Thunderbolts and Whistlers, December 1956 Popular Electronics

The first electronic circuit I remember building as a budding teenage tinkerer was a "whistler" detector, aka a "sferic." Instructions and a schematic came from a project book I bought at Radio Shack. A whistler is a time-varying electromagnetic signal that decays in both frequency and volume over a short time - like sounds made by the eponymous fireworks genre. Having always had an interest in weather phenomena as part of my flying hobbies, it seemed like an apt learning endeavor. To my recollection, the whistler detector was a simple diode circuit with a couple Rs, Ls, and Cs strewn around in a particular configuration, and a long wire antenna. I can't honestly say whether or not any whistlers were ever heard with it. My interest was a layman's curiosity, but elsewhere in the world, professional scientists were expending a lot of effort in their attempts to analyze and quantify a whitler's particulars.

The why and wherefore of eerie-sounding radio signals generated by certain types of lightning

The theoretical path taken by a whistler is demonstrated by Harold E. Dinger with a loop of wire. Because it follows the earth's magnetic lines of force, the path extends into intersolar space.

Pen-and-ink spectrograms are analyzed to determine how the sound changes in frequency as time elapses. The whistler below is represented by the dark band running from left to right.

Bonk!! ... Sheeeeeooooooooo ... ... Bonk! ............ Sheeeeeooooooooo ... Bonk! ............ Sheeeeeooooooooo ...

These odd sounds were emitted over and over by a loudspeaker in The Naval Research Laboratory, Washington, D. C., during a stormy afternoon last summer. For hours the popping, crackling sounds of "sferics," the usual very-low-frequency radio static, had been monotonously pouring out. Then, about midday, a thunderstorm with strong lightning strokes had combined with the proper condition of the atmosphere. "Whistlers" - strange, drawn out, eerie-sounding radio signals - were being generated by lightning stroke after stroke in great profusion.

What, you might ask, could be sillier than a bunch of people listening to static instead of turning the radio off during the storm? But it's not ordinary static they are listening to. The "Bonk!" of the lightning crash is followed a second or two later by a swooping whistle; a weird, downsliding tone like a sigh from the ether. The mysterious cause of these "whistlers" is what our scientists want to fathom.

According to theory, the sighing whistle means that part of the radio wave generated by the lightning flash has zoomed thousands of miles through the upper reaches of the atmosphere and out into space, turning downward again to the Southern Hemisphere. Way down near the Straits of Magellan, it bounces back off the earth and dogs its own track back to its point of origin. With uncanny precision, this errant radio wave seeks out the small storm area where it was "born" from all the immensity of the earth and sky. No homing pigeon could be more faithful than this mysterious, short-lived, v.l.f. radio wave.

For years, scientists at The Naval Research Laboratory have been studying these curious radio waves. From a long, low building on the Laboratory grounds, a coaxial cable runs up to the roof and connects to a long-wire antenna. This is 200 feet long and ends at the top of a 120' radio tower. Radio signals from thousands of sources fall on this receiving antenna. Local broadcast stations "plaster" it, and on 16 kc., the dahdahdit-dahditditdit-ditdahdit of GBR, Rugby, England, can be heard.

To Harold E. Dinger, the NRL expert on v.I.f. radio waves, these man-made signals are simply interference, requiring careful filtering for their elimination. "Whistling atmospherics" presently hold Mr. Dinger's concentrated attention, and he and his associates have surrounded themselves with special electronic apparatus for their study. The long omnidirectional antenna feeds an audio amplifier which covers the frequency range from 800 to 14,000 cycles per second, and can amplify signal voltages by a factor of one million. A high-fidelity tape unit is fed by the amplifier and automatically records whistler activity.

Since a whistler tone is never completely pure and often very ragged, a careful analysis of the taped spectrograms is necessary to determine the dominant frequency. Mr. Dinger and his staff have studied thousands of whistlers, corroborating previous results by other workers, and have discovered new evidence which as yet is not completely explained.

What is the precise mechanism of propagation of these waves? Any attempt to explain even the common types of whistlers must show how the single sharp "Bonk!" of electromagnetic energy from a thunderbolt is changed into a delayed echo many seconds long. Where did the energy go during the one or two seconds between lightning flash and whistler? The delay strongly suggests an echo or reflection from a remote point.

The German scientist, Heinrich Barkhausen, is generally credited with discovering whistlers while intercepting Allied landline telephone conversations in World War I. Later, both Barkhausen and the English scientist T. L. Eckersley offered the theory that the pulse from the lightning flash traveled through the ionosphere. Eckersley further proposed that the earth's magnetic field would cause the energy from a flash to be split and was responsible for the generation of a whistler.

In 1950, in the famous Cavendish Laboratory, University of Cambridge, England, L.R.O. Storey took up the study of whistlers. By 1953 he had calculated that the path length of whistlers was a minimum of fifteen thousand miles. With the help of data obtained from British thunderstorm locating stations, he deduced that the waves were focused into a huge curved beam, closely following a line of force of the earth's magnetic field out into space and down into the Southern Hemisphere, and that they returned after reflection to an area around the initial flash. He proposed that the whistlers sometimes heard without a preceding "Bonk!" or "click" had originated in the Southern Hemisphere during thunderstorms there.

It seems virtually certain that whistlers will provide a valuable tool to assist in determining the nature of the "outer" ionosphere. These studies may also throw new light on partially unexplained geophysical phenomena such as "radio blackouts," magnetic storms, and the aurora.

Thunderbolts and WhistlersDecember 1956 Popular Electronics

Thunderbolts and Whistlers
December 1956 Popular Electronics