Mar. 21, 2001 If humans had radio antennas instead of ears, we would hear a remarkable symphony of strange noises coming from our own planet.
Scientists call them "tweeks," "whistlers" and "sferics." They sound like background music from flamboyant science fiction films.
But this is not science fiction. Earth's natural radio emissions are real. And, although we're mostly unaware of them, they are around us all the time.
"Everyone's terrestrial environment almost literally sings with radio waves at audio frequencies," says Dennis Gallagher, a space physicist with the Space Science Research Center at the National Space Science and Technology Center in Huntsville, Ala. — a partnership with NASA's Marshall Space Flight Center in Huntsville, Alabama universities and federal agencies.
"Our ears can't detect radio waves directly, but we can convert them to sound waves with the aid of a very low frequency (VLF) radio receiver."
VLF receivers are simple, yet uncommon. Consisting only of an antenna and an audio amplifier, they are sensitive to radio waves with frequencies between a few hundred Hertz and 10 kilohertz. For comparison, AM broadcast band radios span the much higher frequency range 540 kilohertz to 1.7 megahertz.
With an Internet connection, you can now listen to a VLF radio anytime. Gallagher and his colleagues recently installed a VLF receiver at the Marshall Center's Atmospheric Research Facility. Twenty-four hours a day, it broadcasts the peculiar songs of Earth live on the Web.
The source of most VLF emissions on Earth is lightning. Lightning strokes emit a broadband pulse of radio waves, just as they unleash a visible flash of light. VLF signals from nearby lightning, heard through the loudspeaker of a radio, sound like bacon frying on a griddle or the crackling of a hot campfire. Space scientists call these sounds "sferics," short for atmospherics.
Even when there is no lighting in a listener's area, they can still hear VLF crackles from storms thousands of miles away. Radio waves can propagate even greater distances by bouncing back and forth between our planet's surface and the ionosphere -- a layer of the atmosphere ionized by solar ultraviolet radiation. The ionosphere, which begins about 55 miles (90 kilometers) above the ground and extends to thousands of miles in altitude, makes a good over-the-horizon reflector of low frequency radio waves.
"The ionosphere and the surface of the Earth form a natural waveguide for VLF signals," explains Bill Taylor, a space scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. Sferics that travel very far through the waveguide become "tweeks," which produce a musical ricochet sound in the loudspeaker of a VLF receiver.
Tweeks sound as they do because "their high frequency components reach the receiver before their low frequencies do," says Taylor. This is called delay dispersion, a result of waves traveling between the ground and the ionosphere.
Sometimes the ionosphere leaks lightning pulses into space. They exit the atmosphere entirely, following magnetic field lines that guide them 6,000 miles (10,000 kilometers) or more above Earth's surface, into our planet's magnetosphere and then back again.
"Lightning pulses that travel all the way to the magnetosphere and back are highly dispersed, much more so than tweeks," continued Gallagher. "We call them 'whistlers' because they sound like slowly descending tones. Whistlers are dispersed, because they travel great distances through magnetized plasmas."
Lightning strikes somewhere on Earth nearly all the time (about 100 times per second), so strange-sounding VLF signals are constantly traveling around our planet. "The best time to listen is usually around sunset or dawn," says Gallagher. "That's when electron density gradients that act as natural waveguides form in the local ionosphere."
Dawn breaks over Huntsville, where the online receiver is located, around 7 a.m. EST. "Nighttime is generally better than the day when you're listening to a VLF receiver," says Gallagher, "so anytime between about 5 p.m. and 7 a.m. is a good time to listen to the online audio."
Gallagher built the online receiver from an INSPIRE VLF radio kit. INSPIRE, which stands for "Interactive NASA Space Physics Ionosphere Radio Experiments," is an educational program based at Goddard led by Bill Pine, a high school science teacher in Ontario, Calif., and Bill Taylor.
Participants build their own VLF radios and they can join a global network of monitoring stations that includes more than 1,500 schools. "Almost anyone who can learn to solder can build one of these receivers," says Gallagher.
Taylor, Pine and others frequently organize experiments for members of the network. Experiments have ranged from monitoring terrestrial VLF radio waves during a solar eclipse to using an INSPIRE receiver aboard a weather balloon to listen for plasma wave emissions from Leonid meteors.
To hear sample VLF radio sounds, or to listen to the online receiver itself, visit SpaceWeather.com's online INSPIRE page at:
The National Space Science and Technology Center is a collaboration that enables scientists, engineers and educators to share research and facilities. Marshall's space science and technology expertise is providing the core for the center, opened in 2000, focusing on space science, materials science, biotechnology, Earth sciences, propulsion, information technology, optics and other areas that support NASA's mission.
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