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A BEGINNER'S GUIDE TO NATURAL VLF RADIO PHENOMENA.Second part.

By Michael Mideke Ragged Point, CA

http://loscrittoio.old.mytrident.net/Pages/MM-0700.html

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 rare occasions. 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 categories:

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 hauntingly beautiful.

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:

http://image.gsfc.nasa.gov/poetry/inspire

http://www.lwca.org/

http://www.triax.com/vlfradio/natradio.htm

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