|
 Listen to this story (requires
RealPlayer)
 January 19, 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 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 time.
"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."
Above:
Lightning strokes like this one are the source of the eerie-sounding radio
emissions that surround us.
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 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
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.
 Left: Electrical engineers use
waveguides like these to confine and direct radio waves. Our planet and
the ionosphere form a giant natural waveguide for VLF radio
signals.
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 cannot.
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."
Above:
This dynamic spectrum shows how the highest frequencies of a VLF whistler
arrive before the lower ones do. Click here
for more information about dynamic spectra and to find out what a whistler
really sounds like.
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 ionosphere."
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, Chaffey
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 online INSPIRE
page. If listening to our online receiver whets your appetite for one
of your own, visit Goddard's
INSPIRE web site for information about ordering a receiver and joining
their program.
SEND THIS STORY TO A
FRIEND | 
