American scientists have created stratospheric balloons that only need sunlight to rise to great heights. Using such balls, they first recorded mysterious infrasound of an unknown nature in the stratosphere. The results of the discovery reported at the American Acoustic Society conference in Chicago.
Geophysicists have long used special sensors near the Earth’s surface to record infrasonic vibrations propagating through the planet after various natural and man-made events – volcanic eruptions, rocket launches, nuclear weapons tests, etc. Scientists led by Daniel Bowman of Sandia National Laboratories (USA) set out to find out what sounds can be heard in the stratosphere, a relatively quiet layer of the atmosphere in terms of the propagation of vibrations. For these purposes, scientists have developed inexpensive and simple balloons with a diameter of up to 7 meters, which do not need light gases and fuel to rise to a great height.
Darielle Dexheimer, Sandia National Laboratories
“Our balls are basically giant plastic bags with charcoal dust applied to the inside, making them dark. We made them with plastic wrap from a hardware store, masking tape, and charcoal. powdered wood from a fireworks store.When the sun shines on a dark ball, the air inside heats up and becomes light.This passive solar energy is enough to lift the ball over 20 miles in height. Each ball is only $50 and can be picked up in the basketball hall,” Bowman explained.
To record low-frequency sounds, the balloons were equipped with microbarometers, originally designed to monitor volcanoes. Also, GPS receivers were installed on the balloons, thanks to which they were tracked during flight and landing.
At more than 20 km altitude, microphones mounted on balloons recorded many different sounds, inaudible on the surface. These include the natural sounds of ocean collisions and thunder, as well as the man-made sounds of wind turbines and explosions. However, in addition to this, scientists recorded sounds of an incomprehensible nature. “There are mysterious infrasound signals in the stratosphere that have been observed several times per hour on some flights, but their source is completely unclear,” Bowman said.
Scientists believe their method of making “solar” balls can be used to study other planets, for example to study seismic and volcanic activity on Venus using stratospheric balloons.
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