Unexplainable, low-frequency sounds in the Earth's atmosphere are leaving scientists scratching their heads.
A group of scientists from Sandia National Laboratories have detected , the origins of which are unknown to experts.
鈥淸In the stratosphere,] there are mysterious infrasound signals that occur a few times per hour on some flights, but the source of these is completely unknown,鈥� said Daniel Bowman, e.
The stratosphere is , and is known to be a relatively calm environment where sounds can be heard that may not be picked up elsewhere, as it is filled with ultraviolet-blocking ozone.
At a height of 21,336 metres, the low-frequency noises were detected by solar-powered balloons sent out to pick up on different sounds.
The balloons, which Bowman describes in a press release as looking like giant plastic bags, were about seven metres wide, and were fastened with infrasound sensors, called microbarometers.
To make the balloons float, they filled them with charcoal dust, making them dark enough to allow sunlight to heat up the air in the balloon.
鈥淲e build them using painter's plastic from the hardware store, shipping tape, and charcoal powder from pyrotechnic supply stores. When the sun shines on the dark balloons, the air inside heats up and becomes buoyant. This passive solar power is enough to bring the balloons from the surface to over 20 km (66,000 ft) in the sky,鈥� said Bowman.
Although the scientists' initial intention was reportedly to record volcanic eruptions, they surprisingly made a new discovery.
The balloons also picked up a mixture of other low-frequency sounds, some familiar ones such as thunder, ocean waves and rocket launches, to name a few. But the mysterious, unknown low-frequency sounds left Bowman and his team without answers - although they have some theories that could potentially explain the noises.
They speculate the sounds could be from a previously undetected form of atmospheric turbulence, or perhaps are echoes of noises from the ground that are now unrecognizable.
More investigation will be done to better understand these sounds in the stratosphere, the researchers said. They鈥檒l also trace more sounds back to where they came from and examine their variability across seasons and in different geographic areas across the world to try and figure out what they are.
