A distant lighthouse flashes for an instant brighter than an entire galaxy. This is a fast radio burst — a mysterious signal from deep space. Some repeat, and astronomers have noticed a peculiarity: these bursts show rapid changes in polarization — a property indicating how the radio wave is twisted in space. It's as if the lighthouse light suddenly changes color or brightness between flashes.

Usually, polarization is altered by magnetic fields and clouds of charged particles along the signal's path. But for two recently discovered repeating bursts, FRB 20201124A and FRB 20220529, very fast, small-scale jumps were detected. Intergalactic space cannot change that quickly, so the culprit must be the source's immediate surroundings. The authors of the paper suggest that special waves in the plasma, the hot gas of charged particles surrounding the source, are responsible. These waves, called Alfvén waves, arise when magnetic field lines oscillate like plucked strings. As such a wave passes, it pushes particles apart, creating regions of lower density — tiny bubbles of emptiness.

In these bubbles, there are fewer charged particles, so the magnetic field has less influence on the passing radio signal. When the burst's beam crosses such a bubble, its polarization changes differently than in denser plasma. This gives rise to the rapid jumps observed by telescopes. The authors showed that the wave strength, plasma density, and temperature together determine how deep these density 'cavities' are and how strongly they affect the signal.

This is like ripples on a pond: when wind drives waves, the reflection of the sky in the water breaks up and distorts. Similarly, Alfvén waves in plasma break up the polarization of the radio burst, leaving their signature. Now astronomers have a physical mechanism explaining the fine trembling in the signals — a key to understanding what goes on near the most powerful radio sources in the universe.