Dubai, United Arab Emirates (CNN) — Giant solar balloons launched 70,000 feet into the air to record the sounds of the stratosphere, and microphones picked up some unexpected sounds.
The stratosphere is the second layer in the Earth’s atmosphere, and the lower layer contains the ozone layer, which absorbs and dissipates the sun’s ultraviolet rays, according to the US space agency “NASA”.
The thin, dry air of the stratosphere, where jet planes and weather balloons reach their highest altitude, is a relatively calm layer of the atmosphere that is rarely affected by turbulence.
During his graduate studies, Daniel Bowman, a principal scientist at Sandia National Laboratories in New Mexico, was inspired to explore the stratospheric acoustic landscape after learning about the low-frequency sounds produced by volcanoes.
This phenomenon is known as infrasound, and it is inaudible to the human ear.
Bowman and his friends had previously installed cameras on weather balloons “to take pictures of the black skies above us and the Earth far away,” and succeeded in building their own solar balloon.
He suggested attaching infrasound recorders to balloons to record the sounds of volcanoes.
But then he and his advisor, Jonathan Lees of the University of North Carolina, “realized that 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.”
The blimps can carry sensors up to twice the height of commercial aircraft.
“Through our solar balloons, we’ve recorded surface and buried chemical explosions, lightning strikes, crashing ocean waves, helicopters, city sounds, semi-radial missile launches, earthquakes, maybe even freight trains and fighter planes, and we’ve also recorded sounds of unclear origin,” Bowman told CNN. .
The results were shared Thursday, during the 184th meeting of the Acoustical Society of America in Chicago.
The recording Bowman shared from a NASA balloon around Antarctica contains the infrasound of crashing ocean waves, which sound like continuous sighs, but there is also a crackle and creak of unknown origin.
In the stratosphere, “there are mysterious infrasound signals happening a few times an hour on some flights, but their exact origin is not known,” Bowman said.
The advantage of the higher altitude at which balloons reach is lower noise levels and increased detection range, which means that the entire ground is available.
However, balloons also pose challenges for researchers.
The stratosphere is classified as a harsh environment with sharp fluctuations between temperature and cold.
“The solar balloons are quite slow, and some of them crashed into trees while trying to launch them,” Bowman said.
He continued, “We had to walk through valleys and mountains to launch our balloon. One time, our colleagues at Oklahoma State University’s balloon landed in a field, spent the night there, and then got back up in the air to fly another full day!”
The process has been made somewhat easier by lessons learned from many balloon flights, but the biggest challenge now for researchers is identifying the radio signals recorded during the flights.
“There are many flights that contain signals that we don’t understand where they come from,” Bowman said.
“It’s probably insignificant, maybe it’s some disturbance, or a severe storm far away, or some kind of instrument that a human uses like a freight train, but it’s hard to know what’s going on sometimes because there’s a lack of data there.”
Sarah Albert, a geophysicist with Sandia National Laboratories, has imaged an “acoustic duct,” a pathway that transmits sounds from great distances through the atmosphere, which is located at the altitudes Bowman is studying.
Her recordings featured missile launches and other mysterious sounds.
“It may happen that the sound gets trapped in the channel and repeats in a way that makes it completely incomprehensible, but if it is close and relatively quiet (such as a weather fluctuation) or far and noisy (such as a distant storm) is not yet clear,” Bowman explained.
Bowman and Albert will continue to investigate the atmospheric audio channel and try to determine the source of the stratospheric hum, and why some flights record these sounds while others do not.
Bowman is looking to understand the acoustic system of the stratosphere and uncover key characteristics, such as changes in seasons and locations.
Helium-filled versions of these balloons could one day be used to explore other planets such as Venus, carrying scientific instruments above or in clouds for a few days as a test flight for larger and more complex missions.
