We have a pretty good template for this one, actually, because the Earth was hit with a very powerful solar storm in 1859. The solar flare that started it off was observed by Richard Carrington, who had a history of sunspot-watching, and his name got attached to it, and so we know it now as the Carrington Event.
1859 is an interesting time; our scientific understanding of the Sun and aurora was advanced enough to recognize basically what had happened, but we humans hadn’t become as reliant on sensitive electronics during our daily lives, so the electromagnetic disruption was not too severe. (A Carrington-scale event occurring today would cause much more severe disruption. You would notice. A lot. More on that later.) 1859 was also recent enough that we have a lot of records of what people saw that evening.
Even with a minor solar storm, if you’re far enough North or South (in other words, if you’re close enough to magnetic north and magnetic south on our planet), it’s not too hard to spot the aurora on a clear night. It helps if there’s no moon, but with dark skies you can spot them without too much trouble. This is one reason some of the most spectacular recent images of the northern Lights tend to come from places near the arctic circle, like northern Scotland, Sweden, Norway, and Alaska. (You can also get excellent images from space, but that's cheating.) It’s harder to spot the Southern lights with the same ease, but that’s just because no one lives in Antarctica, and the next most habitable places are farther removed from the southern pole.
As you crank up the strength of the solar storm, the auroras become visible to more and more of the planet. The Carrington Event’s aurora were visible for three nights, and not just in the North. There are records of people seeing the aurora from New York all the way down to New Orleans, and from many cities in between. And it didn’t stop there - Jamaica and Cuba were also in on the show. In fact, the more you dig into historical records, more and more of the planet appears to have seen the light show in the sky. In 2015, a paper digging into historical records in South America found a record of a bright, S-shaped light in the night sky, which woke up the people living in a town in Columbia. Columbia is only 8 degrees north of the Equator, which means that if they could see it, a huge fraction of the planet was likely exposed to the auroras if their skies were clear.
There are records of a golden or fiery red aurora in China and Japan from the same solar outburst; a number of diary & local news accounts tell of a strange sky that evening. To be specific, the main “Carrington event” is considered to be the main wave which hit the planet while it was nighttime for the Americas and Europe, but there was a bit of a second wave, which hit during the night for Asia. Unfortunately, it appears to have been cloudy in Korea, and possibly cloudy over the middle of Japan as well - the most spectacular light show yet recorded can still be stymied by clouds. (Ask anyone who tried to watch the solar eclipse in the UK in 2015 through thick cloud.) From Japan, the auroras appeared to be a massive fire on the horizon - and many of the writers appeared to have been waiting for news of a giant fire which never materialized.
The aurora would be the least of our concerns if another Carrington-sized solar storm hit us. Auroras are the glow of ionized gas in our atmosphere, the result of a severe bashing by an influx of charged particles from the Sun. Because there’s so many charged particles moving around, they induce a current on wires below them. And this induced current is under no obligation to be small and manageable. In 1859, this meant that people’s telegraph equipment was shocking them, or working even though it had been unplugged, or not working at all.
We use a lot more electronic equipment than we did in 1859. We have a lot more GPS and communication satellites now, which underpin everything from your cell phone’s ability to check the best path between you and your home, to your plane’s ability to track itself through the sky, to some credit card purchases. And, as a solar storm which impacted Quebec in 1989 proved, our power grids do not like extra current; Quebec’s power grid (which has since been improved, I should note) went down in 90 seconds and stayed down for 9 hours, keeping millions of people out of power.
The US in particular is generally considered to be very poorly prepared for the sort of electrical damage that a power surge from a strong solar storm could impose. A report from the National Research Council, produced in 2008, estimates that a Carrington-style event would put more than 130 million people out of power, which, given the population of the US at the time, means that 43% of the population of the US would be out of power. They estimate that $1-2 trillion dollars would be lost (either through cost of replacement or via people not being able to work), which would take 4 to 10 years to recover from.
A “best-case” scenario for weathering a giant solar storm means lots of backup systems in place for our power grids, the ability to handle sudden weird influxes of power, and the monitoring power to know when they’re coming. This requires a healthy set of satellites monitoring our Sun - we’ll only ever have a few day’s warning, but it’s better than no warning - we might be able to set satellites into safe mode, make sure the power grids are able to handle it, and switch off non-vital electronics that can’t handle it.
As nice as it is to dream about a theoretical, bright, planet-wide aurora (from an aesthetic perspective), the reality of those aurora mean that they are harbingers of large scale electronics failures, both on the ground and orbiting above us.
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