Causes of the Aurora

Abstract

The aurora is a phenomenon of Earth’s high atmosphere, occurring at altitudes typically in excess of 100 kilometers. Its light results fromexcitationof tenuousatmospheric oxygen and nitrogen by electrons accelerated in near-Earth space under conditions where the magnetosphere—the volume of space in which terrestrial magnetism has a dominant influence—becomes disturbed. Geographically, the aurora is ever-present in two oval regions, one surrounding either geomagnetic pole. These auroral ovals are normally quite narrow, and lie at high latitudes.Under disturbed conditions, the ovals brighten and broaden, and during severe disturbances—geomagnetic storms—they can be driven down toward the equator, bringing Nature’s awesome light show to the skies of observers in the southern United States or more temperate parts of Europe. Other, less severe disturbances cause frequent substorms at higher latitudes: during these events, observers in Alaska or Canada, for example, will see the aurora brighten and become more active. Even at times when the Sun appears relatively inactive, variations in the solar magnetic field—particularly those associated with the so-called coronal holes—can trigger minor enhancements in auroral activity.