Aurora

Aurora, luminous atmospheric phenomenon occurring most frequently above 60° north or south latitude, but also in other parts of Earth. It is named specifically, according to its location, aurora borealis (northern lights) or aurora australis (southern lights). The term aurora polaris, polar lights, is a general name for both. Auroras occur at altitudes of from about 60 km to 200 km (37 mi to 124 mi) in the ionosphere portion of the atmosphere.

The aurora consists of rapidly shifting patches and dancing columns of light of various hues. Extensive auroral displays are accompanied by disturbances in terrestrial magnetism and interference with radio, television, and telephone transmission. The period of maximum and minimum intensity of normal auroras seems to be almost exactly opposite that of the sunspot cycle, which is an 11-year cycle (see Sun), so the intensity of the auroras is normally low while the Sun is very active. Huge displays that occur farther from Earth’s poles than normal, however, occur more often while the Sun is very active and Earth experiences magnetic storms.

Studies made during and after the 1957 and 1958 International Geophysical Year indicate that the auroral glow is triggered when the solar wind (see Solar System) carries an influx of high-energy atomic particles emanating from the Sun. The electrons and protons penetrate the magnetosphere of Earth and enter the lower Van Allen radiation belt (see Radiation Belts), overloading it. The excess electrons and protons are discharged into the atmosphere over an area centering on the north and south magnetic poles and extending about 20° away from them. These particles then collide with gas molecules in the atmosphere, thereby exciting the molecules and causing them to emit electromagnetic radiation in the visible portion of the spectrum. See Luminescence.

The auroras are, on average, less intense when the Sun is most active. When the sunspot cycle is at its maximum, the Sun emits much more ultraviolet radiation than it normally does. The ultraviolet radiation changes Earth’s magnetosphere, making it better able to handle the influx of particles from the sunspots. The radiation belt is therefore not overloaded as easily and the auroras are less intense. However, the largest and most spectacular displays occur during or soon after the Sun’s peak in activity. The most active period of the Sun’s cycle creates wide variations in the intensity of the solar wind. Sometimes huge blasts of particles hit Earth, creating auroral displays that extend far from the poles.

The auroras assume an endless variety of forms, including the auroral arch, a luminous arc lying across the magnetic meridian; the auroral band, generally broader and much more irregular than the arch; filaments and streamers at right angles to the arch or band; the corona, a luminous circle near the zenith; auroral clouds, indistinct nebulous masses, which may occur in any part of the heavens; the auroral glow, a luminous appearance high in the sky, the filaments diverging toward the zenith; and curtains, fans, flames, or streamers of various shapes. Colors are created when oxygen and nitrogen molecules are excited by charged particles and emit photons of particular energies.

Auroras have also been observed in the atmospheres of the giant gas planets Jupiter, Saturn, Uranus, and Neptune. Smaller auroras have been detected in the atmospheres of the planet Mars and Jupiter’s moons Io and Ganymede.