Aurora Chorus

There is a sound the sun makes when its wind strikes the Earth.

It is not a mechanical sound — space is vacuum, and sound waves cannot propagate through it. But charged particles streaming from the sun at hundreds of kilometers per second interact with Earth's magnetic field in ways that generate electromagnetic emissions in the Very Low Frequency (VLF) range, from roughly 300 Hz to 30 kHz. This falls within the human audible range. With the right receiver and antenna, you can hear it.

What You Are Hearing

The aurora chorus — also called dawn chorus — is a naturally occurring radio emission generated when energetic electrons in Earth's radiation belts interact with VLF waves. The interaction produces a rising-frequency chirp that sounds eerily like birdsong, hence the name.

During geomagnetic storms driven by solar activity, the effect is dramatically amplified. Emissions spread across a broader frequency range and sustain longer. At the same time, the aurora borealis becomes visible at lower latitudes, connecting the visible light phenomenon in the sky to the radio phenomenon in the spectrum.

A related emission is whistlers — a descending tone produced when a lightning strike generates a radio wave that travels along Earth's magnetic field lines from one hemisphere to the other. The dispersion of the medium causes higher frequencies to arrive first, producing the characteristic whistling sound.

How to Receive Natural Radio

VLF natural radio reception requires a simple antenna — a long wire laid on the ground or strung horizontally, connected to a low-noise amplifier and a sound card or SDR. No transmitter license is required because you are only receiving.

The primary challenge is man-made interference. Power lines, computers, and switching power supplies all generate noise in the VLF range. Rural areas away from electrical infrastructure produce dramatically cleaner natural radio reception.

Software like Spectrum Lab can display the audio in real time as a spectrogram, making whistlers and chorus events visible as well as audible.

The Science

Aurora chorus emissions were first formally studied in the 1950s and 1960s using ground-based receivers and, later, satellites. The phenomenon is a significant research topic in space physics because the same wave-particle interactions that produce audible chorus can accelerate electrons in the Van Allen radiation belts to relativistic speeds — speeds approaching the speed of light. These energetic electrons pose hazards to satellites and, during extreme events, to GPS systems and the electrical grid.

What you hear as a pleasantly eerie natural sound is the same physical process that makes geomagnetic storms a genuine infrastructure concern.

HAM Radio and Natural Phenomena

The aurora has direct practical implications for HAM radio operators. During aurora events, VHF propagation is dramatically altered — 6m and 2m signals can propagate over paths of 1,000+ kilometers via aurora scatter. The signals have a distinctive harsh, buzzy quality caused by the rapidly moving ionized trails.

Experienced VHF operators actively monitor space weather forecasts and chase aurora openings. A major aurora event on 6m is comparable to a major tropospheric ducting event on 2m — unusual propagation that makes contacts possible that would otherwise be physically impossible.

The aurora is, in this sense, a radio phenomenon before it is a visual one. The lights are a side effect of the physics that most HAM operators experience primarily through their radios.