A BEGINNER'S GUIDE TO NATURAL VLF RADIO
By Michael Mideke
Ragged Point, CA
BASIC GUIDE TO WHISTLERS. EMISSIONS AND ASSOCIATED PHENOMENA
STATIC - Static is the impulsive crackling
and popping of lightning
generated broad spectrum radio bursts. Static can be heard throughout the
radio spectrum. Its character varies according to the structure of the
lightning producing it, distance from the receiver and the paths over which
it propagates. Static impulses are also referred to as sferics.
TWEEKS - Tweeks are sferics subjected to
dispersive distortion by
subionospheric propagation, They are sharp falling notes with a duration of
25 to 150 milliseconds.
WHISTLERS - Whistlers are descending tones
generated through the
propagation of sferics over very long paths formed by field aligned plasmas
(ducts) m the magnetosphere. Whistler's magnetospheric propagation is
between magnetic conjugate regions in northern and southern hemispheres.
Terrestrial reception of whistlers results from subionospheric propagation
of these signals.
Whistler duration ranges from a fraction of a second to several
seconds. The frequency range of whistlers can extend from above 30 kHz to
below 1 kHz but those readily heard with simple equipment will mostly lie
between 1 and 9 kHz, with their maximum energy usually concentrated between
3 and 5 kHz. Whistlers are categorized according to hops. One hop equals a
single traverse between conjugate regions. A one hop whistler is generated
by lightning in the opposite hemisphere from the listener. It has
traversed the magnetosphere just once and as a consequence, it tends to be
a high pitched whistler of short duration. Since the causative sferic is
very far away, it is rarely heard in association with single hop whistlers.
Two hop whistlers are produced by lightning in the same magnetic
hemisphere as the listener. The signal has traveled to the opposite
hemisphere and echoed back to the region of its origin. Subject to roughly
twice the dispersion of a single-hop whistler, its duration is much longer
than its one-hop cousin. Causative sferics can often be heard in very
distinct association with 2-hop whistlers. Delays of 1.5 to 3 seconds
between sferic and whistler are typical.
Odd order hops (1, 3, 5, etc.) indicate opposite hemisphere
lightning while even order progressions (2, 4, 6, etc.) follow from same
hemisphere lightning. On occasion, whistlers generate multiple echoes or
progressions known as echo trains. While trains exceeding about a dozen
echoes are uncommon, progressions of more than 100 have been observed on
Whistler notes range from extreme] pure tones to breathy, diffuse
swishes. The breathy quality is described as diffuseness. It results from
whistler mode excitation of multiple ducts, with slightly different travel
time for each duct serving to spread or diffuse the signal.
Whistlers were the first studied and most easily understood class
of magnetospheric radio events but they are far from being the only ones
that can be observed by a patient listener using basic tools.
VLF EMISSIONS - VLF emissions are naturally
occurring phenomena found in
the same frequency range as whistlers. In his book WHISTLERS AND RELATED
IONOSPHERIC PHENOMENA, Robert Helliwell divides VLF emissions into 7 basic
HISS - Hiss, as the term suggests, is a
hissing sound. Unlike white noise,
it is more or less band-limited. Its center frequency and bandwidth can
vary widely with different conditions. Hiss may be stable in amplitude and
frequency for minutes or hours. Or it may show distinct short-term
fluctuations which may or may not be periodic in nature. Hiss is often
found in conjunction with other emissions.
DISCRETE EMISSIONS - Discrete emissions
are brief, transient events. They
may be pure or fuzzy tones which rise ('risers") or fall ("falters') in
frequency. Sometimes falters abruptly turn about and rise in frequency as
'hooks". Other descriptive terms that come to mind are 'chirps', "croaks",
"honks" and "barks".
PERIODIC EMISSIONS - When clusters of discrete
emissions form regularly
spaced repeating patterns they are known an periodic emissions. They may
be singular or multiple, relatively frequency stable or drifting.
CHORUS - Multiple closely spaced or overlapping
events are known an chorus.
Chorus may resemble the sound of birds at sunrise but often it is
reminiscent of croaking frogs or seals barking. Chorus is frequently found
rising out of the upper edge of a band of hiss.
QUASI-PERIODIC EMISSlONS - These are events
consisting of discrete
emissions, periodic emissions or chorus which appear at long but fairly
regular intervals - on the order of tens of seconds. They are less regular
than periodic emissions.
TRIGGERED EMISSIONS - Sometimes one magnetospheric
event triggers another.
Triggered emissions are those which appear to be clearly associated with a
triggering source. Whistlers, discrete emissions, manmade VLF signals and
atmospheric nuclear explosions may all serve as triggers. Whistlers and
other signals may also be seen to modify the spectrographic signatures of
other events in the same duct.
THE ORIGINS OF VLF EMISSIONS
VLF emissions appear to arise from interactions
above. the equatorial
region that involve incoming solar wind particles, the planetary magnetic
field and plasma resident on the field lines. At whistler and emission
frequencies the magnetosphere has the potential to perform as an amplifier.
