|Greenland’s ancient ice sheet holds evidence of a solar storm that struck Earth more than 2,600 years ago. Researchers found radioactive evidence of an “extreme solar event” that occurred around the year 660 BC.|
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.