It began as an ordinary summer's evening. But by 8 o'clock on 29 August 1859, the residents of New Orleans knew something was up. The sky was glowing green and sparks were falling like rain.
Soon the blaze turned blood-red, illuminating streets, houses, and sombre faces. The light show was visible from Hawaii to London.
Some people thought it was a message from God, or an omen of famine, or revolution. The more scientifically-minded guessed it had something to do with the weather. Perhaps it marked the beginning of a cold snap.
In fact, it was a new kind of weather. Just 17 hours earlier in England, astronomer Richard Carrington had been sketching spots on the Sun's surface when a patch of intensely bright white light appeared, then vanished.
Carrington knew he'd seen something big, and the timing was uncanny. But at first he was hesitant to link the two events. An electric light show from space, powered by a storm on the Sun, was just too incredible.
Eventually he did draw the link, and over the years it became clear that he was right. We like to think of the Sun as an unwavering beacon, beaming steadily from day to day, century to century. Carrington showed that the truth is far more spectacular and violent.
The blaze that lit the heavens was caused by a super-explosion on the Sun, equivalent to ten billion atomic bombs. In our solar system, it was the biggest stellar outburst of the last 500 years.
Everyday fluctuations in the Sun's activity aren't a problem, because the Earth is protected by its magnetic field
Now known as the Carrington event, it was an unusually extreme bout of "space weather": the conditions arising from the Sun's varying activity. Space weather is a hostile world of magnetic storms and solar wind, of electric rain, plasma clouds, and toxic radiation.
According to a UK government report released in July 2015, a repeat of the Carrington event would likely cause unprecedented chaos. It would disrupt satellite communications and GPS navigation, and trigger global blackouts lasting up to a year. That's no radio, television, or phone signal, internet, mains-powered electrical devices, or heating.
Trains and traffic lights would fail. With no GPS navigation to guide them, there would be no flights, and with most of the world's cash stored in electronic databases, very little money either.
In all, it could be devastating to the modern world, amounting to trillions of dollars' worth of damage.
Everyday fluctuations in the Sun's activity aren't a problem, because the Earth is protected by its magnetic field. You might not be aware it's happening, but you certainly wouldn't want to be without it.
Take it from astronaut Chris Hadfield, who spent 145 days on the International Space Station (ISS) from 2012 to 2013, and kept busy while he was up there.
"In space, if you just close your eyes and wait, every so often you will see a flash of light," says Hadfield.
Each flash is a cosmic ray, a high-energy particle that has slipped through the walls of the ISS and crashed directly into your optic nerve, causing it to fire a signal to the brain.
"It might be from our Sun or from any other star in the universe," says Hadfield. "It's travelled all that immense distance and you can actually see the radiation with your own vision system."
Because we're closer to the Sun than to any other star, it sends us a lot more stuff.
In school we're often taught that the Sun is a ball of gas, but technically this isn't true. Really it is made of plasma: gas that is so hot, the atoms have broken apart into smaller charged particles.
Like the Earth, the Sun has its own magnetic field
Plasma is not as exotic as it sounds. It's the same stuff that fills fluorescent lights and plasma television screens – except that, at the Sun's surface, it's over 5,500 °C.
On a calm day, this destructive "vapour" streams off the Sun in all directions, reaching far beyond Neptune – more than 100 times the distance from the Sun to Earth.
Just occasionally, our star presents its more capricious side.
Like the Earth, the Sun has its own magnetic field. It has a magnetic north and south, and field lines running down between them.
Even at their worst, the Sun's flares are not physically capable of destroying Earth
As the Sun rotates the plasma moves too, and the faster currents at the equator distort the magnetic field lines. In sunspots the field lines get really twisted and loops rise through the surface, pulling bubbles of plasma with them. Occasionally the lines become so strained, one will snap.
Cue one of the most dramatic events in our solar system: a solar flare. These monstrous explosions in the Sun's atmosphere fling out radiation. Travelling at the speed of light, the radiation takes just 8 minutes to reach Earth. This was the burst of white light seen by Carrington.
It was followed by a second solar deluge: a coronal mass ejection (CME), a cloud containing up to a billion tons of plasma. It takes several hours for a CME to break away from the Sun's surface, but once free they travel at up to 7 million miles per hour. In 1859, the CME took about 17 hours to hit Earth.
All this pummelling can have some radical impacts here on Earth.
