Scientists were able to monitor mysterious sounds through giant solar balloons equipped with sensitive microphones sent to an altitude of 70,000 feet from the surface, where they entered the heart of the second layer of the Earth’s atmosphere known as the “stratosphere”.
NASA explained that the thin, dry air in the stratosphere, where jet planes and weather balloons reach their maximum altitude, is a relatively calm layer of air that is rarely affected by turbulence.
And the “stratosphere”, according to the US Space Agency (NASA), is the second layer of the Earth’s atmosphere, and at its lowest level there is the ozone layer, which absorbs and dissipates the sun’s ultraviolet rays, according to CNN.
The human ear does not hear it
Inspired by his study of the low-frequency sounds produced by volcanoes, Daniel Bowman, principal scientist at Sandia National Laboratory in New Mexico, is exploring the acoustic landscape of that layer of the atmosphere. This phenomenon is scientifically known as ultrasound, which the human ear cannot hear.
Bowman and his team previously installed cameras on weather balloons to capture images of the black sky above and the earth below, and then successfully built their own solar balloon.
He also proposed attaching infrared recorders to balloons to record the sounds of volcanoes, in collaboration with his advisor, Jonathan Lees of the University of North Carolina.
“We decided to go ahead and explore what this new platform could do, in collaboration with Jonathan Lees, a geoscientist, ocean and environmental scientist with experience in seismological and volcanological research.
According to Bowman, these balloons are equipped with sensors that are twice as fast as commercial aircraft.
“In our solar balloons we have recorded chemical explosions, thunder, ocean waves crashing, helicopters, city sounds, additional rocket launches, earthquakes, freight trains and jet planes,” Bowman said in an email. “We have recorded other sounds, but their origin is unclear.”
Continuous sob sound
In a recording Bowman shared from a NASA balloon orbiting Antarctica, the infrasound of crashing ocean waves sounds like a continuous sigh, but the explosions and other shocks are of unknown origin.
Bowman said, on Thursday, during that participation, that “inside the stratosphere some planes had mysterious infrared signals a few times an hour, but their source is completely unknown.”
Bowman and his assistants conducted the research using NASA balloons and other aeronautical service providers, but decided to build their own balloons, each 6 to 7 meters in diameter.
He also indicated that materials for making these balloons can be found in hardware and fireworks supply stores, in addition to the possibility of assembling balloons on a basketball court.
“Each balloon is made of painter’s plastic, shipping tape and coal dust, and the cost of one balloon is about $50,” Bowman said.
Coal dust is usually placed inside these balloons to darken them, and when sunlight reflects on them, the air inside them makes them float in the air and fly away.
According to experts, these cheap balloons help researchers release many of them to collect as much data as possible.
Bowman estimates that he launched dozens of solar balloons to collect infrasound recordings from 2016 to April this year 2023.
The researchers tracked their balloons using the Global Positioning System (GPS) as they traveled hundreds of miles before landing in remote locations.
The advantage of the higher altitude the balloons reach is the lower the noise level, the greater the detection range, and the entire planet Earth can be explored.
However, balloons present challenges for researchers, as the stratosphere is a harsh environment where temperatures fluctuate between hot and cold.
“The solar balloons are a bit flimsy, and we destroyed a few in the bushes in an attempt to launch them,” Bowman said, adding, “There are many balloons that detect signals whose origin we do not understand. There is no doubt that they are normal or caused by air turbulence.”
He also added, “Or it may be the sounds of a severe storm far away or some kind of human factor, such as the sounds of a freight train, but sometimes it is difficult to determine what it is because there is no clear or sufficient data.”
Sarah Albert, a geophysicist at Sandia National Laboratories, investigated an “acoustic channel” (a channel that transmits sounds over great distances through the atmosphere) located at altitudes identified by Bowman’s research.
Sarah’s recordings have captured missile launches and other unknown sounds.
In turn, Bowman said: “This sound may be trapped in the channel and reverberate until it is completely distorted.”
Bowman and Sarah Albert will continue to investigate the atmospheric acoustic channel and try to determine the source of the “strange sound” in the stratosphere, and why some flights record it while others do not.