A solar flare is like an explosion in the atmosphere: bright, fast, with different 'voices.' X-rays come from electrons that have been accelerated and then abruptly slowed down, losing energy. But in 2017, detectors on the Fermi satellite caught a different signal — gamma rays that didn't fit this picture. They were too strong at energies of a few million electron-volts, as if someone had switched on a separate light source.
The spectrum of these gamma rays pointed to a strange group of electrons: instead of a smooth decline in energy, they had a peak — a 'bump' around a few MeV. Such a pattern doesn't arise from ordinary braking. The researchers compared Fermi data with microwave radio observations from the EOVSA telescope. It turned out that the shape of the microwave spectrum from the 'bumpy' electrons is quite different from that of ordinary electrons. It's like telling the voice of a trombone from a violin in an orchestra.
Scanning the flare maps, the researchers found a region where the microwave spectrum matched what was expected for the 'bumpy' electrons. This region is in the Sun's corona, near the site where the magnetic field releases energy and accelerates the bulk of the particles. So, the mysterious electrons are not born in place: they arrive from the acceleration zone and change their energy profile along the way. It's transport, not acceleration, that makes them what they are.
For the first time, this allowed the mysterious gamma-ray component to be tied to a specific location on the Sun and to understand that it results from particle travel, not a new acceleration mechanism. The flare turned out to be not just an explosion but a journey with transfers, where some electron passengers get off at another station to shine in the gamma-ray range.
The gamma rays from the solar flare on September 10, 2017, were so bright that they were detected by instruments usually tuned to distant cosmic explosions.