When NASA scientists first picked up a radio signal from the Sun in August 2025, the burst looked unremarkable. Solar radio bursts are common and typically fade within hours or, at most, a few days. This one did not. It kept broadcasting. By the time it finally stopped, the signal had persisted for 19 days, smashing the previous record of five days.
The signal fell into a category called a Type IV radio burst, produced by energetic electrons snared inside the Sun’s magnetic fields. The radio waves themselves travel harmlessly through space. But the same magnetic conditions can unleash solar eruptions that fling damaging particles toward satellites and spacecraft. Figuring out why this particular burst held on for so long became a priority for scientists who monitor space weather.
The findings, reported in The Astrophysical Journal Letters, offer a rare look at the enormous magnetic structures that can trap and release energy over weeks. Reconstructing the event required data from a fleet of spacecraft, a new analysis technique, and a careful timeline of eruptions that appear to have kept the signal alive.
A Fleet of Spacecraft Tracked the Signal Across the Solar System
Following a 19-day signal takes more than one set of eyes. The Sun rotates, so active regions drift in and out of view from any single spacecraft. The research team combined observations from four missions: NASA’s STEREO (Solar Terrestrial Relations Observatory), Parker Solar Probe, and Wind spacecraft, plus the Solar Orbiter mission operated jointly by the European Space Agency and NASA.
Each spacecraft recorded a different chapter. The burst first surfaced in Solar Orbiter data on August 21, 2025, while the source region still sat on the far side of the Sun relative to Earth. Twelve days later, the signal rotated into view of Wind and Parker Solar Probe. A day after that, STEREO-A picked up the final stretch of emission through September 9.
The sequential handoff aligned with the Sun’s rotation rate, confirming the researchers were watching a single persistent structure. They described it as a corotating electron reservoir, a long-lived magnetic trap that became detectable only when the viewing geometry lined up.
The Source Was a Helmet Streamer Stretching Millions of Miles
Locating a low-frequency radio source inside the solar corona is difficult because the corona bends and scatters radio waves. To cut through the distortion, the researchers developed the wavevector-corrected ray sphere (WCRS) method, which adjusted the recorded signal direction to account for how much the solar wind had displaced it.
After correction, the signal pointed back to a helmet streamer, a towering magnetic loop that extends far into the Sun’s outer atmosphere. These structures can confine hot gas and energetic particles for long stretches. The team calculated that the emitting structure sat between 6 and 10 solar radii above the solar surface and estimated its width at roughly 2.5 to 3.0 solar radii, making it a vast cavity by coronal standards.
No radio signal can run for 19 days on stored energy alone. The study points to three coronal mass ejections, or CMEs, that erupted from the same region and likely refreshed the trapped electron population. The first launched on August 21, coinciding with the initial detection by Solar Orbiter.
The second followed on August 30 and arrived with a surge of type III radio storm activity, evidence that the active region was pumping fresh electrons into the surrounding corona. The third CME blasted off on September 4, just before the burst hit its brightest phase.
Each eruption either dumped new energetic particles into the trap or rearranged the local magnetic field enough to sustain the emission. After the third CME, the Wind spacecraft detected an unusually thin solar wind near Earth, with proton densities bottoming out around 0.1 particles per cubic centimeter. The research team noted that such rarefied wakes could reduce radio wave scattering, making the burst easier to spot.
A Flickering Signal and an Optical Illusion
During its brightest stretch, the signal was not steady. STEREO-A data showed the burst brightening and dimming on a regular beat with periods between roughly 45 and 60 minutes. These quasiperiodic pulsations offered a second way to measure the magnetic trap.
The researchers interpreted the rhythmic flickering as standing oscillations of the magnetic structure itself. Using magnetoseismology, a technique that turns wave periods into physical dimensions, they derived a lower-bound size that matched the rotation-based estimate. That agreement between two independent methods gave the team confidence they were looking at a single large magnetic cavity.
Density variations in the solar wind scatter low-frequency radio waves, making the source look much bigger than its true size. The STEREO measurements showed the burst spanning roughly 20 degrees of apparent width. The researchers calculated a broadening factor of roughly 60, meaning the spacecraft saw a smeared, magnified version of a compact source.
That finding has practical weight for space weather forecasting. If long-duration Type IV bursts routinely appear broader than they are, forecasters may overestimate source region sizes unless they apply a correction like the WCRS method.
Why This 19-Day Signal Still Holds Secrets
The authors flagged several gaps. The exact mechanism that confined the electrons for 19 days remains unresolved. Telling apart continuous injection from the low corona versus repeated replenishment by eruptions would require modeling and measurements that current spacecraft cannot provide.
The team also noted that their height estimates assumed the emission was plasma radiation at the fundamental frequency. If the signal instead tracked the electron cyclotron frequency, the absolute heights would shift, though the broad structural interpretation would hold.
The researchers drew a parallel to a long-duration Type IV burst recorded in May 2002, which lasted about six days and showed similar strong polarization and pulsations. In both cases, the radio burst was followed by an unusually low-density solar wind near Earth, suggesting that rarefied CME wakes might help long-lived radio reservoirs stand out from a single vantage point.
The new correction technique and multi-spacecraft tracking approach demonstrated in this study now give forecasters a framework for identifying and measuring future events of this kind before they fade.
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