Propagation Page
Lightning causes the simultaneous emission of a broad band of radio frequencies. The signals can be
heard from a few hertz to hundreds of megahertz. Our AM radio hears these bursts as the snap and
crackle of static. If you were to line up several receivers tuned to different frequencies, it is likely that
you would pick up the same static bursts at the same time.
Most of lightning's effective radio energy is concentrated in the 1 to 30 kilohertz region. This region
of the radio spectrum is considered the VLF, or Very Low Frequency, radio range.
Spectrograms are a visual analysis of sound. They are specifically useful for viewing VLF
recordings. When viewing spectrograms, you will notice dark lines, which represent sounds. The
height (i.e y-axis) of the graph represents the frequency. The time scale is represented by the x-axis.
Sferics
When listening to VLF radio, you will often hear static, like pops and crackles. These cracks and
pops are produced by lightning. Lightning emits a broad spectrum of radio waves; all frequencies
appear at once. These sounds are known as sferics. The character of a sferic varies with the
distance from receiver, the path at which it propagates, and the structure of lightning which produced
it.
Audio of above sferic sample
Audio of more VLF samples
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Tweeks
Sometimes you will hear sferics that end with an abrupt musical ring. These signals are called
Tweeks. The frequency will typically spread out over a few hundred hertz in a few milliseconds.
Tweeks are sferics subject to dispersion. That is the spreading out of frequencies over a certain time
period. The time period for tweeks is from 25 to 150 milliseconds.
Notice the short drop in frequency near the end of each tweek, like a tail. The tail is around 2
kilohertz. Harmonics can be seen around 4 and 6 kilohertz as well.
Audio of above tweek sample
Audio of more VLF samples
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Whistlers
"They are musical sounds that may be heard in a radio receiver tuned to very low
frequencies. Originating in the atmosphere, they provide a new method for exploring its
outlying regions."
-L.R. Storey
Whistlers have been the center of attention of VLF explorers since World War I. Whistlers are
descending tones generated through the propagation of sferics over very long paths formed by the
magnetosphere. Whistlers can extend above 30 kilohertz and below 1 kilohertz, but they will most
likely lie in the 1 to 9 kilohertz region, and are concentrated in the 3 to 5 kilohertz region. Please
refer to our section on magnetosphere and ionosphere.
Audio of above whistler sample
Audio of more VLF samples
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OMEGA is a manmade radio signal. It is used in aircraft as a navigational tool. There are eight
stations around the world which emit the signal. It sounds like a progression of high pitched tones
about 1 second long. When observing the spectrogram, the tones appear as dashes. Our receivers
regularly pick up signals from the Hawaii, Japan, and South Dakota stations. The signals are
produced in the 10.2 kHz and above frequency region. You will see OMEGA as small bands at the
top of a spectrogram. If these signals are heard, it ensures that the receiver is working properly.
OMEGA stations transmit a sequence of four consecutive tones, i.e. not at the same time. A set of
double dashes (or more) indicate there is more than one OMEGA station present.
Audio of above OMEGA sample
Audio of more VLF samples
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