Introduction.
Since the '80s, in the world of the radiowaves research, Mike Mideke made something very important for the worldwide community approaching the very low end of radio frequency: the natural sounds that our atmosphere keeps and creates when particles from outer space arriving.
At that time Mike was living in a remote ranch in central California, San Simeon, where power lines did not arrive close to home and he had to arrange his family life without electricity. A miracle for Natural Radio Sounds. He was able to record 24 hour a day with no problem, collecting one of the most important Very Long Radio Frequency library. Mike soon became a Master of this field. Scientists asked him about those strange sounds. He also recorded a cassette to let people better understand what was going on.
Later Mike Mideke was one of the four people who founded the Inspire Project, with Jim Ericson, William Pine and William Taylor.
This NASA Educational Project was the first to propose to research this field to radioamateurs, physics teachers, students ans scientists, all together, following a William Taylor's idea.
The Guide that now LoScrittoio.it is pubblishing, was written by Mike in the late '80, though appear to be perfect for nowdays too.It will be published in four parts. You'll find very good science in his words, though written in a very clear way.
Some of the final Journals proposed as a reference you'll find out of date.
LoScrittoio.it thanks Mike Mideke who kindly allows to publish his work on these pages.
Flavio Gori
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!
End of the first part. The second one will be published next July.
Please refer to these sites for further informations and on the field experiences: