The northern lights are expected to be visible across several U.S. states this week, according to the University of Alaska Fairbanks.

On Wednesday and Thursday, the Kp-index, or planetary index, will be at five and six, respectively. The scale goes all the way up to nine, and any occurrence above five is considered a geomagnetic storm.

The aurora borealis could be seen across parts of Washington, Idaho, Vermont, Wyoming, New Hampshire, Montana, North Dakota, South Dakota, Minnesota, Nebraska, Massachusetts, Maryland, Ohio, Wisconsin, Michigan, New York, Maine, according to the university.

On Wednesday, the storm will be highly visible "low on the horizon from Seattle, Des Moines [Iowa], Chicago, Cleveland, Boston, and Halifax [Nova Scotia]."

On Thursday, the storm will get stronger and can be seen overhead in Minneapolis, Milwaukee; Bay City, Mich., and on the horizon in Salem, Mass.; Boise, Idaho; Cheyenne, Wyo.; Lincoln, Neb.; Indianapolis, and Annapolis, Md.

What happens during a geomagnetic storm?

During the storm, a coronal hole (the spots that appear black on the sun) prompts high winds, which in turn, trigger coronal mass ejections, or CMEs. A CME projects plasma and pieces of the sun's magnetic field into the atmosphere.

"CMEs typically take several days to arrive at Earth, but have been observed, for some of the most intense storms, to arrive in as short as 18 hours," NOAA's Space Weather Prediction Center says.

What is an aurora?

The sun's activity is volatile, and in some cases, the disturbances are so strong they can pull the Earth's magnetic field away from our planet.

But, like a taut rubber band when it's released, the magnetic field snaps back, and the force of that recoil creates powerful ripples known as Alfvén waves about 80,000 miles from the ground. As those waves get closer to Earth, they move faster thanks to the planet's magnetic pull.

Sometimes electrons hitch a ride on these superfast Alfvén waves, reaching speeds as high as 45 million mph as they hurtle downward.

"Think about surfing," Jim Schroeder, an assistant professor of physics at Wheaton College who has led research on the process. "In order to surf, you need to paddle up to the right speed for an ocean wave to pick you up and accelerate you, and we found that electrons were surfing. If they were moving with the right speed relative to the wave, they would get picked up and accelerated."

When the electrons reach Earth's thin upper atmosphere, they collide with nitrogen and oxygen molecules, sending them into an excited state. The excited electrons eventually calm down and release light, which is what we see as the aurora.

How to view the aurora

The Space Weather Prediction Center says the best time to view the aurora is usually between 10 p.m. and 2 a.m. local time.

You don't need any special equipment to see auroras.

• Pick a spot where there is little light pollution.
• Get to a higher elevation if possible.
• Check the forecast for signs of clouds or precipitation, which could block your view.
• Scan the skies — while northern is in the name, they can appear from all directions.

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