Naturally-occurring VLF Radio sounds descriptions:
Lightning-stroke "static": If you've already listened to ANY of these recordings, you will have certainly noticed (or may even be fed-up with) the nearly constant crackling and popping noises on each and every one of these CD's tracks. An unavoidable part of Natural VLF Radio, lightning static is ALWAYS audible, though, depending on the location and time of year, the amount of lightning static can widely vary.
Generally, recordings made in local summer are plagued with lightning- storm static and those made in mid-winter tend to be wonderfully quiet. While a nuisance to some listeners, VLF lightning static is trying to tell us something. Imagine a bolt of lightning--say-a bolt of lightning that strikes the ground from a cloud above. The length of this awesome spark can be many miles long and as wide as an automobile.
Between 10,000 to over 100,000 volts are generated in this instantaneous jolt. Furthermore, a single lightning bolt rarely fires just once, but as much as 100 times a second, giving it that odd "flickering" effect. As such; each and every one of those innocent "pops" evident in these recordings is one of those huge sparks just described. But as you may have already observed, there are seemingly HUNDREDS of them per second occurring in many of the recordings, some of them really loud, but most quite moderate to faint. They seem to permeate the background--sort of like an old, worn vinyl record.
Obviously, there is A LOT of lightning going on "out there!" And there are-a couple million lightning strokes (flashes) occur each day, worldwide, from approximately 1500-2000 lightning storms in progress at any given time. A VLF receiver is quite good at picking up lightning from as far as 3000 miles distant (perhaps more), and gives you a nice idea of the SHEER amount of lightning strokes firing off in any given second! You may experience days or weeks of sunny, delightful weather where you live, but the VLF receiver NEVER lets you forget that lightning is lurking all 'round you!
Tweeks: You might have already noticed a lot of the lightning static ("sferics") seems to have odd pinging and ringing characteristics. This "tweeking" effect, sometimes quite beautiful sounding (such as in the Fish Rock Road Whistlers track), is generally a nighttime effect, with a few tweeks audible in the late afternoon/early evening and reaching their best and most numerous around midnight, and finally tapering off once sunrise occurs.
At about 50-55 miles in altitude (80-88 km), the E-layer of Earth's ionosphere (a layer of charged particles, called "ions") acts similar to a mirror to VLF radio waves. The same goes for Earth's surface (more- or-less) and the two "sides" form a sort of pipeline which channel VLF radio signals, especially lightning stroke static impulses. Static impulses from very distant lightning storms (thousands of miles) can travel better at night in this huge radio wave pipeline of Earth, but, below a certain frequency, there is an abrupt cut-off, whereby the pipeline effect ceases. This is at about 1700 Hz audio frequency, which is also the frequency which most of the ringing and pinging sounds of tweeks are taking place.
Tweeks slowed down about 10 times almost begin to mimic low-pitched whistlers! Like Whistlers, one can get lost in the explanation of what causes a Tweek, and so it's sometimes more fun just to enjoy their odd sounds. Also like Whistlers, Tweeks can sound very different from night-to-night, sometimes very pure and ringy, other nights they have a "crusty" sound. During those (frequent) times no other Natural Radio sound can be heard besides incessant static, listening to Tweeks themselves can be mesmerizing!!
Whistlers: Most people get introduced to Natural Radio by hearing a recording of a whistler. Indeed, whistlers are the most common Natural VLF Radio sound besides lightning static, especially for those listening in middle latitudes. The term "Whistler" broadly defines downward- falling sounds which range from nearly pure whistling tones to windy/breathy sounds more similar to a "sigh" than a whistle. Between these extremes are a vast variety of whistler types.
In the case of the whistlers recorded in the eastern Nevada high desert, I called those whistlers "growlers," since they sounded more like growls than whistles. Of course, there are many samples of whistlers in these CD's. Whistlers are the direct result of a lightning stroke firing off, and usually occur 1-2 seconds after an initiating lightning flash. Very few of any lightning strokes ever produce whistlers, but enough do to make things very interesting on the good days, and sometimes whistlers are so numerous as to be called "Whistler Showers" or even "Whistler Storms."
