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Every hour, when the hands of a clock align at twelve, something happens in Earth's magnetosphere. It's as if an invisible conductor raises a baton, and waves race along magnetic field lines toward the surface. They're called pearls: on recordings, they look like a string of beads strung along time.

Pearls are ultra-low-frequency electromagnetic waves with frequencies from 0.2 to 5 hertz. They are born in the outer radiation belt — a region around Earth where the magnetic field traps charged particles. From there, the waves travel to Earth as Alfvén waves — what physicists call oscillations that propagate along magnetic field lines, like a tremor along a taut string.

Sometimes pearls arise on their own, and sometimes something triggers them, like a match igniting a fire. The article proposes a classification of such 'triggers': two types, four classes, and eight kinds. Among them are natural and artificial ones. For example, lightning or the operation of industrial installations can give a nudge that results in a series of pearls.

But the strangest thing is the Big Ben effect. That's the name for the phenomenon where a pearl sequence starts right after the hourly mark in universal time. It's as if someone checks their watch and, exactly at the chime, presses an invisible button. The connection between pearl excitation and hourly marks is not random: it's confirmed by observations. But what exactly serves as the trigger remains a mystery.

The authors suggest that the Big Ben effect arises from the influence on the radiation belt of some as-yet-unknown periodic artificial trigger. Perhaps it's the technosphere — the sum of all human-made systems, from power lines to radio transmitters. But the exact mechanism has yet to be found.

So every hour, when you hear the chime, somewhere high above Earth, a pearl wave may be running. And we still don't know who or what sets it off.