A remaining mystery of the aurora is the reason why the thin auroral arc is the most common shape seen. Arcs stand in the sky like giant ribbons set on edge. In length they often extend from horizon to horizon, a distance of a thousand kilometers or more; sometimes arcs are several thousand kilometers long. Arcs reach upward from their lower borders in the direction of the magnetic field, which at auroral zone latitudes is nearly vertical. The height extent of the arc is usually a few tens of kilometers and sometimes far in excess of a hundred kilometers

Considering the great length and height extent of arcs, it is truly amazing that they sometimes are less than a hundred meters thick. Furthermore, the brighter an arc becomes the thinner the structure is. As yet, there is no universally accepted theory that explains this extreme thinness.

Seeing an arc low in the northern or southern sky, one is unable to discern the arc's thickness. Only when the arc sweeps up into the zenith region can this aspect of the arc's shape be recognized. A precise measurement of arc thickness can be made only when the arc appears exactly in the observer's magnetic zenith. Every observer has his or her own private magnetic zenith.

If one holds a compass needle suspended by a string tied to its center of mass, the needle will turn to point upward to the magnetic zenith. The north- seeking end of the compass needle will point downward and northward. Thus the south-seeking end is upward and is pointing somewhat southward of the true zenith, the true zenith being that point in the sky vertically above the observer's head. In Alaska and western Canada the magnetic zenith appears 10 to 20 degrees to the south of the true zenith, the exact angle depends upon where the observer is located.

One has no trouble locating the magnetic zenith if there is aurora overhead since it is the point toward which ray structures within auroral arcs appear to converge, and if an arc sweeps through the zenith, the arc will appear thinnest there. Part of the fun of watching the aurora is picking out the location of the magnetic zenith and seeing how auroras appear to change shape--and exhibit their thinness--as they sweep through the zenith.