Whistlers and the Dawn Chorus

Summary

Put a long wire in the ground and listen to it, and you will hear the sky.

You will hear whistles: single descending tones, sliding smoothly from high to low over about a second. And at dawn, sometimes, you will hear something that sounds so exactly like a wood full of birds waking up that the people who first heard it called it the dawn chorus, and the name has stuck for over a century, because there is no better one.

Telephone operators heard them in the 1880s and had no idea what they were.

They are lightning. And working out how they get to you turned out to reveal that the space around the Earth is not empty.

What is documented

The sounds. Whistlers are descending tones, lasting on the order of a second, heard on very low frequency radio receivers and, historically, on long telephone and telegraph lines.

The dawn chorus is a rising and falling profusion of tones, most often heard around dawn, and it sounds like birdsong. Not vaguely. Precisely.

When they were heard. Long-line telephone and telegraph operators reported them from the 1880s. They became far more widely noticed during the First World War, when both sides listened on long antenna wires in an attempt to intercept the other's field telephone traffic, and heard, instead, the sky.

Barkhausen. The German physicist Heinrich Barkhausen heard and described them in 1919, having encountered them while eavesdropping on Allied lines.

Storey. Owen Storey established the mechanism in his Cambridge doctoral work, published in 1953.

Leading explanations

Lightning, dispersed by the magnetosphere. Established.

A lightning stroke is an electrical event of enormous power and very short duration. It radiates a broadband pulse of radio energy across a wide range of frequencies at once, all of it emitted in the same instant.

Some of that energy does not stay near the ground. It escapes upward and enters the plasma that surrounds the Earth, and there it is guided: it follows the lines of the Earth's magnetic field, arcing out into space, over the equator, and down into the opposite hemisphere. Sometimes it comes back.

And the plasma sorts it. This is the mechanism, and it is the whole answer.

The medium is dispersive: high frequencies travel through it faster than low frequencies. The pulse left the lightning stroke as a single instantaneous crack, with all its frequencies together. By the time it has travelled tens of thousands of kilometres through the magnetosphere, the high frequencies have arrived first and the low ones are still on their way.

A crack has been stretched into a slide.

That is why a whistler descends. You are listening to a single flash of lightning, smeared out in time by the plasma it travelled through, arriving in order of pitch.

What Storey's work actually established, and it is much bigger than the whistle.

To account for the timing, Storey had to work out what the pulse had travelled through. And the answer was that the region around the Earth, out to many Earth radii, is filled with plasma at densities nobody had expected.

He did this in 1953, four years before Sputnik. Nobody had put anything into orbit. There was no way to go and measure it.

The structure of near-Earth space, the plasmasphere, was deduced from the shape of a noise on a wire.

The dawn chorus. A different mechanism, related. It is produced by whistler-mode chorus waves: emissions generated in the magnetosphere by energetic electrons, particularly during the disturbances that follow a geomagnetic substorm. It remains an active research subject, because those same waves accelerate the electrons in the radiation belts that damage satellites.

Current status

Later explained. The mechanism is understood and has been for over seventy years.

The registry keeps this record as its best example of an anomaly whose explanation was worth immeasurably more than the anomaly.

Somebody heard a strange noise on a wire, and asked why, and the answer was the magnetosphere.

Last reviewed: July 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.