How a St. Patty’s Day solar storm turned on the Northern Lights

On Sunday, two giant bursts of ionized gas spewed forth from the sun. For two days, these charged particles hurled toward Earth at millions of miles per hour, eventually colliding with magnetic field in the Earth’s upper atmosphere that stretches from the upper atmosphere deep into space. The result was a severe, but ultimately harmless, geomagnetic storm and a spectacular show of lights across the Northern Hemisphere.

From Scotland to Alaska, during Tuesday’s pre-dawn hours, people reported seeing the Aurora Borealis glowing bright green, appropriate for St. Patrick’s Day.

Scots enjoy stunning display of #NorthernLights as #AuroraBorealis transforms the skies http://t.co/ygcbW92iRb pic.twitter.com/t2ztUq8JSF

— ScotlandNow (@ScotlandNow) March 18, 2015

But how does an eruption from the sun cause a brilliant light show on Earth? And what is the source of the colors?

Think of Earth’s atmosphere as a neon light bulb, and solar wind as a giant battery, said Terry Onsager, a physicist at the Space Weather Prediction Center in Boulder, Colorado. When electrical current flows through a neon light bulb, the atoms in the gas get excited. As they settle down, they glow.

Did anybody manage to see the Northern Lights last night? #AuroraBorealis pic.twitter.com/I7OpDehDQP

— Clark Weightman (@clarkweightman) March 18, 2015

Like a river flowing across rocks in a riverbed, solar wind blows constantly around Earth, shielded by its magnetic field, Onsager said. And when the magnetic field of the solar wind is polar opposite to that of the Earth, the two connect, creating electrical current, just like the opposite ends of a magnet. Then through a series of complex processes, electrical current can flow between the solar wind and Earth’s atmosphere.

Imagine Earth as a bubble with this flow on the edges and the edges like the terminals of a battery.

At a distance of 62 to 300 miles above the Earth’s surface, the atmosphere is just thick enough for the current from the charged solar wind to flow easily. As it flows, it energizes the particles in the atmosphere, and they glow like a light bulb, he said.

But there’s more. The colors of the Northern lights correspond to different atoms. Oxygen atoms glow green and yellow; nitrogen produces the brilliant reds and blues.

Kind of have to post one picture from the solarstorm yesterday #Iceland #AuroraBorealis #NothernLights pic.twitter.com/8zRihFcxx3

— Bragi Kort (@Bkort_photos) March 18, 2015

Below 62 miles, the atmosphere is too thick for the current to continue, so the current zig-zags sideways, zipping along the atmosphere and back out into space along Earth’s magnetic field, Onsager said.

“Often when you look at the Aurora, it appears to be vertical curtains,” he said. “Where you see the Aurora, that’s the footprint of that magnetic field line carrying the current out into space.”

Did you catch the #AuroraBorealis last night? @Spicey_Spiney did from Tackley here in Oxfordshire! She's on at 1415. pic.twitter.com/5OC6Z41bLR

— BBC Oxford (@BBCOxford) March 18, 2015

There’s always some current from the daily solar wind flowing through the atmosphere, he said, but it’s usually too minimal to generate much light compared to the moon and the stars. But when coronal mass ejections occur, they generate a stronger magnetic field, and travel millions of miles faster than a normal solar wind. That makes for a stronger “battery”, Onsager said. And the stronger the battery, the brighter the show.

But if the polarity of the solar wind changes, he said, it flips the lights off like a switch.

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