Mysterious sounds in the stratosphere cannot be attributed to any known source

Solar-powered balloons floating in the stratosphere have recorded low-frequency sounds of mysterious origin.

“When we first started flying balloons years ago, we didn’t really know what we were going to hear,” says Daniel Bowman at Sandia National Laboratories in New Mexico. “We learned to identify the sounds of explosions, meteor collisions, planes, thunderstorms and cities. But practically every time we send balloons, we find sounds that we cannot identify.

Bowman and his colleagues measured infrasound signals – sounds so low in frequency that they are inaudible to the human ear – using solar balloons floating 20 kilometers high.

The researchers built Balloons about 7 meters wide and made of thin plastic. They filled the balloons with charcoal powder, which heats up in sunlight and makes the balloon float. Unlike weather balloons, which rise until they burst, these crafts solar energy Balloons coasted in the stratosphere for hours, carrying infrasound sensors hundreds of miles. Researchers deployed more than 50 balloons over a seven-year period starting in 2016.

The data they collected showed that the sound of the stratosphere is very different from that of the Earth’s surface. On the ground, infrasound sensors picked up signals that were deflected by winds as they descended, but the balloons were floating above those winds – they recorded turbulence signatures in other parts of the atmosphere and infrasonic sounds of sea storms. However, Bowman says many infrasound signals from the stratosphere had no obvious origin. He presented the work at a meeting of the Acoustical Society of America in Chicago, Illinois on May 11.

These mysterious signals could be related to types of atmospheric turbulence that have never been recorded before, but infrasound in the stratosphere has only rarely been explored before, so it’s hard to make educated guesses, Bowman says.

He says one of the first such balloon studies was a US Army Air Forces experiment codenamed Project Mogul, which sought to detect infrasonic signals from Soviet Union nuclear weapons testing in the 1940s. One of Project Mogul’s balloons crashed in Roswell, New Mexico, exposing the top-secret program to the public. Concealment to conceal the goal of the ball triggered UFO conspiraciesand most of the data from subsequent balloon flights, ending in the 1960s, has been classified, Bowman says.

Roger Waxler at the University of Mississippi says he’s not surprised by the enigmatic infrasound signals appearing in recordings from the stratosphere. “On the ground, you can place sensors in arrays and know exactly where they are relative to each other, which helps calculate where infrasound is coming from. With balloons, they just go where they go,” he says.

Bowman is collaborating with NASA to develop similar balloon technology for an even less explored location: the clouds of Venus. He and his colleagues want to adapt their solar-powered balloons to record infrasound above the surface of Venus, which could help chronicle the planet. seismic activity.