During the Middle Ages the Earth was struck by two giant solar storms, far larger than anything we have observed. The explosions left marks in polar ice and in the trees that date to the era. With communication systems and electricity grids vulnerable to major storms, the findings suggest we may be at more risk than we realized.

The sun sometimes engages in powerful eruptions, spitting out bursts of charged particles that, if directed towards the Earth, can result in magnificent auroras above the poles. The Earth’s magnetic field usually protects us against damage, but in 1989 a powerful Coronal Mass Ejection blacked out much of North America.

The most powerful eruption recorded is thought to have been the Carrington Event of 1859, but Raimund Muscheler of the University of Lund says this was dwarfed by what happened around 774/775 and 993/994. Tree rings from those dates record a spike in radioactive carbon, but there are multiple plausible explanations for this, although all suggest some sort of extraterrestrial event. Muscheler and colleagues from five nations set out to explain the cause.

In Nature Communications they report finding Beryllium-10 and Chlorine-36 spikes in ice cores laid down around the same time in Greenland and the Antarctic. Like Carbon-14, Beryllium-10 and Chlorine-36 are formed when protons from the Sun or more distant sources strike particles in the atmosphere.

“With these new results it is possible to rule out all other suggested explanations, and thereby confirm extreme solar storms as the cause of these mysterious radiocarbon increases,” Muscheler said in a statement. Gamma-ray bursts, an alternative favored theory would be expected to produce Chlorine-36 but not Beryllium-10, given the likely distribution of proton energies.

The ratio of radioactive elements observed in these ice cores, combined with the previously collected tree ring data indicate that, “These solar events were characterized by a very hard energy spectrum with high fluxes of solar protons with energy above 100 MeV,” the authors argue.

Neither event can be dated precisely, as the radioactive particles are spread over 2-3 years’ ice. The paper acknowledges that the event probably only lasted a few days, arguing the difference “can be explained by the duration of the transport of the radionuclides from the stratosphere, where they are mostly produced”.

Solar storms are tied to the Sun’s magnetic cycle, making now a good time to hunt auroras. However, cycle intensity varies. Efforts have been made to predict future event sizes. However, these are based on a data that only goes back six decades, and more unreliable proxies.

The authors conclude the 774/775 explosion was five times more powerful than the biggest event we have seen since measurements of solar outbursts began in 1956, while the one two centuries later was around half as large as its predecessor.

Muscheler said, “These solar storms by far exceeded any known events observed by instrumental measurements on Earth. The findings should lead to a reassessment of the risks associated with solar storms.”

Journal Reference: Florian Mekhaldi, Raimund Muscheler, Florian Adolphi, Ala Aldahan, Jürg Beer, Joseph R. McConnell, Göran Possnert, Michael Sigl, Anders Svensson, Hans-Arno Synal, Kees C. Welten, Thomas E. Woodruff. Multiradionuclide evidence for the solar origin of the cosmic-ray events of ᴀᴅ 774/5 and 993/4. Nature Communications, 2015; 6: 8611 DOI: 10.1038/ncomms9611