Under the right conditions, the VLF signal travels out
away from earth and returns by traveling along a magnetic field line. During
this long path, dispersion is much greater than with tweeks. While tweeks
might disperse a few hundred kilohertz over a few thousandths of a second,
whistler show a dispersion of a second or more over several thousand kilohertz.
The sound of a whistler is a musical descending tone that lasts for a second
or more. On the spectrogram, whistlers appear as long sweeping arcs showing
the sequential arrival of the frequencies. It is important to remember that
all of the frequencies start out at the same time (a sferic), but the path
taken by a whistler is so long that the dispersion of the frequencies is
quite pronounced.
Pure Note Whistler
A pure note whistler has traveled along a signal magnetic
field line. It is heard as a clear whistling sound and appears on the spectrogram
as a strong single curve. The following shows two whistlers: one at :09
seconds and a stronger one at :13 seconds. The horizontal dashes are OMEGA
signals. Several other faint whislters are audible in the sound sample.
Diffuse Whistlers
Diffuse whistlers have traveled along a set of magnetic
field lines that are not all of the same length. The sound is "breathy"
or "swooshy". The spectrogram shows the whistler as a broad region
rather than a narrow curved line.
- More Whislters
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-*- More whistlers (65k)
-*- Another whistlerEcho Chain (101k)
2-hop Whistlers
Two-hop whistlers originate near the receiver site. The
signal that travels along the magnetic field line bounces off the ionosphere
in the other magnetic hemisphere and returns to be heard as a whistler near
where the original lightning stroke occurred. Two-hop whistlers can be identified
by the presence of a strong "local" sferic between one and two
seconds before the whistler is heard. Remember, local lightning is within
about 2000 kilometers of the observing site.
Whistler Echo Train
Echo trains result when the radio wave bounces back and
forth between magnetic conjugate points. Each time the signal bounces off
the ionosphere, some of the energy leaks down in the lower atmosphere and
is heard as a whistler. All of the whistlers in the train are the result
of a single lightning stroke. Successive "hops" of the whistler
are seen with increasing dispersion time as the distance travelled grows
with each bounce.
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