Traces of an enormous solar storm that battered the atmosphere and showered Earth in radioactive particles more than 2,500 years ago have been discovered under the Greenland ice sheet.
Scientists studying ice nearly half a kilometre beneath the surface found a band of radioactive elements unleashed by a storm that struck the planet in 660BC.
It was at least 10 times more powerful than any recorded by instruments set up to detect such events in the past 70 years, and as strong as the most intense known solar storm, which hit Earth in AD775.
Raimund Muscheler, a professor of quaternary sciences at Lund University in Sweden, said: “What our research shows is that the observational record over the past 70 years does not give us a complete picture of what the sun can do.”
The discovery means that the worst-case scenarios used in risk planning for serious space weather events underestimate how powerful solar storms can be, he said.
Solar storms are whipped up by intense magnetic fields on the surface of the sun. When they are pointed toward Earth they can send highly energetic streams of protons crashing into the atmosphere. The sudden rush of particles can pose a radiation risk to astronauts and airline passengers, and can damage satellites, power grids and other electrical devices.
Scientists have come to realise over the past decade that intense solar storms can leave distinct traces when they crash into the planet. When high energy particles slam into the stratosphere, they collide with atomic nuclei to create radioactive isotopes of elements such as carbon, beryllium and chlorine. These can linger in the atmosphere for a year or two, but when they reach the ground they can show up in tree rings and ice cores used to study the ancient climate.
Muscheler’s team analysed two ice cores drilled from the Greenland ice sheet and found that both contained spikes in isotopes of beryllium and chlorine that date back to about 660BC. The material appears to be the radioactive remnants of a solar storm that battered the atmosphere.
The scientists calculate that the storm sent at least 10bn protons per square centimetre into the atmosphere. “A solar proton event of such magnitude occurring in modern times could result in severe disruption of satellite-based technologies, high frequency radio communication and space-based navigation systems,” they write in Proceedings of the National Academy of Sciences.
Britain’s emergency plans for severe space weather are based on a worst-case scenario that involves a repeat of the 1859 Carrington event. This was a powerful geomagnetic storm set off by a huge eruption on the sun known as a coronal mass ejection. A 2015 Cabinet Office report anticipated only 12 hours warning of a similar storm that could lead to power outages and other disruption.
The discovery of more powerful solar storms in the past 3,000 years suggests that space weather can be worse than the UK plans for. “The Carrington event is often used as a worst-case scenario, but our research shows that this probably under-estimates the risks,” said Muscheler.
