The polar aurora is perhaps the most fascinating and mysterious of the many spectacles that Nature puts up for us to observe. Only direct observation can reveal its three-dimensional grandeur, but some of the beauty can be captured in color photographs such as those included with this article (taken by Gustav Lamprecht of College, Alaska). The structure generally consists of one or more thin arcs stretching east to west from horizon to horizon, as is seen in the cover photograph. Periodically, the quiet arcs break up in a burst of activity called a substorm in which they brighten and spread across the sky, followed by a gradual return to quiet forms. Green is the most common color, both because of the strong (forbidden) emissions at 5577 A from atomic oxygen at altitudes from 100 to 150 km and because the maximum response of the human eye occurs in the green. Type A aurora is characterized by strong red emissions at high altitudes (inside back cover) due to another forbidden transition of atomic oxygen at 6300 A, and Type B has red lower borders due to emissions from molecular nitrogen at lower altitudes (inside front cover). Rayed structure is common in all a uroras, and high-speed television recordings show small-scale structures of considerable complexity that often escape the human eye. The active forms present during substorms are accompanied by geomagnetic and ionospheric disturbances.