Mysterious sounds in stratosphere can’t be traced to any known source

A solar-powered balloon floating in the stratosphere recorded a low-frequency sound of mysterious origin.

“When I started flying balloons many years ago, I had no idea what I was hearing,” says Daniel Bowman of Sandia National Laboratory in New Mexico. “We’ve learned how to identify the sounds of explosions, meteor strikes, aircraft, thunderstorms and cities.

Bowman and his colleagues measured infrasound signals (sounds with frequencies too low for the human ear to hear) using solar-powered balloons floating 20 kilometers high.

The researchers made balloons about 7 meters wide made of thin plastic. Fill a balloon with charcoal powder and heat it in bright sunlight to make it float. Unlike weather balloons that rise until they burst, these DIY solar-powered spheres sailed for hours in the stratosphere, carrying infrasound sensors for hundreds of kilometers. Researchers have deployed more than 50 balloons over seven years, starting in 2016.

The data they collected showed that sounds in the stratosphere are quite different from those on the Earth’s surface. On the ground, infrasound sensors pick up the signal deflected by the descending wind, but the balloon hovers above the wind and picks up traces of turbulence in other parts of the atmosphere and infrasound from ocean storms. Recorded. But many of the infrasound signals from the stratosphere had no clear origin, says Bowman. He presented the work on May 11 at the Acoustical Society of America meeting in Chicago, Illinois.

These mysterious signals may be related to a type of atmospheric turbulence that has never been recorded, but stratospheric infrasound has been largely unexplored so far, so knowledge Bowman says it’s hard to make an educated guess.

One of the first balloon studies of its kind, he said, was the U.S. Army Air Corps, codenamed “Project Moguls,” which sought to detect infrasonic signals from nuclear weapons tests conducted in the Soviet Union in the 1940s. It is said that it was an experiment. One of his Project Mogul balloons crashes in Roswell, New Mexico, bringing the top-secret program into the public eye. A cover-up to hide the balloon’s purpose sparked a UFO conspiracy, Bowman said, after which most of the balloon flight data, which ended in the 1960s, was classified.

Roger Waxler of the University of Mississippi says he is not surprised by the mysterious infrasound signals appearing in stratospheric recordings. “On the ground, we can put sensors in an array and know exactly where they are relative to each other, which helps us calculate where the infrasound is coming from. is,” he says.

Bowman is working with NASA to develop similar balloon technology for an even less explored location: the clouds of Venus. He and his colleagues hope to adapt a solar-powered balloon to record infrasound on the surface of Venus, which could help record the planet’s seismic activity.