In Depth: The Sun and Superflares

How trees hold the key to understanding the Sun's past - and future.

Coronal loops arc over the surface of the Sun, shortly after a solar flare erupted. Credit: NASA/GSFC/TRACE. Image shared under CC BY 2.0.

It all began on October 22nd, when a part of the Sun suddenly cooled and darkened. A cluster of sunspots appeared, blemishing the face of our star and slowly growing in size. Over the span of a week astronomers tracked it drifting across the disk of the Sun, until, on the 28th, it was pointing almost directly at Earth.

Then, as the Sun shone over the Americas, the cluster erupted in a violent flash of light. Solar monitoring satellites picked up a powerful burst of X-rays, a signal that they instantly beamed back to Earth. The eruption threw out a cloud of solar plasma: a swarm of energetic particles rushing towards our planet.

NOAA, the organisation that watches the Sun, categorised the event as an X1 solar flare - the highest level possible on their scale. Aurora, they knew, would certainly light up the planet’s skies. Satellites, bombarded with radiation, could spin madly out of control. Radio networks across South America would briefly collapse, suffocating under the power of the Sun.

Chances are, however, that you had absolutely no idea – apart, perhaps, from a few news stories – that anything untoward was happening. Flares, even fairly big ones, are short-lived events; a sudden surge of energy washing over the planet, disappearing almost as soon as they come. Most - those classed as M or C - are little more than ripples; inconsequential events that endanger no one.

The further up the X class you go, however, the worse the impact can be. An X2 flare has twice the power of an X1; an X3 has three times the power. Since the scale is open-ended there is, in principle, no limit on how high the number can get. The worst we’ve ever seen - in a few decades of watching - came in at X28 - almost thirty times stronger than October’s flare.

So bright was it, however, that the satellite watching - GOES - was left temporarily blind. Its true power may, therefore, have been far higher, perhaps hitting as high as X40 or X50. On the day it erupted - bursting out from the Sun at thousands of miles per hour - the Earth was fortunately placed far from its path. The flare, the most powerful we’ve ever seen, never even came close to hitting us.

If things had been different – as they almost were in 2012, when a massive cloud of solar plasma came within days of hitting us – the consequences on Earth could have been catastrophic. A powerful pulse of magnetic energy would have swept across the planet, sending wild currents surging in power girds and communications networks.

Transformers – vital components of the electrical system – would overheat and fuse, leaving entire continents without power for weeks. In a worst case scenario the Internet itself might collapse, creating a global communications blackout lasting months. The implications for our globalised planet are almost too awful to ponder.

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In 2010, a few years after GOES saw the Sun erupt in a powerful flare, another telescope was carefully watching a faraway star. In many ways that star resembled our own. It burnt yellow, just as the Sun does, spun at roughly the same rate, and seemed, more or less, to be made of the same kind of stuff. In other words, the star – KIC9944137 – was, like the Sun, astronomically boring.

That, astronomers were hoping, meant that the star might have planets in orbit around it; planets, indeed, that might look something like Earth. The telescope – Kepler – was directed to check it out, part of a regular monitoring of thousands of stars that may host planets. If it did, Kepler would see a repeated pattern of dimming, a sign of a planet moving across the disk of the star.

On March 1, however, KIC9944137 did something unexpected. Instead of dimming, it suddenly brightened. A massive burst of energy – a dozen times more powerful than anything ever emitted by our Sun - erupted from its surface and raced outwards. We have no idea if any planets stood in the path of that energy – but if they did, they were surely battered by a magnetic and radioactive storm unlike any we could have imagined.

Had such a storm hit Earth, the consequences would have been horrific. Swept by intense magnetic energy, the power systems of entire continents would fall silent; vital and expensive equipment devastated by surging electrical power. Satellites – from GPS to television – would be destroyed, their circuits fried by an overwhelming burst of radiation.

KIC9944137 is not the only sun-like star we’ve seen erupt in violent flares. Time and time again astronomers have spotted them bursting out in rage; pouring scorn and fury on their solar systems. That such events are possible is not in doubt. The only question is whether our own star is capable of doing the same.

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This, however, is not an easy question to answer. Unlike many other natural disasters, solar flares leave little trace of their passing. There is no impact crater, no swathe of devastated land. Their occurrence – at least in the technologically innocent past – would have been marked only by strange lights shimmering in the night sky.

And though astronomical records stretch back millennia – ancient civilizations watched the night carefully – they rarely mention aurora. Even when they do, the recordings are not clear. Ancient Babylonian texts speak, for example, of a night two and a half millennia ago when the sky glowed red and priests foretold omens of the future. Was that really an aurora? Or did it glow red because of fire, or ice crystals glittering high in the atmosphere?