Let's get one thing out of the way immediately. NASA is clear that "even at their worst, the Sun's flares are not physically capable of destroying Earth."
The visual effects of the Sun's power were flowing between my legs
The most visible effects are spectacular auroras, colourful light displays in the sky. They are normally only visible close to the north and south poles, but a CME can trigger them worldwide.
When the plasma from a CME reaches Earth, it slams into the atoms of gases in the upper atmosphere. The impact strips the electrons from these atoms, briefly turning them into plasma. The atoms then recombine, and emit light in the process.
"Normal" auroras are induced by the steady solar wind and occur year-round. They are best appreciated from outer space.
"When I was out for one of my spacewalks we actually went through the Aurora Australis, the southern lights," says Hadfield. "The visual effects of the Sun's power were flowing between my legs."
The other impacts of a solar superstorm are significantly less appealing.
The plasma shock wave affects the Earth's magnetic field, creating a violent distortion called a "geomagnetic storm" that lasts 6-12 hours. As a result, things get electric.
The rapidly-changing magnetic field generates an electrical current in any conductive material near the ground, including telephone and power lines, undersea cables, and oil and gas pipelines. Once the electrical surges reach the power grid, they overload transformers and trip their circuit breakers, causing widespread blackouts.
The blast created a geomagnetic storm and powerful electric currents
In 1859 there wasn't an electrical grid. Instead the storm burned out telegraph wires, the 19th-century equivalent of the internet, all over the world. Pylons threw sparks, telegraph paper caught fire, switchboards blew, and a few unlucky operators were thrown across the room.
On the American telegraph line, electrical currents disrupted the service at several points. Elsewhere, operators found they could disconnect the batteries and transmit their signals via the geomagnetic current alone.
"How do you receive my writing?" read a telegraph from an operator in Boston. "Much better than with the batteries on," came the reply from Portland.
In 2015 the world is very different. Over 80% of people now rely on electric power. What would happen if a solar superstorm hit?
We can get a hint of the answer from a blast with one-third of the power of the Carrington event, which struck in 1989. It doesn't look good.
The blast created a geomagnetic storm and powerful electric currents. These currents found a weak spot in Hydro-Québec's electrical grid and shut down the power for the entire province of Quebec within 2 minutes. The blackout lasted 12 hours, closing schools and businesses, leaving people stranded in elevators, and halting all trading on Toronto's stock market.
A strong geomagnetic current would melt the insides of these transformers and destroy them
A bigger surge might be strong enough to destroy the national grid itself.
The vulnerable components are called extra-high-voltage transformers, which are used to convert the high-voltage electricity from power stations into lower-voltage electricity that we can use in our houses. They are the foundation of civilisation as we know it.
Unfortunately they are also heavy, tricky to transport, and must be custom-designed. They can take months to make and up to a year to arrive, and cost about $10 million each. Understandably, power companies don't keep many spares.
A strong geomagnetic current would melt the insides of these transformers and destroy them.
A 2011 report estimated that a storm half the strength of the Carrington event would destroy 300 transformers in the US alone, leaving 130 million people without power for up to 12 months. Backup generators would only have enough power to keep water and sewage systems going for a few days. After that there would be no clean water or sanitation.
If the stations don't fire their engines, they may simply fall out of the sky
We might also lose a lot of money. Banks store people's cash in databases, so if a geomagnetic current found its way to a data centre it could delete millions of people's savings.
Up in space, the satellites we use for communications, navigation, and weather forecasting are at risk. Plasma particles can collide with crucial electronics, disabling them.
Hadfield says this isn't a problem for the ISS. "We're really, really careful with our power. It's something we've thought about and planned for," he says. "If we have a short because of a huge burst of energy, then the solar arrays have ways to disconnect themselves from the batteries".
There's another risk. All satellites in low orbits, including manned stations like the ISS and China's Tiangong-1, could get dragged back down to Earth.
After a big solar event, the extra energy from the Sun swells the Earth's atmosphere, pushing it higher into orbit and thickening the air.
Solar flares emit all kinds of radiation, from visible light to ultraviolet and X-rays
"The furthest atoms out, they actually drag the station down, like tiny little tendrils," says Hadfield. "So we either need to stay further away from the Earth or we need to fire our engines more often to re-boost our orbit and push us up higher."
