On the night of March 9–10, 2024, a cloud of plasma ejected by the Sun raced away from the star. It had already passed a distance of five solar radii when a ground-based array of 352 antennas in California's Owens Valley caught a brief radio signal from it. This signal was not a burst or noise; it had a shape that physicists recognize as the fingerprint of a magnetic field.

Magnetic islands inside the ejection, like fireflies in a cloud, reveal their field through short radio glows. More precisely, not glows but steady emission at specific frequencies. Electrons trapped in magnetic loops gyrate around field lines at a frequency that depends only on the field strength. This is gyroresonance emission: thermal electrons spiraling in a magnetic field radiate at their gyration frequency. If you can catch this signal, you can measure the field directly, without waiting for the ejection to reach Earth.

Until now, the magnetic field of coronal mass ejections in the middle corona remained a mystery. It was estimated from indirect signatures: the splitting of bands in type II radio bursts or the circular polarization of moving type IV bursts. Such methods give only a line-of-sight averaged field, not local values in individual structures. In the new work, for the first time, a spectrum characteristic of gyroresonance emission was observed, and from its break the field was calculated. The break frequency changed from 57 to 43 MHz as the ejection moved from 4.9 to 7.5 solar radii, corresponding to a field from 7.9 to 5.6 G. The observed brightness temperature of about 0.1 MK was an order of magnitude lower than expected for a filled source. This means the emission comes from compact magnetic islands about 0.03 solar radii in size, the very fireflies whose field we can now discern.

This result opens a new chapter in space weather forecasting. The magnetic field of a CME determines how strongly the ejection will hit Earth's magnetosphere, but until now it was measured only in situ, when the ejection was already near the planet. Remote diagnostics in the middle corona provide a lead time of tens of hours. The gyroresonance diagnostic method could become a routine tool for tracking the evolution of a CME's magnetic field from the Sun to Earth. Open questions remain: how small-scale magnetic structures form inside the ejection, and how universal the law of field decay with distance is. But one thing is already clear: fireflies in a solar cloud are no longer a metaphor. They have become a measurable signal.