Earth
Songs
NASA
Science News
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.
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, 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 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.
Related Links:
INSPIRE
Home Page -at the NASA Goddard Space Flight
Center
The
online INSPIRE VLF receiver -at NASA
Marshall's Atmospheric Research Facility
For lesson plans and educational
activities related to breaking science news, please
visit Thursday's
Classroom
Author: Dr.
Tony Phillips
Production Editor: Dr.
Tony Phillips
Curator: Bryan
Walls
Media Relations: Steve
Roy
Responsible NASA official: Ron
Koczor
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