Earth's magnetic-field, which keeps compasses nicely pointing in one direction only (hopefully!), plays a major role in the formation of whistlers. Not fully understood to this day, the traditional theory assumes that SOME of the radio energy from SOME of the lightning strokes in just the RIGHT location get "ducted" into channels formed along the lines of Earth's magnetic field, traveling out into near space and to the opposite hemisphere, where they are heard as a short, fast whistler (explained in more detail in the accompanying booklet with this CD set).
If conditions are favourable, some of the energy from these short, fast whistlers rebounds back the way it came to "arrive near" (within several thousand miles of) the point of its initiating lightning stroke, and becoming magically louder and longer. Essentially, during its globe- hopping round trip, the "all-frequencies-at-once" radio signal of a lightning "pop" gets the privilege of being pulled and stretched apart, with its higher audio frequencies arriving sooner than its lower frequencies, hence the downward-falling tone.
Some, if not MOST days are DEAD--entirely devoid of the sounds of whistlers, but there can be those days where whistlers rain down too many to count, like a huge switch was thrown by somebody "up there." Listen to the recordings, and you get the idea...
Chorus: Another general term used to define a number of Natural VLF Radio sounds, chorus defines several types of sounds when they occur in a rapid, intermixed form. The individual squawks, whoops, barks, and chirps of "triggered emissions" tend to get lumped into the general term of "chorus" when they occur in large amounts together. Depending on the time of day, location of event (or at least where it was heard), Chorus becomes "Auroral Chorus" (it was occurring near auroral sources or during visual displays of aurora), or around the pre to post-sunrise period, when it is called Dawn Chorus." Both sound generally similar, though chorus can manifest itself in endless variety.
Chorus is a product of magnetic storms, when events on the Sun, such as a solar flare, or "holes" on the Sun's outer atmosphere (the Corona) allow a barrage of high-speed charged particles to impact Earth's outer magnetic field (magnetosphere), causing it to deform and pulsate, much like air currents deform the thin film of a soap bubble. Phenomena such as Aurora (Northern and Southern Lights) also increase dramatically during magnetic storm periods, as do such natural VLF Radio sounds such as chorus.
Notice the similarities of the various Chorus events presented on these two CD's, yet also notice the variations. Short- lived repeating bursts of the individual sound components of chorus are sometimes referred to as "Chorus Trains." Auroral Chorus tends to be heard more often and at generally higher latitudes than whistlers, except for the widespread Dawn Chorus, which, when heard at lower-middle latitudes, is strictly a magnetic-storm time phenomena.
Hiss: Also called "Hissband," is a VLF radio emission partially arising directly from Aurora, with some of it emitted right from the same location as where the visible light (usually greenish in cast) is produced and usually very high-pitched.
Other hissband is generated farther out in Earth's magnetosphere including the bow-shock region of the magnetosphere facing the incoming Solar Wind. Hiss can vary in its frequency band, sometimes it has a high-pitched sound like a slightly open water valve or toilet-tank filling up, and on other occasions can sound much like the low-pitched roar of a waterfall. While generally stable in characteristic, it can sometimes abruptly change in volume and/or pitch, indicating some sudden change has occurred in the geo-magnetic field.
Periodic Emissions: Other sounds different than whistlers or chorus get lumped into this category, but is the term implies, they tend to occur only occasionally (periodically) and in repetitious fashion with a predictable repetition time (period). A fine example of this sound is in the "Kenai Crazy Whistlers" (sound-file link to U. Iowa server) Track on the "Electric Enigma" CD1, Track 7 (recorded on the Kenai Peninsula in southern Alaska ) which actually are NOT true whistlers at all but are rarer Natural Radio emissions arising from magnetic storm/auroral phenomena and heard this strongly only at higher latitudes such as Alaska, central or northern Canada, Iceland, northern Scandinavia, or Antarctica. Out of humour I entitled this phenomena with the above name!
Notice that the Periodic Emissions in this "Kenai" track seem to trigger subsequent ones (rather like a good tennis volley) until it winds down.
Tonal Bands: Strange-sounding hissy noises, or a multitude of whistling sounds which abruptly begin and end, usually for only 5-10 seconds in duration. Many of these can be heard in the "Alberta Auroral Chorus" track (CD1, Track 9 and CD2, Track 1), particularly the longer recording.
