Whistlers

Lacking radio receivers built into them, human beings are unable to hear radio waves without special electronic devices. However, these devices need not always be the radio receivers we normally think of.

For example, some people who have two or more different kinds of fillings in their teeth are able to hear high-power AM broadcast stations when located within a few hundred feet of the stations. In such cases, the strong radio waves act upon the teeth fillings in such a way that the electromagnetic oscillations get transformed to mechanical vibrations in the person's head, and these are heard as sound.

A quite different but equally curious phenomenon was discovered before the end of the last century. People listening to telephones sometimes have heard strange whistling noises in addition to whatever other sounds were coming over the wires. The whistling noise starts out with a high-frequency tone and steadily decreases to lower tone, over a period of a second or two. It sounds very much like the second of the two-syllable whistle that appreciative boys sometimes emit when seeing attractive girls, or visa versa.

It is now known that these sounds, called whistlers, are created by lightning strokes traveling over a surprising path. A lightning stroke emits a wide range of radio waves, the most powerful being at about 5,000 Hertz. Some of the radio waves pass through the ionosphere and travel along geomagnetic field lines. Guided by the magnetic field, the waves pass high above the earth's equator and then follow along the field lines back down to the opposite hemisphere. The higher-frequency signals travel fastest and consequently get back to earth sooner than the lower frequencies, hence the decreasing tone of the whistler when it is heard.

Any long wire, such as a fence or a long telephone line makes a good antenna for receiving these space-traveling signals. An audio amplifier such as is contained in a home music system, a radio or a tape deck hooked to a long wire makes a satisfactory receiving system for whistlers.

With such a system, a person in Alaska or western Canada can hear lightning strokes in the South Pacific Ocean. Each stroke sounds not like a crash, but rather is a whistle with tone decreasing with time. One can also hear a whistler that starts out from a local lightning stroke. When this happens, one first hears the crash of the lightning stroke in the receiving system, followed a few seconds later by the whistler. In this case, the radio waves creating the whistler travel along the magnetic field lines to the other hemisphere where they bounce off the ionosphere and speed back along the same path to the point of origin. Such a two-hop whistler is more drawn out in time than one caused by the radio waves making a single passage.

When auroras are active, other sounds, often semi-musical, also are heard on the receiving equipment. Sometimes both ascending- and descending-tone whistlers are heard, at other times the sound is a jumble of murmurs and rustling. Because of their musical nature, the name 'dawn chorus' is given to some of these sounds. Since all these sounds come in as radio waves, they apparently have no direct relation to the auroral sounds many people report when observing active auroral displays.