The Arctic traveler sees the auroral displays as faint curtains of blue-green light stretching across the sky. Sometimes stationary, sometimes slowly swaying back and forth as from a light breeze. Often with considerable detail, where distinct columns of light stretch upward the entire height of the curtain. Features are sometimes so clear and sharp, these curtains appear to be only a few miles away. Our view is somewhat that of an ant looking up at the curtains of a huge stage.
But when we look at the subject in more detail, we find that the bottoms of these majestic curtains are some 100 km above us. The tops of the curtains stretch upwards yet another some 200 to 300 km. The aurora is the largest terrestrial structure we can see from the surface of the earth. And a curtain or series of curtains can extend from the eastern horizon to the western horizon. Satellite studies have photographed a single curtain stretching from Alaska to Hudson Bay, literally across the entire North American continent.
But this magnificent visual display we see is really only a small, almost insignificant, side effect of the big show. Which starts out as a flux of subatomic particles flowing out from the sun, especially during solar flares. A very small fraction of those particles get caught by the earth's magnetic field. These particles are focused and spiral down along the earth's lines of magnetic force towards the North and South poles. As the particles come closer to the surface of the earth, the magnetic field becomes stronger and the spiral becomes tighter and tighter.
A basic law of physics is that changing the direction of motion of a charged particle in a magnetic field causes it to give off electromagnetic radiation - which in turn agitates some nitrogen atoms and they retaliate by radiating light at 5577 angstroms - hence the blue-green auroral signature color. Eventually the spiral becomes so tight something has to give and the particles get reflected and begin spiralling back up along the magnetic lines. They then cross the equator some 50,000 to 100,000 km up and then spiral back down toward the other pole where, a few minutes later, they perform the same frenetic dance and produce another similar spectacular display. After bouncing back and forth from pole to pole several times, and loosing a bit more energy each time, they fade away and let another group come in to continue the performance.
Copyright 1990 (c) E. Stiltner