Auroral Arcs and Bands

The sky provides no guides to give the visual observer depth perception. Consequently an observer of the aurora cannot tell how high the aurora is above him nor can he determine the true shape of the configurations being seen.

However if one knows ahead of time the normal shapes and orientations of auroral forms, an observer can more readily follow and understand the changes that occur in a display, and also better appreciate the true size of auroral phenomena.

Those long arches that extend roughly east-west (actually magnetic east-west) from horizon to horizon are called arcs. If of nonuniform curvature, these forms are called bands. No really meaningful difference exists between arcs and bands, except that the more convoluted form, the band, is often brighter than the arc, and the appearance of bands usually signifies that the overall display is becoming more active.

Arcs and bands are thin ribbons set on edge parallel to the ground. The thickness of an arc or band may be as little as 100 meters (100 yds). The lower edge is typically 80 to 120 km (50 to 75 miles) above the earth and the upper edge is usually 10 to 100 km above that. Off out to the east or west, arcs and bands appear to meet the horizon. Still roughly 100 km above the surface, the aurora there is more than 600 miles distant. Hence, the aurora seen to meet the eastern horizon at Fairbanks is actually nearly directly overhead at Whitehorse, and visa versa. Similarly, an arc or band seen from Fairbanks to be 20 degrees above the north horizon is directly overhead Fort Yukon.

The ray structure often seen in arcs and bands marks out the orientation of the magnetic field, nearly vertical at high latitude. The vertical extent of arcs and bands is also along this direction. Though the rays appear to converge upward, they are, in reality, essentially parallel shafts of light.

If rayed aurora is directly overhead, the point to which the rays appear to converge is the magnetic zenith. A line from that point to the observer marks out the local direction of the earth's magnetic field.