Solar Storms in the Middle Ages

Scientists studying ice-cores extracted from Greenland and Antarctica have found further evidence of two very powerful solar storms that took place in 774 and 993 AD. In 2012, researchers found traces of a rapid increase of radioactive carbon in tree rings in Japanese cedar, precisely dated to AD 774-775. It's the same year that, according to a text in an ancient Anglo-Saxon chronicle, a "red crucifix" appeared in the heavens after a sunset.

Spikes in the amounts of radioactive elements like chlorine-36, beryllium-10, and carbon-14 were later found all over the world, including in ice-cores drilled in Greenland and Antarctica. Those ice-cores provide scientists with an abundance of exact information about the composition of the atmosphere going back hundreds of thousands of years. The spikes in radioactive isotopes point to two huge events in the years 774 and 993, and are created by high-energy protons slamming into our air, which means the source must have been from space.

Whatever this source was, it must have been extremely violent to create that many particles. One possible candidate was that the Earth got caught in the beam from a gamma-ray burst (GRB), the incredibly powerful demise of a very high mass star. However, GRB impacts don’t usually create beryllium-10 due to the detailed physics of GRB's, and moreover, they’re very rare events so having two happen in 220 years is rather unlikely.

Now researchers led by geologist Raimund Muscheler of Lund University in Sweden say they have solved the mystery of both the 774-775 AD event as well as the one in 993-994 AD. In addition to carbon-14, they have linked beryllium-10 to both events, firmly establishing them as solar flares.

Large solar flares can cause coronal mass ejections and subsequently a solar storm that may reach Earth in a few days. The charged particles cause beautiful auroras but are also capable of disturbing Earth's magnetic field, causing a geomagnetic storm that, when powerful enough, induces currents in conductors on the ground. In 1859, numerous sunspots were observed on the Sun causing auroras which were visible even in places like Cuba or Italy, in what became known as the Carrington event. Telegraph wires in both the United States and Europe experienced voltage increases, in some cases even delivering shocks to telegraph operators and igniting fires.

Since then, less severe storms have occurred, notably the aurora of 1882 and the 1921 geomagnetic storm, both of which disrupted telegraph services and initiated fires, and of 1960, when widespread radio disruption was reported. The March 1989 geomagnetic storm caused auroras as far south as Texas, and knocked out huge transformers in North America as well as the Québec power grid. In 2012 the Sun blew out a flare similar to the one in 1859, but happily for us it was sent off in another direction, and missed the Earth.

It can now be concluded that powerful solar storms like the ones in 774 and 993, which were several times stronger than the Carrington event, may be more common than previously thought. In 2013, Lloyd's of London and the Atmospheric and Environmental Research Center estimated that the duration of power outages during a Carrington-like event today could last five months or longer and cause economic costs of up to 2 trillion dollar. Moreover, were an event the magnitude of the 774/ 993 solar storms to occur today, it could destroy satellites and means of communication and subject astronauts to potentially lethal doses of radiation.

While there’s not much we can do to prevent a surge of powerful solar radiation from striking Earth, there are steps that we can take to mitigate the damage. The first step is predicting when one might hit, as it takes at least 12 to 15 hour before it strikes Earth. For this specific purpose the NOAA Space Weather Prediction Centre is constantly monitoring the Sun. ESA is working with scientists across 14 European countries on developing a warning network as well.

Recently, the White House’s National Science and Technology Council has released a National Space Weather Action Plan, in which it warns that with electrical systems becoming increasingly interconnected, if one were to be knocked out, it could cause a cascade of system failures. It recommends a federally-coordinated approach to a number of procedures, from reducing the vulnerability of those infrastructures deemed most at risk, to increasing forecasting and communications abilities.
(November 2016)