One of the strongest known solar storms blasted Earth in 660 B.C.

One of the strongest solar storms ever to hurtle toward Earth blasted the planet in 660 B.C., researchers say, based on traces of the storm preserved in both ice cores and tree rings. The study was published online March 11 in Proceedings of the National Academy of Sciences.

As the sun’s magnetic field shifts, it occasionally releases bursts of charged particles into space. In the most extreme solar storms, called solar proton events, these charged particles are dramatically sped up by interactions with other solar emissions: solar flares or coronal mass ejections. Even Earth’s protective magnetic field can’t deflect such swift, energetic particle streams. The radiation is particularly hazardous to modern technology and astronauts.

It’s unclear how common such extreme events are; satellite- and ground-based instruments have tracked them for only about 70 years. To look farther back in time, researchers hunt for spikes in cosmogenic radionuclides such as carbon-14 — recorded in tree rings — or beryllium-10 and chlorine-36 — preserved in ice cores. Such radionuclides form when cosmic rays interact with molecules in Earth’s atmosphere.

In 2017, scientists identified a sudden spike in tree ring carbon-14 dating to about 660 B.C. that might indicate a solar proton event. But a carbon-14 spike can also signal a supernova or a solar flare.

In the new study, researchers led by geologist Paschal O’Hare, then at Lund University in Sweden, examined two Greenland ice cores. O’Hare, now at Heathgate Resources in Adelaide, Australia, and his colleagues found spikes in beryllium-10 and chlorine-36 dating to the same time.

The relative abundance of the radionuclides in the ice suggested that the 660 B.C. event was about 10 times more powerful than a 1956 event, the strongest solar storm recorded by instruments. The only known solar storm to rival the ancient storm’s power occurred in A.D. 774–775, an event also recorded in tree rings and ice cores.