In a surprising and yet mysterious way, large solar balloons deployed for an experiment recorded sounds containing some unexpected elements at an impressive altitude of 70,000 feet in the Earth's stratosphere, that researchers cannot identify.
Daniel Bowman and other researchers at Sandia National Laboratories have been sending solar-powered balloons with microphones to capture the sounds present in this atmospheric layer.
The results of this intriguing study were recently presented at the 184th Meeting of the Acoustical Society of America held in Chicago. During the event, Bowman shared a recording from a NASA balloon that had circled Antarctica.
What is the stratosphere?
The stratosphere, the second layer of Earth's atmosphere, harbors the ozone layer, which is responsible for absorbing and dispersing the sun's ultraviolet radiation, as described by NASA. This region, characterised by its thin and dry air, serves as the maximum altitude reached by jet aircraft and weather balloons. Typically, the stratosphere remains undisturbed by turbulence, presenting a relatively calm atmospheric environment.
Bowman's experiment with Solar Balloons
The Solar Balloon | Sandia National Laboratories
The inspiration for exploring the soundscape of the stratosphere came to Daniel Bowman, a principal scientist at Sandia National Laboratories in New Mexico, during his time in graduate school. He was introduced to the concept of infrasound, which refers to low-frequency sounds generated by volcanoes that are inaudible to the human ear.
Bowman and his colleagues had previously utilised weather balloons equipped with cameras to capture images of the vast expanse of space and the Earth's surface from above. Building upon this experience, they successfully constructed their own solar balloons.
Daniel Bowman and other researcher at Sandia National Laboratories | Sandia National Laboratories
Originally, their intention was to attach infrasound recorders to these balloons, aiming to capture the sounds emitted by volcanoes. However, they soon realised that no one had attempted to affix microphones to stratospheric balloons for over fifty years. This revelation prompted them to redirect their focus and explore the capabilities of this novel platform. Jonathan Lees, Bowman's advisor from the University of North Carolina, Chapel Hill, is a respected professor specialising in Earth, marine, and environmental sciences, with a specific emphasis on seismology and volcanology.
Bowman and his collaborators build their own balloons, each spanning about 19.7 to 23 feet (6 to 7 meters) across, with painter’s plastic, shipping tape, and charcoal dust.
They cost about $50 to make, and a team of two can build one in about 3.5 hours. One simply brings it out to a field on a sunny day and fills it up with air, and it will carry a pound of payload to about 70,000 ft.
Sandia National Laboratories
The charcoal dust is used inside the balloons to darken them, and when the sun shines on the dark balloons, the air inside them warms up and becomes buoyant. The inexpensive and easy DIY design means the researchers can release multiple balloons to collect as much data as possible.
These solar balloons possessed the capability to ascend to altitudes twice as high as commercial jets. Bowman, in an email statement, described the diverse range of sounds recorded during their experiments.
The captured audio included surface and buried chemical explosions, thunder, colliding ocean waves, propeller aircraft, city sounds, suborbital rocket launches, earthquakes, and potentially even sounds associated with freight trains and jet aircraft. Additionally, the researchers encountered unexplained crackles and rustling noises that left them uncertain about their origins.
In this recording, the infrasound of colliding ocean waves produced a continuous, sighing-like sound. However, alongside these recognisable sounds, the recording also contained mysterious crackles and rustling, leaving the researchers intrigued and curious about their source.
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