Balloon Detects First Signs of a 'Sound Tunnel' in the Sky

Atmospheric analog to ocean's acoustic channel could be used to monitor eruptions and bombs. From a report: About 1 kilometer under the sea lies a sound tunnel that carries the cries of whales and the clamor of submarines across great distances. Ever since scientists discovered this Sound Fixing and Ranging (SOFAR) channel in the 1940s, they've suspected a similar conduit exists in the atmosphere. But few have bothered to look for it, aside from one top-secret Cold War operation. Now, by listening to distant rocket launches with solar-powered balloons, researchers say they have finally detected hints of an aerial sound channel, although it does not seem to function as simply or reliably as the ocean SOFAR. If confirmed, the atmospheric SOFAR may pave the way for a network of aerial receivers that could help researchers detect remote explosions from volcanoes, bombs, and other sources that emit infrasound -- acoustic waves below the frequency of human hearing. "It would help enormously to have those [detectors] up there," says William Wilcock, a marine seismologist at the University of Washington, Seattle. Although seismic sensors in the ground pick up most of the planet's biggest bangs, "some areas of the Earth are covered very well and others aren't." In the ocean, the SOFAR channel is bounded by layers of warmer, lighter water above and cooler, denser water below. Sound waves, which travel at their slowest at this depth, get trapped inside the channel, bouncing off the surrounding layers like a bowling ball guided by bumpers. Researchers rely on the SOFAR channel to monitor earthquakes and eruptions under the sea floor -- and even to measure ocean temperatures rising from global warming. After geophysicist Maurice Ewing discovered the SOFAR channel in 1944, he set out to find an analogous layer in the sky. At an altitude of between 10 and 20 kilometers is the tropopause, the boundary between the troposphere, the lowest layer of the atmosphere (where weather occurs), and the stratosphere. Like the marine SOFAR, the tropopause represents a cold region, where sound waves should travel slower and farther. An acoustic waveguide in the atmosphere, Ewing reasoned, would allow the U.S. Air Force to listen for nuclear weapon tests detonated by the Soviet Union. He instigated a top-secret experiment, code-named Project Mogul, that sent up hot air balloons equipped with infrasound microphones.

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