Curtains of Light in the Night Sky: The Northern Lights (Photo Credit: Sharon Sullivan)

Northern lights lit up the night sky over Mendenhall Glacier in Juneau, Alaska shortly after 4 am, November 2017.

30,000 years ago, early humans in France illustrated natural lights in the sky through cave paintings. The Inuit of Alaska believed that the lights were the spirits of animals they hunted. As early as the 1600’s, Galileo Galilei used the name “aurora borealis” to describe the phenomenon, after the mythical Roman goddess of dawn, Aurora, and the Greek name for the wind of the north, Boreas. Particles may escape the sun from sunspot regions on its surface, sending charged particles into space and toward Earth (known as the solar wind). These particles interact with the Earth’s magnetic field and are funneled toward the poles, where they set off colorful displays of light (aurora borealis in the Northern Hemisphere or aurora australis in the Southern Hemisphere).

 Auroras can come in a variety of colors from pink, green, blue, violet, white, and most commonly green. Green lights are produced when particles collide with oxygen and may appear above 150 miles of the surface. They can be seen in arcs, rays, streamers, bands, or rippling curtains. Auroras are even thought to make sounds similar to radio static or hissing during strong displays due to charged particles that rapidly discharge (like static electricity) when they slam into the Earth’s atmosphere. 

Magnetometers around the world constantly measure the effect of the solar wind on Earth’s magnetic field and give an indication of auroral activity up to 30 minutes in advance. In order to know your chances of seeing an aurora, the Kp index (from German “Kennziffer Planetarische” or “planetary index number”) gives a range of auroral activity on a scale of 0 to 9 for a 3-hour period. Anything above a Kp5 indicates a geomagnetic storm, which can interfere with GPS signals and even the electrical grid. When geomagnetic activity is low (Kp1-2), auroras are typically located around 66 degrees magnetic latitude and may expand toward the equator as activity increases. To see the Northern Lights in parts of southern Canada and southeastern Alaska, generally a Kp3 level is needed and anything greater than Kp6 for the Boston area to have a chance of seeing this amazing light show. (Winter is usually the best time to see them, due to lower levels of light pollution and the clear, crisp air). The Kp forecast for the rest of this week shows low auroral activity (Kp2-3), with a high chance of auroral activity to kick off the week.

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© 2018 Meteorologist Sharon Sullivan

Northern lights over Juneau, AK, September 2018.