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A huge solar storm created dazzling auroras worldwide on St. Patrick's Day

Aurora borealis seen in Legal, Alberta, Canada on March 17, 2015.
Aurora borealis seen in Legal, Alberta, Canada on March 17, 2015.
Jeff Wallace/Flickr

On Tuesday, lots of people across the United States and Europe got to witness truly spectacular auroras in the sky after sunset:

The northern lights over the still-frozen Lake Ontario, taken from western New York. (darosenbauer/Flickr)

Normally, bright auroras are only visible close to the poles — in places like Alaska or Iceland. (That's why they're colloquially known as the Northern Lights.) But on Tuesday, people could see the lights as far south as Illinois, New York, New Jersey...

The International Space Station also got an eyeful:

Auroras were also visible in the southern hemisphere, in parts of Australia and New Zealand. You find see many, many more pictures on SpaceWeather.com.

Why did this happen? Auroras typically occur when charged particles from the Sun are deflected along the Earth's magnetic field and collide with gas atoms in our upper atmosphere, causing those atoms to emit light. Every now and again, however, the Sun will flare up and send a lot of charged particles our way, causing especially vivid auroras that can be seen much further south.

That's what happened this week. On Sunday, March 15, the Sun fired off two coronal mass ejections — fast-moving clouds of charged particles — that hit Earth's magnetic field on Tuesday and caused a G-4 geomagnetic storm (the scale goes from 1-5). It was the strongest solar storm since 2013.

These strong storms can create dazzling auroras but can also potentially disrupt the electric grid and satellite communications. A truly massive geomagnetic storm could potentially cause a lot of chaos — in the worst case, leaving millions without power. Below is a more detailed explanation of where solar storms come from, and why they produce such dazzling displays.

Where do auroras come from? It all starts with the Sun.

The Sun isn't just a static ball of hot gas. It goes through 11-year solar cycles in which its magnetic field fluctuates. When the sun is near a solar maximum — something that's happening now — a large number of sunspots build up on the Sun's surface, near the mid-latitudes.

Sunspots are basically places where the Sun's magnetic field lines have become distorted. Those magnetic distortions inhibit convection, preventing some energy from reaching the Sun's surface and creating spots that are a bit cooler than the surrounding region (though they're still extremely hot, at 2,700°C or more). That's why sunspots are visibly darker:

A gigantic sunspot – almost 80,000 miles across --can be seen on the lower center of the sun in this image from NASA’s Solar Dynamic Observatory captured on Oct. 23, 2014. This active region is the largest of the current solar cycle. (NASA/SDO)

Every so often, these areas of the Sun erupt into massive solar flares. When those tangled magnetic lines reconnect, they cause an acceleration of charged particles that interacts with the Sun's plasma and can release a staggering amount of energy — NASA estimates that big flares produce "the equivalent of millions of 100-megaton hydrogen bombs exploding at the same time."

These solar flares can come in different sizes, and usually occur near active sunspot regions. A really behemoth sunspot that appeared in 2014, called AR 2192, fired off six large X-class solar flares and four smaller M-class flares.

An eruption on April 16, 2012 was captured here by NASA's Solar Dynamics Observatory in the 304 Angstrom wavelength, which is typically colored in red. (NASA/SDO/AIA)

These solar flares send energized particles hurtling out toward space at extremely high speeds — they can reach Earth in about eight minutes. The magnetic field around the Earth tends to protect us from most of these particles. But large X-class flares can trigger radiation storms in Earth's upper atmosphere that can disrupt radio communication.

That's not all. Some truly massive solar flares are also followed by what's known as a coronal mass ejection (CME) — when the Sun essentially shoots out part of its upper atmosphere. This NASA video captures in fascinating detail a coronal mass ejection leaving the sun on July 23, 2012:

During the high point of its solar cycle, the sun can fire off as many as five of these CMEs per day. Most are aimed far away from Earth. But every so often, one is pointed our way, and a fast-moving cloud of charged particles can reach us within 1 to 5 days.

When those charged particles hit Earth, they create auroras

When one of these coronal mass ejections does hit Earth's magnetic field, strange things start to happen.

First, the charged particles from the Sun are carried along the magnetic lines of the Earth's magnetic field. As those particles collide with gas atoms in the Earth's upper atmosphere, those atoms emit light, creating auroras. Here's a stylized graphic from NASA, though not to scale:

Not to scale. (NASA)

Most of the time, these auroras are relatively small, visible only in places like Alaska or Iceland.

But really severe geomagnetic storms caused by major coronal mass ejections can create auroras that are visible much further south. That's what happened on Tuesday.

Geomagnetic storms might also mess with our power grids

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A 2012 blackout in New York City after Hurricane Sandy. (brklyn is dead/Flickr)

Then again, it's not all good news.

If a very large coronal mass ejection hits the Earth's magnetic field in just the right way, it can also induce a strong ground current that can travel through power lines, pipelines, and telecom cables. If those currents are large enough, they can overload electric grids — which is exactly what happened in Quebec in 1989, when a geomagnetic storm fried the grid for several hours.

Some experts have warned that if a truly massive storm ever hit us, it could fry a significant number of high-voltage transformers. Those can often take years to replace, as many weigh up to 400 tons and are custom built, with intricate supply chains. In the meantime, millions of people could go without power.

Risk-of-outage

Lloyd's of London

In a worst-case scenario, a massive solar storm could leave 20 to 40 million people in the Northeast United States without power, according to a sober assessment last year by Lloyd's of London. (The region is one of the places most at risk thanks to its aging power grid and unique geologic features.)

The good news is that we're not totally helpless. As I've detailed before, businesses and government agencies have been devising plans to cope with disruptive space weather — from hardening power grids to rerouting flights that might get disrupted by geomagnetic storms. But even so, it's hard to protect against a truly massive storm, and it doesn't help that we're about to lose some key observational satellites.

Further reading: NASA: Earth almost got hit by a solar storm in 2012 that could have left millions without power.