fill it with air, it can carry a one-pound payload to about 70,000 feet.
Charcoal dust is used inside the balloons to darken them, and when sunlight hits the darkened balloons, the air inside them heats up and floats. The cheap and easy DIY design means researchers can release multiple balloons to collect as much data as possible.
Mentor Star Engineering LLC/Sandia National Laboratories
This view of one of Sandia National Laboratories’ solar-powered hot air balloons was taken about 13 miles (21 kilometers) above Earth’s surface.
“Essentially, a group of high school students with access to a school gym A solar balloon can be madeAnd there’s even a cell phone app called RedVox that can record infrasound,” Bowman said.
Bowman estimates he launched several dozen solar balloons to collect infrasound recordings from 2016 to April of this year. Microbarometers, originally designed to monitor volcanoes, were attached to balloons to record low-frequency sounds.
The researchers tracked their balloons with GPS as they traveled hundreds of miles and landed in inconvenient locations.
The longest flight to date on a NASA helium balloon was 44 days, recording 19 days worth of data before the batteries in the microphone died. Meanwhile, solar balloon flights last about 14 hours in summer and land after sunset.
01:22 – Source: CNN
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The advantage of the high altitude that balloons reach is that the noise level is low and the detection range is increased – and the entire Earth can be accessed. But the balloons present challenges for researchers. The stratosphere is a harsh environment with wild temperature fluctuations between hot and cold.
“The solar balloons are a bit flimsy, and we’ve wrecked a few in the bushes trying to launch them,” Bowman said. “We had to go down through valleys and mountains to get our payloads. Once, our Oklahoma State colleagues actually landed a balloon in a field, spent the night, and launched it back into the air for another full day of flying!
Lessons learned from many 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 planes with signals that we don’t understand the origin of,” Bowman said. “They’re certainly mundane—perhaps turbulence, a distant severe storm or some sort of human factor—like a freight train—but sometimes it’s hard to say what’s going on because there’s no data there.”
Sarah Albert, a geophysicist at Sandia National Laboratories, studied a “channel of sound” — a pathway through the atmosphere that carries sounds over great distances — at the altitude Bowman is studying. her Records capture rocket launches and other unidentified rumblings.
Randy Montoya/Sandia National Laboratories
Sandia National Laboratories geophysicists (from left) Daniel Bowman and Sarah Albert show the infrared sensor and the enclosure used to protect the sensors from extreme temperatures.
“The sound gets stuck in the channel and reverberates until it’s completely distorted,” Bowman said. “But whether it’s nearby and very quiet (like turbulence) or distant and loud (like a distant storm) is still unclear.”
Bowman and Albert will continue to study the airborne sound channel to try to determine where the sounds in the stratosphere are coming from — and why some planes record them and others don’t.
Bowman is interested in understanding the soundscape of the stratosphere and unlocking key features such as variation 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, more complex missions.