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 that stretches from the Earth’s 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.

But how does an eruption from the sun cause a light show on Earth? And what explains the brilliant 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 light bulb, the atoms in the gas get excited. As they settle down, they glow.

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. 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.

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.”

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.