VLF - The power and the glory

Below the good old long-wave is a range of frequencies used as National Standard's for frequency accuracy and time. Nuclear technology has made these very accurate indeed so to convey the pure engineering of these stations, no mathematical shorthand has been used. Down here, we are talking real numbers. Sad to say, these stations are closing down.

The ELF ranges, where the frequencies are so low we could hear them if they were vibrations in air, contain submarine navigation signals. These require antennas so vast that an entire geological feature such as an atoll is used, soaking up many megawatts of power to get a signal through the Earth, not over its surface.

Geostationary satellites are today's more economical solution.

The Dawn Chorus

Occasionally, shortly after sunrise and even extending into the mid-morning, a phenomenon called "Dawn Chorus" may occur. Dawn chorus can resemble the sound of a flock of birds singing and squawking, dogs barking, or sound like whistlers raining down by the hundreds per minute (called a "whistler storm"). Dawn Chorus results from hundreds of overlapping, rapidly upward rising tones that can be continuous or appear in bursts, called chorus trains. Chorus trains sound fascinating--the bursts of chirps and squawks (risers) seem to suddenly commence, and over the course of two to five seconds, weaken and fade away, then repeat over again, often in different pitches. Bursts of chorus trains happening at different octaves can overlap in a beautiful cacophony.

Dawn chorus occurred several times a month during years of high sunspot activity after solar flares and/or coronal mass ejections on the Sun send a barrage of charged particles into the Earth's magnetic field, causing a geomagnetic storm and also producing Aurora. In years of low-sunspot counts and few solar flares, coronal mass ejections from the Sun can still cause magnetic storms once or twice a month. Chorus doesn't always only occur at dawn, especially for listeners located at higher latitudes, particularly in southern and central Canada, Alaska, and in northern Europe. This auroral zone is source to a vast amount of natural VLF phenomena. During auroral displays, chorus is often heard, as well as "hiss" of various pitches, "sliding-tone emission" which eerily and weirdly rise in pitch slowly over one to several seconds duration. The chorus which occurs during displays of Aurora is called "Auroral Chorus".

Sferics are the snap, crackle...

...and pop of lightning-stroke electromagnetic impulses from lightning storms within a couple thousand miles of the receiver; the more powerful the lightning stroke or the closer it is to the VLF receiver's location, the louder the pops and crashes of sferics will sound in the headphones.

Several million lightning strokes occur daily from an estimated 2000 storms worldwide, and the Earth is struck 100 times a second by lightning. At times the receiver's output is a cacophony of crackling and popping sferics from lightning strokes originating in storms near and far.

These huge sparks of lightning strokes are powerful sources of electromagnetic (radio) emission throughout the radio frequency spectrum - from the very lowest of radio frequencies up to the microwave frequency ranges and the visible light spectrum. However, most of the emitted electromagnetic energy from lightning is in the very lowest part of the radio spectrum, from 0.1 to 10 kHz. The radio pulses produced by lightning strokes travel enormous distances at these very-low radio frequencies, following the surface of the Earth as "ground waves." It is interesting how generally quiet and lightning sferic-free the hours are from just after sunrise to mid- morning, when thunderstorms tend to be at their minimum. Later, the crackling and popping of lightning sferic activity picks up as afternoon thunderstorms build in numbers and intensity. Weather monitoring agencies employ special receivers and direction-finding equipment in order to determine where lightning strikes are occurring and the potential for wildfire ignition, hazards, to aviation, and damage to electric power utilities from those lightning strikes.

VLF Whistlers

Whistlers are magnificent sounding bursts of ELF/VLF radio energy initiated by lightning strikes which "fall" in pitch. A whistler, as heard in the audio output from a VLF "whistler receiver", generally falls lower in pitch, from as high as the middle-to-upper frequency range of our hearing downward to a low pitch of a couple hundred cycles-per-second (Hz). Measured in frequency terms, a whistler can begin at over 10,000 Hz and fall to less than 200 Hz, though the majority are heard from 6,000 down to 500 Hz.

Whistlers can tell scientists a great deal of the space environment between the Sun and the Earth and also about Earth’s magnetosphere. The causes of whistlers are generally well known today though not yet completely understood. What is clear is that whistlers owe their existence to lightning storms. Lightning stroke energy happens at all electromagnetic frequencies simultaneously that is, from "DC to Light". Indeed, the Earth is literally bathed in lightning-stroke radio energy from an estimated 1,500 to 2,000 lightning storms in progress at any given time, triggering over a million lightning strikes daily.

The total energy output of lightning storms far exceeds the combined power output of all man-made radio signals and electric power generated from power plants.

Whistlers also owe their existence to Earth’s magnetic field (magnetosphere), which surrounds the planet like an enormous glove, and also to the Sun. Streaming from the Sun is the Solar Wind, which consists of energy and charged particles, called ions. And so, the combination of the Sun’s Solar Wind, the Earth’s magnetic field surrounding the entire Planet (magnetosphere), and lightning storms all interact to create the intriguing sounds of whistlers.

More Like Old Times.

The writer remembers a Practical Wireless project in the late Sixties for an ELF radio. The new Ferroxcube transformer cores formed the base of the untuned coils working somewhere below 9KHz. The gain came from three stages of OC71's, an OA47 detector and my last OC71 to drive the headphones. The thing was alive. Screaming whistles and whale-like howls tracked the course of electrical storms across entire continents - they showed me the magnetic changes brought on by the movement of the Earth's tectonic plates could be heard, but by now I was too scared to listen. The Thing was assigned to The Twilight Zone in the attic.