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A BEGINNER’S GUIDE TO NATURAL VLF RADIO PHENOMENA

By Michael Mideke
Ragged Point, CA

The emergence of new technologies has a way of revealing unexpected
aspects of the natural universe. The telephone, which effectively shrank
our world by superimposing its electronic dimension upon our existing
physical space, has played a coincidental role in showing us the same world
larger, more mysterious, more wonder-filled than we’d imagined.
The story goes back to the earliest long telephone lines, clear
back to the 1880s and operators who reported strange chirps and whistles
that had no obvious connection with the telephone system and its traffic.
It was a mystery, but one which attracted little serious attention until
much later, when it emerged to complicate life in the entrenched
battle-fields of World War I Europe.
Though primitive by today’s standards, there was a mature telephone
technology when the war began, and that technology soon found its way into
the trenches. Electronic Counter Measures arrived immediately thereafter
in the form of high-gain vacuum tube amplifiers which each side employed to
intercept “leakage” from the other’s communications. The general idea was
to run wires from widely separated ground stakes to the input of a
sensitive amplifier sampling stray or induced currents from the enemy’s
telephone system. For the most part, this worked well
enough to be worth the trouble but now and then eerie descending whistling
tones appeared in the monitors’ headphones. Some likened them to the sound
of phantom shells passing over-head. At times the cacophony of these
“whistlers” became so thick that eavesdropping efforts were completely
foiled.
The German scientist H. Barkhausen became quite intrigued with
these peculiar sounds. His initial assumption was that they were an
artifact of the amplifiers, but his attempts to reproduce the whistler
phenomenon in the laboratory were fruitless. About this time he did make
basic discoveries relating to electronic oscillators. and it is
interesting to speculate that his pursuit of whistlers may have played
this. Barkhausen’s next conclusion about whistlers was that they were a
natural phenomenon which could not be explained on the basis of current
knowledge. It was a fascinating puzzle, and over the years he and other
researchers pecked away at it, arriving by late 1920s at fairly general
agreement that lightning was responsible. A correct interpretation, but
the ‘how” of it remained elusive until the 1950s.
What does the sudden flash and roar of lightning have to do with
these musical electromagnetic signals that glide from as high as 30 kHz
down to less than 1000 Hz? How can the one event lead to the other? The
key lies in the nature of electrical sparks.
Lightning is a spark discharge – a huge spark, embodying peak
currents of thousands of amperes, potentials on the order of 250 million
volts. Any electrical spark is a source of electromagnetic energy. Not
only light but ultraviolet, infrared and radio. The latter is the basis of
Heinrich Hertz’s experimental verification of Maxwell’s equations, and the
source of whistlers.
Rather than being a coherent signal confined to a particular
frequency or band of frequencies, lightning’s radio emission is a broad
spectrum burst – all frequencies appear in it at once, from hundreds of Hz
through hundreds of MHz. On our conventional AM and short-wave radios we
hear these bursts as the snap and crackle of static. Were you to line up
several radios tuned to different frequencies, chances are good they would
all register the same static bursts at the same time. (The experiment is
not guaranteed to produce this result because radio waves propagate quite
differently at different frequencies – radios- tuned to widely separate
parts of the spectrum might be responding to static from completely
different areas of the world.)

A large percentage of lightning’s effective radio energy is
concentrated in the 1 to 30 kHz region loosely defined as the VLF or Very
Low Frequency region. At these frequencies the static
bursts propagate with particular efficiency in the “waveguide” formed by
the earth’s surface and the ionosphere. Tuning through most
of this frequency range, you will hear static that sounds pretty much like
what you hear on your AM receiver. But If you tune below about 5 kHz
you’ll discover that sometimes (not always by any means), the crackle
becomes a liquid musical “pinging”, each pop of static producing a rapid
descending note. These sounds are called tweeks. Typically, they drop a
few hundred Hz in a fraction of a second, then cut off abruptly.
The mechanism of tweeks is quite well understood. When radio
signals pass through a non-vacuum medium, those of higher frequencies
usually travel faster than those of lower frequency. Since an impulse of
lightning static starts out as high and low frequencies produced
simultaneously, propagation in the earth-ionosphere waveguide necessarily
sorts its frequency components; the highs arrive first, the lows later. The
signal becomes dispersed over time; thus is referred to as dispersion. The
degree of dispersion or “tweeking” is an indication of how far signals have
traveled. Because of the nature of “waveguide” propagation, this is not
necessarily an indication of point-to-point geographic distance.

