| definition |
-
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.
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.
[Summary provided by NASA]
|
