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The Webfooted Astronomer - September 2000

 

The Northern Lights: The Science Behind the Sights

By John Waters

LIKE many others in our area, I witnessed the spectacular Aurora Borealis that occurred on August 11. I was a few miles from Winthrop in north central Washington, spending the time with about 20 others at a cabin. At about midnight a few of us went outside to watch the Perseid meteor shower. The moon was full, but I kept my pessimism to myself. Soon we began seeing meteors, which seemed determined to put on a good show in spite of the moon. It was clear overhead but there were a few scattered clouds on the horizon. Then another fellow and I noticed some clouds quickly forming and disappearing. We soon realized that we were seeing the Northern Lights, although I had never seen them before.

I didn't see any color in the aurora; it just looked like a muted white light. We saw all manner of shapes and sizes of light. I saw serpentine shapes that looked like jet contrails from a commercial airliner that had tried to behave like a stunt plane. There were small forms that looked like typical individual clouds. Sometimes a large curtain-like shape would appear. I remember one extended non-descript form about 15 degrees off the zenith that had a small crisply-defined edge where there was a dark patch, like a dark nebula in the Milky Way. It faded from view and then returned, perhaps three or four times in a period of about five seconds, maintaining the same sharp edge next to the dark area. The whole affair reminded me of a cosmic disco. Illuminated forms would appear or vanish in just a fraction of second in widely separated areas. Don't let a full moon discourage you from observing.

The Science

The Northern Lights phenomenon, known as the Aurora Borealis in the northern hemisphere and Aurora Australis in the southern, are exciting images of the interactions between the streaming solar plasmas and the Earth's magnetic field and atmosphere.

Some of the earliest known references to them appear in the Old Testament, which portrayed the lights as fire raining down from heaven—which, as it turns out, is not that inaccurate. Canadian Eskimos believed the lights were torches, illuminating a pathway to heaven. Even Aristotle and Ben Franklin puzzled over them. We've learned more about them in the last 20 years than in the previous 2,000. But even today scientists don't completely understand the phenomenon.

Here is a summary of what causes this beautiful celestial illumination. The Earth's magnetic field is roughly spherical in shape and extends far beyond the atmosphere into the vacuum of space. The sun spews out charged particles into space called the solar wind. These consist mostly of electrons and protons from hydrogen atoms that have come apart due to the extreme temperatures in the solar coronal region. The solar wind collides with the Earth's magnetic field. The solar wind strength changes, sometimes rapidly, especially during periods of high solar activity. Since it is electrical in nature, the collision of the solar wind with Earth's magnetic field distorts the field greatly. Changes in the solar wind thus cause the Earth's magnetic field to move and change shape. This causes a generator-like effect producing up to 10 million megawatts of electrical power, which ultimately causes the movement of substantial numbers of charged particles in the ionosphere, at about 80-130 km. Particles move best here, as opposed to the atmosphere next to the Earth because a large concentration of the particles exists in the ionosphere. And the number of air molecules is small enough so that collisions with them do not greatly hinder charged particle motion. However this motion still causes a great number collisions involving atoms that have not lost any electrons, as well as ions. A collision may cause an electron in the jostled atom or ion to gain energy and jump momentarily up to a higher energy level. The higher energy level is unstable so the electron soon returns to its normal level, giving up the gained energy in the form of light, which we see as the Northern Lights. Because electrons can only occupy specific energy levels in atoms or ions, only specific energies are released. Thus the Aurora is not a continuous spectrum of colors but rather consists of specific colors. For example a green color occurs when an electron falls back to its normal level in an oxygen atom. Red is caused by a nitrogen atom. An electron returning to its normal level in a nitrogen ion may create blue and violet colors while that in an oxygen ion may be dark red.

Resources

See http://blitzen.sprl.umich.edu/PHAYS/Chap_4/Chap_4b.html
for an interesting map that shows that the maximum chance of seeing the aurora is in a circular band passing somewhat south of the geomagnetic pole, through northern Canada and Finland.

Here are some other good Web sites for Aurora information:
http://abcnews.go.com/sections/science/DyeHard/dye81.html
http://www.uit.no/npt/nordlyset/waynorth/00-innhold.en.html

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