It turns out, however, that an accurate and detailed record of the Sun’s activity has been kept – just not by humans. Instead the record lies buried in the hearts of trees and trapped deep in the polar ice. There, thanks to the interaction between the Sun and the atmosphere, traces of ancient oxygen and carbon can reveal the Sun’s distant past.

That story begins high above the Earth. When a stream of charged solar particles flow towards our planet – as happens almost every day, as part of the solar wind – they first encounter the magnetosphere. This – a vast magnetic shield extending far out into space – traps the particles, directing them towards the polar regions.

The result is the aurora: colourful displays of light dancing far above the ground. A second consequence, however, is a slight rise in levels of carbon-14, a radioactive form of the carbon atom. This, in the form of carbon dioxide gas, tends to make its way into plants and trees, or ends up trapped in bubbles frozen into the polar ice sheets.

Over the centuries, then, trees build up a record of how much carbon-14 is present in the atmosphere.  Rises and falls follow the steady cycle of activity the Sun traces – a peak every eleven years, a trough five years later. Every now and then, however, that steady cycle is punctured by a sudden spike, the mark of a big solar flare.

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In 2012 Fusa Miyake, a researcher at the University of Nagoya, took a delivery of two Japanese cedar trees, each of which lived and died during the eighth century. In most of the tree rings the carbon-14 levels were steady, a sign of normal solar activity. In one of the rings, however, she saw an abrupt spike, a spike bigger than any other seen.

Something enormous, it was obvious, must have happened in the year 775. Two possibilities stood out: either a massive solar storm, one far bigger than anything we’ve seen before, or a supernova, one startlingly close to Earth. Since nobody noticed a supernova erupting in the sky – and one like that would have bright; bright enough to outshine the Moon – the solar storm seems a likelier story.

Later studies, based on trees collected from around the planet, confirmed the event. Something big really did hit the Earth in 775, something that appears to have been unmatched in at least the last few thousand years. Something, indeed that was fifty times stronger than any solar flare we have ever seen in modern times. Could it, perhaps, have been a superflare, erupting violently from the Sun?

Even if it was, however, the event seemed unmatched in recent history. Studies of tree rings stretching back two thousand years or so showed only one other event - in the tenth century - that even come close. Nobody alive at the time seems to have noticed the flares - or if they did, they certainly didn’t realise what was happening.

Earlier this year, however, researchers pushed the solar record back by almost ten thousand years - reaching the end of the last ice age. Two more events, both enormous and ancient, turned up: one in 7176 BC and the other in 5259 BC. More, the researchers think, are probably buried in the data, or lost in fallen trees sunk deep into ancient swamps.

In all, astronomers now reckon such big flares could happen as much as once every four hundred years. That – while not common – is frequent enough that we should worry. After all, our industrialised and wired-up world has already been around for two centuries. Our time may soon be up.

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It would start with a warning flashed far from Earth. A handful of satellites maintain a watchful vigil on our Sun, monitoring its every twist and turn. When the flare erupts those satellites will see it, catalogue it and send a message back home. A superflare would be unmissable, a flare so enormous it would blind our satellites – offering, in other words, a brief warning that something big was on the way.

Within hours a surge of x-ray radiation would smash into the planet, disrupting and heating the upper atmosphere. Moments later, a tsunami of high energy particles would surge across the magnetosphere, lighting up skies around in planet - in places so bright that night would turn to day. Even at the equator – a place that rarely sees aurora – shimmering lights would dance in the sky.

The beauty of that display, however, will come with a terrible price. A magnetic storm of unmatched intensity would rip across the planet, sending electrical grids into chaos. Wild currents would overwhelm transformers, knocking them – and the rest of the system – offline. The electrical systems that power the Internet – especially the amplifiers that boost signals making long ocean crossings – would fry, and with them the Internet itself would be in danger.

Riding in the storm’s wake would be a cloud of radioactive particles, each of which the Earth’s magnetic field would send hurtling towards the poles. Astronauts would be forced to run for shelter, desperately praying their radiation shields can block the storm. Satellites would fail, or go mad; their computer chips garbled by the intense radiation. Carbon-14 levels would spike - a signature that would, before long, leave a permanent mark in trees across the world.

The longer-term consequences would be even worse. Without electricity, nuclear power station cannot keep fuel rods cool, and that – eventually – leads to explosions and nuclear fallout. Transformers themselves are expensive and difficult to replace. Repairing dozens of damaged ones would take months, or perhaps years; leaving entire nations to face the prospect of life without reliable supplies of electricity.

The damage to global communications systems and satellite networks would lead to financial chaos, and then – as messages slow to a crawl – to supply chain disruption. An economic depression would be almost inevitable, civilization itself may be in peril.

That of course, is only if the flare is not too bad. The superflares seen around other stars can be utterly devastating events, powerful enough to destroy the ozone layer and cause a mass extinction of life. Nothing like that has ever been seen in the historical record – but nothing, either, yet rules out the possibility.