Tweeks are generally heard at night (though they will often tend to
appear late in the afternoon), and winter is probably their best season.
If you spend long enough listening to static and tweeka below 10 kHz you
are almost certain to hear a few whistlers come howling through. These too
are descending notes, but they occupy seconds rather than milliseconds, and
they can be extremely loud, commanding the listener’s attention in no
uncertain terms!
Beginning with Barkhausen, early researchers toyed with the idea
that whistlers, like tweeks, are dispersed lightning static. There was a
great deal to recommend this explanation, but also one bad problem – nobody
could find signal paths on earth that were anywhere near long enough to
account for the huge amount of dispersion seen in whistlers. Propagation
around and around the world was hypothesized, as were strange
radio-reflective clouds somewhere out in space. But these theories didn’t
fit the observed phenomena very well. Since they seemed to resolve far
fewer questions than they raised, nobody was very happy with them as
explanations for whistlers.
Whistler research lapsed into a sort of limbo from the mid 1930s
through WW-11. Whistlers remained: “Natural Phenomena, cause and mechanism
unknown.”
The war engendered unprecedented technological leaps. New
techniques for recording and spectrum analysis emerged from the conflict to
play central roles in the unraveling of whistler mysteries during the
1950s. L.R.O. Storey, R.M. Gallet, R.A. Helliwell, M.G. Moran and others
were successful in applying new tools and careful observation techniques to
whistler research. In the process, they developed a new view of earth’s
near-space environment and laid foundations for the field of magnetospheric
physics.
As it turned out, the long dispersive whistler paths were found
neither in terrestrial propagation nor in the depths of space – rather,
they were traced to an intermediate region known as the magnetosphere.
This is the region where earth’s magnetic field interacts with the
continuous (but varying) influx of charged particles known as the solar
wind.
The solar wind consists of charged particles (electrons and ions)
moving outward from the sun. Solar wind, magnetosphere and ionosphere are
plasmas, hot, partially ionized gases. These charged particles in motion
develop magnetic fields. Since magnetic fields are subject to interactive
forces of attraction and repulsion, as the solar wind particles encounter
earth’s magnetic field both particles and the planetary field are
perturbed. Energy is transferred, distorting the geomagnetic field into
its now familiar teardrop shape, and solar wind particles are captured in
spiraling courses aligned with the field. The plasma densities and the
dimensions of the magnetospheric plasma environment happen to be suitable
for the effective propagation of radio energy at whistler frequencies.
Broad spectrum VLF radio energy generated by lightning bursts under
some circumstances escapes the earth-ionosphere waveguide to encounter
field-aligned discontinuities (generally described as “ducts”) in the
magnetospheric plasma. The ducts extend between northern and southern
hemispheres, arching to their maximum distance (several earth radii) from
earth over the equatorial regions. Field-aligned
ducts within magnetospheric plasmas do in fact yield paths long enough to
account for the dispersion of whistlers.
Simultaneous monitoring in northern and southern hemispheres has
revealed that specific static impulses m one hemisphere do correlate with
whistlers heard in the conjugate region of the opposite hemisphere.
Moreover, whistlers may rebound back and forth along a duct (or even
multiple ducts) many times, generating progressions of echoes that become
ever longer in duration, lower pitched.
Scientists were quick to realize that the study of whistler
dispersion could yield valuable data about the characteristics of the
magnetosphere. Every whistler is a magnetospheric probe!

About paoleb

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