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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

February 1999

When conditions are right, a wave of color and light may wash over the northern skies. © Scott Nielsen

Celestial shimmer

The northern lights are crackling in a sky near you.

When conditions are right, a wave of color and light may wash over the northern skies.

© Scott Nielsen

by Robert "Dock" Stupp

How to recognize the northern lights
When and where to see the lights

The science behind one of nature's most spectacular mysteries is demonstrated daily in one of life's most mundane chores – laundry! As your clothes twirl around in the dryer and rub against each other, static electricity builds up. A sock sticks to a T-shirt. When you pull the shirt and sock apart, electrons crackle and spark, then snap back to the way they were.

Similarly, protons and electrons whipped up in solar winds smash into the gases of the Earth's atmosphere. The atoms become "excited" – they move faster, and some of the gases in the atmosphere, like oxygen, become agitated. Electrons held in orbit around these oxygen atoms skip away from their centers. When they snap back, light is given off. The light given off by billions of such atoms colliding with air molecules in the atmosphere creates a colorful light display, an aurora.

In 1621 French scientist Pierre Gassendi referred in his writings to the lights in the northern sky as the aurora borealis, or northern dawn. Aurora was the name of the Roman goddess of dawn; Boreas was the god of the north wind.

The direction "north" is the key, for every aurora needs three things: particles from the sun, atmospheric gases, and a powerful magnet. Here in northern Wisconsin we are near such a magnet, the North Pole. (There are also southern lights, but we will concern ourselves with the ones you can see without traveling well south of the equator.)

The sun's tremendous heat is the source of its powerful magnetic field. Temperatures in the atmosphere just above the sun (the corona) can reach several million degrees. At such temperatures gas particles collide so violently that the atoms split into electrons and protons. These high-energy particles (called plasma) escape through holes in the sun's magnetic field. As the particles escape, they are thrown out by the rotating sun in an ever-widening spiral. This is called the "garden-hose effect," from the similar pattern of water droplets formed if one swings a garden hose around and around above one's head.

This solar plasma escapes as a stream of particles with enormous velocity. As the particles speed through space, they act like miniature magnets. In two to five days, when they reach Earth's magnetic field, the two forces push against each other like opposite poles of two magnets. The sun's particles sweep around Earth's field and fan out like water passing around a rock in a stream.

When the solar particles arrive at the far side of the rock – Earth's magnetic field – many of the particles will be trapped in the field. Some particles flow almost around the end of the field. Others spiral down toward the North Pole.

There they collide with oxygen, nitrogen, argon and other gases, releasing some of this energy as light – the aurora borealis. We see the auroras near the Earth's poles because the magnetic fields are open only at the poles. At lower latitudes near the equator, the magnetic fields are "locked."

How to recognize the northern lights

Sometimes the lights look like a faint glow in the sky. At other times, the lights appear as a sparkling curtain, a twisting, twirling ribbon, or pockets of color. The colors undulate across the sky. Arcs, streaks, and whirlpools may splash the evening sky with reds, greens and, sometimes, dark purples in a spectacular colorama.

Where do the colors come from? The answer lies in the atoms and molecules of gases at different levels of the atmosphere.

We see these same colors on the ground in neon signs. By removing all the air from inside a glass tube, we can fill the void with a small amount of different gases found in the atmosphere. We seal the tube and wire it for electricity. Then, just as in an aurora, a stream of electrons is sent through the tube. When the stream strikes the gas, the electrons in the gaseous atoms get excited. As the atoms and molecules calm down, they snap back to their normal state and release energy which creates a glowing red light.

Neon produces a red light, but so-called neon lights do not always contain neon. If a mixture of argon and mercury is used, a green, blue, or gold light will shine from the tube.

Red is the result when electrons and
oxygen atoms collide very high in the sky.

© Scott Nielsen
Red is the result when electrons and oxygen atoms collide very high in the sky. © Scott Nielsen

With the northern lights, however, the colors we see depend not only on the gases that are struck, but also on how far they are from Earth.

If electrons in solar winds spiraling down towards Earth strike oxygen atoms at 150 to 100 miles from the ground, a deep red appears in the dark night sky.

Closer to Earth, but still much higher than jet airplanes travel, oxygen atoms flash a greenish white, greenish yellow, or green light when struck. Because oxygen atoms are more "excitable" than other atoms, this is the most common color we see.

When and where to see the lights

The most spectacular displays usually appear around midnight. Your chances are enhanced at times of a new moon or moonless cycle when the sky is clear. Here's a tip: If you hear a weather report mention that flares are exploding on the sun, set up watch two days later. If this is the case, you may see splashes of color earlier than midnight.

For more enthusiastic viewers, a phone recording at the Space Environment Service Center in Boulder, Colorado – (303) 497-3235 – can help you decide when to set up a night watch. The recording gives callers a forecast of solar activity and geomagnetic activity including a "K" index. This is a measure of the disturbance in the Earth's magnetic field. If the "K" is five or higher, you can count on seeing the lights well south of Canada.

The best place to watch for a beautiful light display is on top of a hill in the countryside, away from city lights. If you are on or near a lake, look toward the horizon. A glow may appear and you may dismiss it, thinking it is only light from town. Stop and wait! Keep watching and you'll be in for a colorful surprise.

For Wisconsinites, the closer you are to the northern half of the state, the greater your chances of seeing the northern lights.

A kaleidoscope of adventures awaits the night sky viewer. The wavy motion is mesmerizing, and perhaps will bring you back to a time when the Menominee people of Wisconsin believed that the lights were torches used by great friendly giants of the North to help them spear fish on a moonless night.

What will you picture in the northern lights? Use your imagination, adjust your daylight eyes, and prepare yourself for a magnificent midnight surprise.

Robert "Dock" Stupp writes from Crystal Falls, Michigan.