If the stations don't fire their engines, they may simply fall out of the sky. That is what happened to Skylab, a $2.2 billion space station that preceded the ISS - although it had long since been abandoned. Similarly, the Solar Maximum Mission fell out of orbit shortly after the 1989 storm, which was ironic because it was designed to investigate space weather.
The extra heating also expands a layer of charged particles called the ionosphere, which radio signals back to Earth must pass through. The thicker ionosphere creates turbulence, delaying the signals. For tasks like GPS, which rely on transmissions that are timed to a billionth of a second, this disruption can be devastating.
Then there are the risks to life itself.
Solar flares emit all kinds of radiation, from visible light to ultraviolet and X-rays. This onslaught is usually deflected by the Earth's magnetic field, but in the aftermath of a solar storm the field is weakened – just when the radiation becomes more intense.
People high up in the sky, for instance on planes, get more radiation than people on the ground. After a large solar flare in 2003, one study into the radiation exposure of passengers in the air at the time found that those on polar flights, such as Washington to Beijing, may have received up to 70% of the recommended yearly limit all in one go.
Robins can literally see the Earth's magnetic field
However, this isn't necessarily as bad as it sounds. Recommended doses are deliberately conservative, and it is actually not possible to identify the impacts of such a low dose because they are so subtle. Based on a 2008 study of radiation risks, the radiation from the solar storm would have carried a risk of death from cancer equivalent to driving 186 miles in a car, something most of us would do without hesitation.
Astronauts are more at risk. During the same event, radiation levels spiked on the ISS, so the astronauts retreated deep within the body of the station.
Other life may be affected, too. Many animals are thought to navigate by sensing the Earth's magnetic field. Could a solar storm throw them off course?
Few people have tried to find out but it certainly looks like it. A 2015 study of the impacts of space weather on honeybee colonies found that, over a 3-year period, the number of lost bees increased by a factor of three on storm days.
Some migratory birds are also known to have a magnetic sense: for instance, robins can literally see the Earth's magnetic field. Nobody has studied whether solar storms affect their ability to navigate. Just in case, many pigeon racers call the NOAA Space Weather Prediction Center before big races to check for any unusual solar activity.
It seems clear that another big storm would trigger a whole string of consequences on Earth. But we don't know when it will happen.
The Sun goes through an 11-year cycle, going from a quiet "solar minimum" to a tempestuous "solar maximum" and back again. You might think that a big flare would be more likely at solar maximum, but that doesn't seem to be the case.
The risk of another Carrington event is about 12% over the next 10 years
"There's absolutely no correlation," says Catherine Burnett, the space weather programme manager at the Met Office in Exeter, UK. "Extreme solar events could happen at any time. In fact, Carrington was at solar minimum."
The rarity of super-storms like the one Carrington saw means there's very little data with which to assess how often they occur. However, in 2012 Pete Riley of Predictive Science in San Diego, California, US had a go anyway.
Riley looked for patterns in the severity of milder space weather events and extrapolated to more severe ones. He estimates that the risk of another Carrington event is about 12% over the next 10 years.
The good news is, if one does happen, we will get some warning.
Solar flares travel at the speed of light, so by the time we see them, they're here. The only way to forecast them is by guessing the probability that a sunspot will flare. Because tangled magnetic fields are more likely to snap, we can estimate the risk of a flare by gauging how magnetically "complex" a sunspot is.
If it's oriented southward, it will impact the Earth
Happily, CMEs can take up to 96 hours to arrive. To find them, forecasters scour high-resolution images of the Sun for signs of solar flares, which usually happen at around the same time.
Next the CME's speed and density are modelled using supercomputers, to forecast when it will arrive and if it's likely to be a big one. But it's an imperfect process. "Actually we don't know whether or not it will impact the Earth until it travels almost all the way," says Burnett.
The space weather community has a satellite called the Advanced Composition Explorer (ACE) that can measure the magnetic orientation of CMEs. By the time a CME passes ACE, it may be just 15 minutes from Earth.
"If it's oriented southward, it will impact the Earth," says Burnett. "If it's northward it won't, it will just pass round the Earth's magnetic field and we won't feel anything."
With our reliance on modern technology increasing by the day, we're certainly upping the stakes of another solar mega-event. Without computers, electric lights, or flushing toilets, it could send large parts of the world back to the 18th century. It might be worth stocking up on batteries and stashing some cash under the mattress.