These are the sounds of the worldwide Omega Radionavigation System. Omega--now inactive as of 30 September 1997 thanks to the advanced GPS (Global Positioning System) satellite navigation system. Omega consisted of eight 10,000 watt transmitters in the following locations: Australia (Victoria), Japan; Hawaii (Oahu); North Dakota (USA); Liberia (Africa); La Reunion Island; Argentina; and Norway. Each transmitter transmited eight "pulses" of 0.8 to 1.2 seconds duration, repeating the process every 10 seconds. During each cycle, each transmitter occupied a unique frequency, and over the course of each 10 second cycle, all of the eight transmitters "hit" on several common frequencies spanning 10.2 to 13.8 kHz.
Also, each transmitter transmitted on its own "unique" frequency on 2 of its 8 transmitted pulses. Omega receivers used to sample the relative phase and timing of each Omega signal. Best results were obtained when at least 4 Omega transmitters were received and analyzed, and the nearest resolution, called a "lane," is about 5-6 miles (8-10 km), though in critical regions, supplemental transmitters of very low power may be used in order to increase navigation accuracy. Omega was subject to the same VLF propagation disturbances which affect Natural Radio, particularly during magnetic storms. When Dawn and Auroral Chorus roar, Omega is probably experiencing accuracy problems!
There were also daily (diurnal) variations in the accuracy and phase of the Omega signals as day becomes night, which for the most part were taken into account within an Omega receiver's internal microprocessor. GPS does not suffer any of these propagation errors as does Omega.
As such, Omega is no longer on the air, though Russia still operates a smaller-scale version of Omega called "Alpha." Alpha has only three transmitters sending three short pulses of approx. 100 milli-second duration followed by a pauseof a couple seconds' duration. Alpha is much weaker in strength than Omega, particularly for listeners previously within a couple thousand kilometres of an Omega transmitter.
Power line "hum" from alternating current electric wires: Switch on a VLF receiver within your home or office, and you will not hear anything BUT this sound! Today, all electricity generated at power plants is "alternating-current (AC)," as opposed to "direct-current (DC)" produced from batteries in your watch and portable radios. With AC, the polarity changes a many times a second. In Europe, Asia, most of Africa, and Australia/New Zealand, the electric mains power changes polarity 50 cycles-per-second, or 50 Hz. In North America and in most Central and South American countries, it is at 60 Hz.
While convenient for long- distance transmission and easy voltage transformation, AC generates hum in poorly filtered audio equipment and especially in whistler receivers! If this wasn't bad enough, most electrical "grids" seem to cause the 50 of 60 Hz current to generate harmonics--multiples of 50 of 60 Hz, causing hum/buzz THROUGHOUT the VLF radio spectrum. Those immense, high-voltage, high-tension electric wires sagging between the tall metal pylons and marching off toward the horizon can generate impossible amounts of hum and buzz if you try to listen with a VLF receiver too close to them-and I'm talking about miles or kilometres near to them!
To have "hum-less" recordings of VLF phenomena (such as the Eve's River or Alaska Auroral Chorus or Kenai Crazy Whistler segments), you have to find listening sites far removed from above-ground power lines. It's fairly easy to find absolutely quiet hum-free listening spots in desert and mountainous or tundra regions of North America, Australia, or the remoter parts of Europe and the British Isles (Scottish Highlands particularly) if you're willing to make a few days of it, but finding quiet spots to listen close to home and/or in populated regions such as the English Midlands or U.S. east and midwest (including farmed areas away from towns) usually mean pesky power lines will be around somewhere, usually alongside the road you're traveling along.
Willingness to walk/hike into listening sites greatly increases your chances that a quiet spot will be found. In most cases, you have to live with SOME background hum, as must I. Thus, some of the recordings on these CD's or your potential favorite VLF listening sites have some weak background hum.
Surprisingly, reasonably quiet natural VLF radio listening spots can be found in places such as large ball fields, large urban/suburban parks away from light poles, farm fields where wires are hidden behind trees, along many beaches (especially if electric wires are below-ground), and so forth. It has been found that within southwest London's Richmond Park and Battersea Park, quite a few remarkably low-hum listening spots exist. This is also true for San Francisco's Golden Gate Park's soccer/football playing fields. Certainly, other big-city parks must also have low-hum VLF listening spots you could listen with your WR-3 or other portable VLF receiver.
Stephen P. McGreevy, March 1996, revised October 1997E-mail Stephen P. McGreevy: firstname.lastname@example.org
READ and enjoy VLF STORY, a presentation of my experiences listening to natural radio in-the-field between 1989 to summer 2001.