This is a great time of year to watch for aurora borealis, otherwise known as the northern lights.
Actually, the northern lights don’t really favor one season over another, but you have the best chance to see them this time of year simply because the nights are so much longer.
I wish the northern lights were a more predictable phenomenon, but aurora are still what I call the wild card of the night sky. That’s because they are ultimately caused by storms on our closest star, the sun.
The sun’s 10,000-degree surface is a cauldron of chaos. Constant storms result in sunspots and flares, and really large storms called coronal mass ejections fling out huge eruptions of a highly charged plasma of protons and electrons. These charged particles move like celestial bats out of solar hell at speeds of millions of miles an hour.
These particles interact with the Earth’s magnetosphere to create the northern lights.
Because coronal mass ejections are difficult to predict, it’s hard to know when we’ll see northern lights. The only thing that we do know for sure is that the sun goes through an 11-year cycle of solar turbulence and storms.
The sun is coming out of the quieter period of that cycle, so the chance of seeing the northern lights will increase in the next few years.
When these charged particles reach the vicinity of our planet, the magnetic field of the Earth directs these particles toward the north and south geomagnetic poles.
These geomagnetic poles don’t exactly coincide with the terrestrial poles but they’re close. The northern geomagnetic pole is in far northern Canada.
Once these charged particles get within 70 to 100 miles of the Earth’s surface, they work their magic with our atmosphere. They temporarily disrupt the structure of atoms and molecules, resulting in the discharge of light.
As billions and billions and billions of collisions occur, the brilliant colors of the aurora dazzle us surface dwellers. Most of the time we see greenish white colors when oxygen atoms are excited, but sometimes bluish, purplish and even reddish tinges will appear when the charged particles react with various states of nitrogen in our atmosphere.
Northern lights seem to float across the sky in waves and curtains as they follow the lines of Earth’s magnetic field. Most of the time northern lights are restricted to the northern part of the sky, but during really active displays they can spread all over the sky.
There are several good Web sites you can check that will give some fairly reliable short-term forecasts of aurora. My favorite is from the University of Alaska Geophysical Institute at www.gi.alaska.edu, then click on “Aurora Forecast.”
This usually works well, but I have to tell you that some of the best displays I’ve seen were completely unexpected, and some of the ones predicted never really panned out. To me, that’s what makes aurora so much fun. They’re the wildcards of stargazing.
Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis and author of the new book “Washington Starwatch,” available at bookstores and on his Web site, www.lynchandthestars. com.
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