Scientists detected a mysterious radio signal that lasted nearly three weeks, shattering previous records and pointing to an enormous magnetic structure that trapped energetic particles in space high above the sun.

The signal came from a so-called "type IV radio burst," a kind of radio noise associated with solar eruptions. Scientists usually watch these bursts flare up and disappear within mere hours. But this one persisted for about 19 days — almost four times longer than the previous record-holding event.

Several NASA and European Space Agency spacecraft observed the phenomenon, which happened last August, as the sun rotated. The Solar Orbiter saw it first. Nearly two weeks later, the Parker Solar Probe and Wind spacecraft near Earth detected the same signal. STEREO-A, short for the Solar Terrestrial Relations Observatory,  picked it up a day after that. This pattern hinted it was one long-lived source rotating with the sun rather than a succession of unrelated eruptions.

The signal also pulsed in a surprisingly regular rhythm. Roughly every 45 minutes to an hour, the radio emission brightened and dimmed. Researchers think enormous magnetic structures above the sun may have vibrated, almost like a ringing bell, periodically affecting the trapped electrons.

"Those vibrations probably squeezed and stretched the trapped electrons in a repeating cycle, which changes the brightness of the radio signal," according to NASA. 

The observations suggest the sun can maintain huge, organized reservoirs of energetic particles for weeks at a time. By studying this extraordinary event, scientists think they have gained a way to estimate the location of solar radio sources using only one spacecraft, a technique that could eventually improve space-weather forecasts. That's valuable because, while solar storms aren't harmful to human health on Earth, they can wreak havoc on satellites, GPS systems, radio communication, and power grids.

The researchers, who published their findings in the Astrophysical Journal Letters, believe the source sat near a "helmet streamer," a giant arching magnetic structure that rises from the sun's atmosphere and stretches far into space. They estimate that the structure measured more than 1.2 million miles wide — enormous even by solar standards.

The event coincided with three fast coronal mass ejections, or CMEs, massive eruptions that blast plasma and magnetic fields into space. The scientists suspect those eruptions repeatedly supplied fresh electrons or reshaped the magnetic environment in ways that kept the signal alive.

The radio waves themselves showed an unusually strong polarization, meaning the waves oscillated in an ordered way rather than a chaotic one. That clue suggested the electrons moved through a surprisingly organized magnetic environment.

Scientists still don't know exactly how the sun made the radio waves, but they have two ideas. One explanation involves plasma emission, a common process in which energetic electrons moving through thin gas naturally create radio signals. Another possibility involves a rarer mechanism that behaves somewhat like a natural microwave laser in space and may operate inside unusually low-density magnetic cavities.

While the researchers don't have a clear understanding of the phenomenon, their evidence points toward a giant, rotating magnetic trap that survived for weeks while solar eruptions continued feeding it energetic particles.

The study also tackles a longstanding technical problem. Low-frequency solar radio waves bend and scatter as they travel through space, making their true source elusive. The researchers developed a correction method to account for some of that distortion. Using the new technique, they traced the source to heights between 2.5 to 4.3 million miles above the sun. 

The scientists say future missions could combine the technique with improved radio imaging and particle measurements to track solar eruptions more accurately.