Giant solar balloons equipped with sensitive microphones were able to capture unexpected sounds after they reached an altitude of 70,000 feet from the surface, where they entered the “stratosphere”, according to the “CNN” news network.
And the “stratosphere”, according to the US Space Agency (NASA), is the second layer of the Earth’s atmosphere, and at its lowest level is the ozone layer, which absorbs and dissipates ultraviolet rays of the sun.
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.
Principal scientist at Sandia National Laboratory in New Mexico, Daniel Bowman, is focused on exploring the acoustic landscape of that layer of the atmosphere, having been inspired by his studies of low-frequency sounds produced by volcanoes.
This phenomenon is scientifically known as ultrasound, which the human ear cannot hear.
Bowman and his team had previously installed cameras on weather balloons to take pictures of the black sky above and the earth below, and then successfully built their own solar balloon.
Bowman suggested attaching infrared recorders to balloons to record volcanic sounds, but he and his advisor, Jonathan Lees of the University of North Carolina, later realized that no one had tried to put microphones in stratospheric balloons for half a century.
“So we decided to go ahead and explore what this new platform could do, in collaboration with Lees, who is an earth, 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 crashes, helicopters, city sounds, additional rocket launches, earthquakes, freight trains and jet planes,” Bowman said in an email.
He continued, “We have recorded other sounds, but their origin is not clear.”
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.
On Thursday, during that engagement, Bowman said, “In the stratosphere some aircraft were having mysterious infrared signals a few times an hour, but their source is completely unknown.”
Bowman and his collaborators conducted the research using balloons from NASA and other aeronautical service providers, but decided to build their own balloons, each 6 to 7 meters in diameter.
He pointed out that the 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 charcoal dust, and costs about $50 each,” Bowman said by email.
And he added: “A two-person crew can build one balloon within a time not exceeding 3 and a half hours, and if it is launched on a sunny day after it is filled with air, it can carry a weight of one pound and reach a height of 70,000 feet.
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.
“cheap”
These cheap balloons, according to experts, 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.
Accurate gauges, originally designed to monitor volcanoes, have been attached to balloons to record low-frequency sounds.
The researchers tracked their balloons using the Global Positioning System (GPS) as they traveled hundreds of miles before landing in remote locations.
The longest flight so far on NASA’s helium balloon was 44 days, and it recorded 19 days’ worth of data before the batteries in the microphone ran out.
Meanwhile, solar balloon flights last about 14 hours in the summer and land after sunset.
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 trying to launch them,” Bowman said.
He added, “We had to walk through valleys and mountains to get our loads, and once our colleagues in Oklahoma state had to spend the entire night in a field to find a balloon before releasing it once to fly all day.”
“Doors learned”
Lessons learned from the many balloon flights have made the process somewhat easier, but the biggest challenge now for researchers is identifying the signals recorded during the flights.
Bowman said: “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.”
And he 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.
“This sound may be trapped in the channel and reverberated until it is completely jammed,” Bowman said.
And he added: “But whether it is close and somewhat quiet (like a turbulent spot) or far and loud (like a distant storm), this is not yet clear.”
Bowman and Sarah Albert will continue to investigate the atmospheric acoustic channel and try to determine the source of the stratospheric “gurgling” and why some flights record it while others do not.
Bowman is eager to understand the sonic landscape of the stratosphere and unlock key features, such as variability across 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 within the planet’s clouds for a few days as a test flight for larger and more complex missions.
