Auroral Rays

One of the aurora's mysteries that took a long time to solve was the nature of the ray structure often seen in auroral arcs and bands.

Standing in the aurora like pickets in a fence, the rays sometimes move sideways across the arcs and bands at high speeds. Sometimes one even sees them appear to move past each other both to the left and the right.

Rays line up along the direction of the earth's magnetic field, which points nearly vertically and somewhat to the northeast over Alaska and western Canada. To recognize the cross-sectional shapes of the rays, one needs to see them directly overhead in the sky. When they are in that position, they don't look like rays anymore; one reason why it took so long to discover their true shapes.

Not until very sensitive, high-speed television cameras were aimed at the bottoms of rays overhead was the mystery resolved. This solution to the enigma yielded a masters degree for now-a-Ph.D. Tom Hallinan of the Geophysical Institute. He found that the rays were tightly wound up spirals only a kilometer or two across. Their form is difficult to recognize with the naked eye because the curled up shapes develop so quickly--sometimes in a second or so--and they often move very rapidly.

With a television camera capable of taking 30 pictures each second, it was possible to record the development of the spiral-shaped rays and measure their motion. Sometimes they move across the sky at speeds one hundred times that of a jet aircraft. To the observer on the ground, they do not appear to move quite that fast because the rays are so far away.