A couple of days ago, I wrote about how the radio waves from the aurora borealis can be converted into sound waves and listened to, as an example of sonification. I now know, thanks to an email from a BBC researcher, that there is also audible sound like someone hitting a metal gate with a hammer.
Researchers at Aalto University in Finland used three microphones to pick up the sound when the northern lights are at low altitude. The source is about 70 metres above the ground. How the sound is being generated by the geomagnetic disturbance is unknown.