- Our planet is a natural source of radio waves at audio
frequencies. An online receiver at the Marshall Space Flight Center is
playing these songs of Earth so anyone can listen.
- 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 a flamboyant science fiction film,
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
- "Everyone's terrestrial environment almost literally
sings with radio waves at audio frequencies," says Dennis Gallagher,
a space physicist at the Marshall Space Flight Center (MSFC). "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 kHz . For comparison,
AM broadcast band radios --like the ones in most automobiles-- span the
much higher frequency range 540 kHz to 1.6 MHz.
- If you have an internet connection you can now listen
to a VLF radio anytime you wish. Gallagher and colleagues recently installed
an <http://image.gsfc.nasa.gov/poetry/inspire/index.html INSPIRE VLF
receiver at the MSFC Atmospheric Research Facility in Huntsville, AL. It's
broadcasting the peculiar songs of Earth live on the web 24 hours a day.
- Listen to the VLF sounds of Earth <http://www.spaceweather.com/glossary/inspire.html
- 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 if there is no lighting in your area, you can still
hear VLF crackles from storms thousands of kilometers away. Some sferics
travel all the way around the Earth! Radio waves can propagate such great
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 90 km above the ground and extends to
thousands of kilometers 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 the Goddard Space Flight Center. 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. We call
this delay dispersion, and it's a result of propagation through a waveguide,"
says Taylor. Every waveguide has a low-frequency cutoff set by its physical
size. The closer a wave is to the cutoff, the slower it travels. The cutoff
frequency of Earth's planet-sized natural waveguide is around 3 kHz --
that's the frequency where half a wavelength will fit between our planet's
surface and the bottom of the ionosphere. Waves with frequencies above
the cutoff can travel through the waveguide, but lower frequency waves
- Sometimes the ionosphere leaks lightning pulses into
space. They exit the atmosphere entirely, following magnetic field lines
that guide them 10,000 km 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, not because of the
waveguide cutoff effect, but rather because they travel great distances
through magnetized plasmas (a plasma is an ionized gas), which are strongly
dispersive media for VLF signals."
- Lightning is striking somewhere on Earth nearly all the
time (about 100 times per second), so strange-sounding VLF signals are
constantly propagating 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
- Dawn breaks over Huntsville, AL, where the online receiver
is located, around 6 o'clock Central Standard time, which is 1200 Universal
Time. Sunset is ten hours later at this time of year. "Nighttime is
generally better than the day when you're listening to a VLF receiver,"
continued Gallagher, "so anytime between about 2200 UT and 1200 UT
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
NASA's Goddard Space Flight Center led by Bill Pine, a high school science
teacher in Ontario, CA, and Bill Taylor.
- Participants build their own VLF radios and they can
join a global network of monitoring stations that includes more than 1500
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. For example, in 1994, listeners across North
America monitored terrestrial VLF radio waves during a solar eclipse. The
observations revealed how a temporary decline in solar ultraviolet radiation
affected Earth's ionosphere. In 1999 and 2000, an INSPIRE receiver floated
to the stratosphere on a weather balloon to listen for plasma wave emissions
from Leonid meteors. Students monitored the meteor shower from ground stations
at the same time.
- To hear sample VLF radio sounds, or to listen to the
online receiver itself, point your web browser at SpaceWeather.com's <http://www.spaceweather.com/glossary/inspire.html
online INSPIRE page. If listening to our online receiver whets your appetite
for one of your own, visit <http://image.gsfc.nasa.gov/poetry/inspire/index.html
Goddard's INSPIRE web site for information about ordering a receiver and
joining their program.
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