Auroras

Changes with time

Auroras change with time. Over the night they begin with glows and progress toward coronas, although they may not reach them. They tend to fade in the opposite order. Until about 1963, it was thought that these changes were due to the rotation of the Earth under a pattern fixed with respect to the Sun. Later, it was found by comparing all-sky films of auroras from different places (collected during the International Geophysical Year) that they often undergo global changes in a process called auroral substorm. They change in a few minutes from quiet arcs all along the auroral oval to active displays along the dark side and after 1–3 hours they gradually change back. Changes in auroras over time are commonly visualized using keograms.