Light from the Sun is usually unpolarized: its waves oscillate in all directions, like a crowd where everyone waves their arms as they please. But on May 3, 2023, astronomers in Locarno, Switzerland, noticed that during a flare, the light from helium became organized — the waves aligned in one direction, like soldiers on parade. This is the first time such polarization has been seen in the helium D3 line (a specific "signature" of light emitted by this gas).
The flare was class M7—fairly strong, but not record-breaking. The ZIMPOL instrument, a highly accurate polarimeter, tracked the helium light for several minutes. It turned out that polarization and absorption of light first grew rapidly, reached a maximum about 5 minutes after the peak of the flare's X-ray emission, and then slowly declined. It's like an echo: first a powerful strike, then a response in the form of ordered light.
The scientists found that the polarization arises not from particle impacts (as previously thought), but from light scattering: helium atoms absorb light from the flare and re-emit it in an orderly fashion, like flashlights all shining in the same direction. This helps understand how flare energy is distributed through the solar atmosphere.
Helium was first found on the Sun, not on Earth: in 1868, its spectral line was seen during an eclipse, and only 27 years later was the gas discovered in terrestrial minerals.
In Arthur C. Clarke's novel "A Solar Storm," solar flares threaten Earth — our work shows how we can read their signals to better predict such events.