(Gains of 20 to 50 dB have been observed.) This amplifier is subject to
instabilities which are regulated by (among other things) the time
constants of whistlers and other signals echoing back and forth along
magnetospheric ducts. To take a simplistic view, the whole system can be
considered as a gigantic electronic synthesizer programmed by solar and
terrestrial processes. The resulting music can be complex, sustained and
ARTIFICIALLY STIMULATED EMISSIONS (ASES)
In the 1950s and 60s, powerful military
VLF Morse Code
transmissions were observed to stimulate emissions resembling elements of
the rather mysterious "chorus" phenomenon. This led to the idea that it
might be possible to perform active experiments in order to better
understand the actual mechanisms involved in the production of whistlers
and emissions, thereby refining our knowledge of the magnetosphere.
Research employing a powerful VLF transmitter at Siple Station,
Antarctica, was carried out in the 1970a and 80s. Transmissions from Siple
generated a variety of magnetospheric phenomena that were heard by a
monitoring station in the magnetic conjugate region near Roberval, Quebec,
and by a variety of satellite monitors operating within the magnetosphere.
These experiments in the controlled excitation of events within the
magnetosphere succeeded in greatly advancing scientists' understanding of
the interactions taking place in the near space environment. They have also
suggested many avenues for future research.
With the magnetosphere well established as the sensitive region
within which the mechanisms governing whistlers and emissions operate,
there has been considerable interest in discovering the effects of
stimulation applied directly to that region. During 1989-90, the Soviet
satellite ACTIVE attempted to accomplish this by passing large 10.5 kHz
currents through a 20 meter loop antenna. Unfortunately, the loop
apparently accidentally deployed in a very inefficient configuration.
Several months of monitoring by NASA, Soviet observers and private
experimenters in the US found no effects, either on the ground or in space.
These joint experiments were nonetheless successful in that they provided
the occasion for participation by a number of amateurs and high school
groups. Had ACTIVE performed as hoped, their data would have made a
valuable contribution to the research.
School and amateur participation will continue in the spring of
1992, when the Space Shuttle based ATLAS 1 (ATmospheric Laboratory for
Applications and Science) will deploy a VLF modulated electron beam
instrument known as SEPAC (Space Experiments with Particle ACcelerators).
The SEPAC electron beam will be modulated at frequencies between 50 Hz and
7 kHz as researchers attempt to analyze its propagation and interactions
with magnetospheric plasmas. The high school and amateur ground stallion
program, INSPIRE (Interactive NASA Space Physics Ionosphere Radio
Experiments) will provide a large network of ground stations to determine
the "footprint" and other characteristics of the SEPAC signal.
Not only has the past four decades of active and passive whistler
research enormously enhanced our understanding of the vast interactions
taking place in the magnetosphere, it has given us glimpses of similar
processes on other worlds. Whistler-like signals have been heard in the
vicinity of Jupiter, Saturn and Neptune. Even Venus and Mars make odd
noises on VLF. It appears that planets having magnetic fields and
turbulent atmospheres are likely to produce whistlers and related
phenomena. The coming decades should signal the emergence of a new branch
of science devoted to the analysis of planetary electromagnetic signatures
at VLF and ELF. But as we speculate along these lines we should remember
that we've only begun to understand the complexities of our own world's
natural electromagnetic environment.
For the amateur observer, pursuit of whistlers represents a unique
combination of challenges and rewards. Highly sophisticated receiving and
recording equipment is not required - the basic tool kit needn't cost a
fortune, and the parts of that kit which need to be constructed (or
commissioned) by the user are not particularly complicated. Even a rank
beginner at electronic construction can build a workable whistler receiver.
The greater challenge probably lies in seeking out radio quiet locations
far from power lines and waiting patiently (or returning again and again)
until something happens.
The rewards are inextricably bound up with the challenges. There
is a genuine thrill to hearing these things for the first time (or the
first hundred times!), particularly if one accomplishes the feat with tools
built "from scratch". With increasing experience it becomes apparent that
the variety of natural VLF radio phenomena is enormous. Many listening
expeditions will be duds, but of those that produce results, no two are
likely to be the same. If one makes recordings, the task quickly evolves
from a quest to collect samples of the various phenomena to an ongoing
process of gathering better samples. With over 60 hours of whistler tape
on the shelf, I foresee no end to that particular process!
The scientifically inclined amateur will find numerous areas that
invite research, including topics and phenomena that have been touched upon
only lightly, if at all, by professional researchers. For instance, the
possibilities of a large network of coordinated monitors have never been
explored simply because there has never been a large number of monitors.
There is a great deal to be done in the area of electronic design
and signal processing. Prototype 'comb filters' have been developed to
remove power frequency harmonics from receivers, but there is clearly more
to be done in this area if the whistler hunter is to be freed from the
necessity of going far from the power lines to do his thing. An
alternative approach to the power line interference problem lies in the
development of remote receiving and recording systems that can be automated
or remotely controlled. Impressive hardware and software for spectrum
analysis exist in the professional world, and the well-to-do amateur can
certainly acquire a rudimentary signal analysis capability "off the-shelf'.
However, the field is wide open for development and innovation.
End of second part. Third one will be published in the August issue.
Please refer to these sites for further information and on the field experiences: