You can hear lightning that struck in the other hemisphere
2026-05-05 · EA5IYR
Everything on this site is about signals somebody transmitted. This one is not.
Below about 20 kHz there is radio that nobody sent. It comes from lightning, from the solar wind, from the Earth's own magnetic field — and because those frequencies fall inside the range of human hearing, you do not need to demodulate any of it. Feed the antenna into a sound card and you are listening to the signal itself. Four things you will hear
Sferics. The basic unit: the radio crack of a lightning stroke. Thousands per second are happening worldwide, and a receiver picks up a continuous frying-bacon crackle.
Tweeks. A sferic that has bounced a long way between the ground and the ionosphere, which act together as a waveguide. The higher frequencies get there first, so the crack acquires a short musical ricochet on the end.
Whistlers. The good ones. A lightning stroke whose energy escapes upward, follows a magnetic field line out into space, and comes down in the opposite hemisphere — travelling tens of thousands of kilometres. Along that path the dispersion is extreme, so what arrives is a slow descending glide lasting a second or more. A single flash, smeared into a falling whistle.
The dawn chorus. Not lightning. Electrons in the magnetosphere, energised by the solar wind, radiating as they spiral along field lines. It sounds startlingly like a field of birds at sunrise, which is exactly how it got the name. What you need
Very little, and this is the appealing part.
A long wire or a loop, an amplifier, and a sound card. Software is any audio spectrogram. There are also public streams if you want to hear it before building anything.
What you actually need is quiet. Mains hum at 50 or 60 Hz and its harmonics will bury everything, so a working receiver in a house is nearly hopeless. Go to a field, away from power lines, and take a battery.
Best around dawn and through the night. Whistlers are more common at higher latitudes and in seasons when the far hemisphere is having thunderstorms. Why it is worth doing once
Because it is unmediated. A VLF submarine transmitter puts out 1.8 megawatts to reach a boat; this reaches you for nothing, and it has been doing it since long before there was anyone to listen.
The whistle you hear started as a lightning stroke over Africa, went out past the orbit of most satellites, and came back down over you. That is a real journey, happening in about a second, and you can hear it with a wire.
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Sources: Wikipedia — Whistler (radio)), Hackaday — sferics, whistlers and the dawn chorus, Institute of Physics — sferics, tweeks and whistlers, NASA Marshall — listening to Earth's songs.