Oct. 18, 2023: In a quiet room, you can hear a pin drop. Norwegian citizen scientist Rob Stammes just heard a pin drop on Earth’s magnetic field.

“It was very quiet when it happened,” says Stammes, who runs a space weather observatory in Lofoten, Norway. On Oct. 17th, his magnetometer was monitoring Earth’s magnetic field as it does every night, and the instrument’s needle had settled itself into a straight line, indicating very low geomagnetic activity.  Suddenly, Earth’s magnetic field began to ring.

“A very stable ~25 second magnetic oscillation appeared in my recordings, and lasted for more than 20 minutes,” he says. “It was fantastic to see the magnetic field swing back and forth by about 0.1 degrees, peak to peak.”

This kind of pure tone is rare, but it has happened before. Researchers call it a “pulsation continuous” — or “Pc” for short. Pc waves are classified into 5 types depending on their period. The waves Stammes caught fall into category Pc3.

The “pin dropping” was a gentle gust of solar wind. Imagine blowing across a piece of paper, making it flutter with your breath. The solar wind can have a similar effect Earth’s magnetic field. Pc3 waves are essentially flutters propagating down the flanks of our planet’s magnetosphere excited by the breath of the sun.

A NASA animation of waves fluttering down the sides of Earth’s magnetosphere [more]

Stammes is a longtime observer of Pc waves. Usually he catches them during Solar Minimum when “the room is quiet” for months at a time. “Recording one now so close to Solar Max is unexpected,” he says. “Lately, my magnetometer traces have been too noisy for such delicate waves–so it came a surprise!”

Pc3 waves, which can only be heard in moments of quiet, can also bring the quiet to an end. The oscillations sometimes flow all the way around Earth’s magnetic field and cause a “tearing instability” in our planet’s magnetic tail. This, in turn, sets the stage for magnetic reconnection and geomagnetic storms.

That didn’t happen on Oct. 17th, though. The pin dropped, the magnetosphere rang, and quiet resumed. Stammes is already listening for more. Stay tuned!