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.
as written by the authors arXiv:2601.01148
Ultra-low-frequency electromagnetic waves Pc1 (0.2-5 Hz), widely known in the literature as pearls, are excited in the outer radiation belt and propagate to the Earth along the geomagnetic field lines in the form of Alfven waves. The study of pearls is of considerable interest for magnetospheric physics. Both spontaneous and stimulated pearl excitation are observed. The paper examines stimulated (trigger) excitation of pearls. A classification of triggers acting on dynamic systems of the magnetosphere is presented. 2 types, 4 classes and 8 species of triggers have been introduced. Examples of triggers of natural and artificial origin are given. The concept of a trigger cascade is introduced. Particular attention is paid to the anthropogenic periodic trigger of pearls. It manifests itself in the form of the so-called Big Ben effect. The essence of the effect is that a series of pearls is often excited immediately following the hour marker according to world time. It is claimed that the connection between the excitation of pearls and hour markers is not accidental, but represents a rather mysterious geophysical phenomenon. It is assumed that the Big Ben effect occurs as a result of the impact on the radiation belt of an unknown type of endogenous periodic artificial trigger. Key words: oscillations and waves, Alfven waves, ion-cyclotron resonator, radiation belt, instability, classification, natural and artificial, technosphere, Big Ben effect.
The frequency of pearls ranges from 0.2 to 5 hertz. That's lower than the lowest note of a double bass: the human ear can't hear such oscillations, but instruments record them as rhythmic bursts.