An auroral chorus is a group of electromagnetic waves that produce sounds similar to chirps, whistles, and musical tones when converted into sound. These sounds are created by geomagnetic storms, which also cause auroras. Each sound lasts about 0.1 to 1.0 seconds. Other auroral sounds include hissing, swishing, rustling, and cracking.

These electromagnetic waves are a type of natural radio wave. They are vibrations of electric and magnetic energy that occur at the same frequency as sound.

Detection

Auroral chorus is most often found near the magnetic equator, in two separate frequency ranges: one above and one below the equatorial half gyro-frequency. The gyro-frequency range is between 0.6 kHz and 1.6 kHz. On high-quality spectrographs, the wave strength increases steadily and then changes to a more complex pattern. Auroral chorus is most clearly observed by ELF/VLF radio receivers in middle latitudes, between 30 and 60 degrees north. The Iowa Plasma Wave Group has collected the most recordings of auroral chorus. They have shared many audio versions of these recordings online, along with graphs showing wave patterns.

Historically, the Inuit people of Canada linked these sounds to spiritual events, as they often heard auroral chorus during cold, windless nights.

Explanation

The exact cause and origin of auroral chorus, a type of sound heard during auroras, remain an ongoing mystery in space and atmospheric science. Observations from the Cluster satellite mission suggest that these sounds may come from many moving sources.

Research shows a clear connection between the presence or absence of auroral chorus and changes in solar wind activity and the southward direction of the interplanetary magnetic field (which is linked to auroras). The loudness of the sounds is tied to the strength of geomagnetic activity in Earth's ionosphere. Other studies find a strong link between auroral chorus and very low frequencies below 20 Hz. A delay between the peak of auroral sounds and the peak of electromagnetic activity matches the time it would take for sound to travel from auroral heights (80–100 km) to Earth's surface. However, local electric field signals do not match sound signals well.

Some scientists think the auroral chorus may not start at the aurora itself but instead change from tiny wave ripples in the air into audible sounds through objects near the observer. It is still unclear whether actual sound waves exist or if electromagnetic waves somehow affect the human ear.