For nearly three weeks, a strange radio signal appeared to circle the sun. One spacecraft would watch the signal fade from view, only for another elsewhere in the Solar System to detect it days later as the sun rotated. Instead of disappearing like most solar radio bursts, the signal kept returning. By the time it finally faded in September 2025, the burst had lasted 19 days, making it the longest Type IV solar radio burst ever recorded. The previous record lasted only about five days.
The event was tracked by several NASA missions, including Parker Solar Probe, Wind, and STEREO, as well as the joint ESA-NASA Solar Orbiter mission. The findings, published in The Astrophysical Journal Letters, trace how the sun may have sustained the radio burst for so long.
A Magnetic Structure Near the Sun May Have Kept the Radio Burst Alive Type IV radio bursts form when fast-moving particles become trapped inside the sun’s magnetic fields. Researchers think this 19-day signal may have survived near a helmet streamer, a looping region of plasma extending above the sun’s atmosphere.
The radio waves themselves are harmless, but the same solar eruptions can also fling charged particles into space that interfere with satellites, spacecraft, and communications systems.
During the event, three coronal mass ejections (huge eruptions of solar plasma and magnetic fields) blasted out from the same region of the sun. Those eruptions may have repeatedly supplied fresh particles into the structure, allowing the radio burst to continue longer than normal. The study describes the system as a possible “corotating electron reservoir,” essentially a rotating magnetic trap filled with energetic particles.
The signal also pulsed every 45 to 60 minutes. Those repeating pulses may have come from magnetic waves rippling through the structure, almost like vibrations echoing through a stretched cable.
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The Sun’s Rotation Let Multiple Spacecraft Follow the Signal Solar Orbiter detected the signal first while the source region was still hidden from Earth’s perspective. Nearly two weeks later, NASA’s Wind and Parker Solar Probe spacecraft picked it up as the sun’s rotation carried the burst into a more favorable viewing angle. One day later, STEREO-A detected it as well.
As the event unfolded, the signal slowly shifted toward lower radio frequencies and narrowed over time, suggesting that the magnetic structure itself was changing shape.
Additionally, the radio waves appeared larger by the time they reached the spacecraft than they likely were at the source. Turbulence in the solar wind could have scattered and blurred the signal while it traveled through space, similar to how Earth’s atmosphere can distort starlight.
The Event May Help Researchers Better Track Solar Activity To analyze the event, the team developed a new method to estimate the origins of solar radio bursts using measurements from a single spacecraft. The technique allowed them to trace the source to a region millions of miles above the sun, near the helmet streamer.
Long-duration Type IV bursts remain extremely rare, and researchers still do not fully understand how magnetic structures can hold energetic particles for so long. But as the sun moves through one of the most active phases of its roughly 11-year cycle, events like this are becoming more important to study.
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This article includes information from a study published in The Astrophysical Journal Letters: Unprecedented 19 Day Type IV Radio Burst as a Corotating Electron Reservoir
https://www.discovermagazine.com/nasa-and-esa-track-record-breaking-19-day-solar-radio-burst-49129
