AN AURORA IS a colored display of light usually seen at night. They are natural, and occur more commonly in the polar regions than in temperate southern regions in the Northern Hemisphere or more temperate northern regions in the Southern Hemisphere. Auroras occur most frequently in the Northern Hemisphere around the time of the autumn equinox in September and October and at the spring equinox in March and April. Auroras are sometimes called polar auroras (aurorae polaris), or in the Northern Hemisphere, aurora borealis. A popular name in the Northern Hemisphere is the northern lights; in the Southern Hemisphere they are the southern lights or the aurora australis (Latin for southern is australis).
The aurora borealis begins with a magnetic storm on the sun. If the sun emits an extra mass of particles in a solar wind to the Earth, this can then produce a magnetic storm on earth. If the conditions are right, the aurora borealis can be seen not only at the poles, but far to the south in the Northern Hemisphere or far to the north in the Southern Hemisphere.
The light displays of auroras are caused by charged ion particles reaching the upper atmosphere of the Earth in solar wind. If the particles arrive as a cloud, they are called solar plasma. If the particles collide with one another when they are in an excited atomic state, light is emitted. This means that there may be more electrons surrounding an atom than normal. The excess energy can be emitted from the particle as visible light. Usually, the light is in wavelengths of 22 in. (557.7 mm.), which is in the visible light range of green. Some may be at lower or higher wavelengths and, thus, seen as red or other colors. The particles follow the Earth's magnetic field and may appear to glow as wavering sheets or curtains. Other forms of aurora activity occur as arcs of light, which can also be in folds that appear to have stri-ations. Contributing to the shape of the aurora borealis
are the rotation of the Sun and the rotation of the Earth. As the solar stream hits the Earth, it meets the Earth's rotation. The two combine to make a spiral shape.
Contributing to the frequency of the aurora is the sun's 11-year cycle. Sunspots and the emission of solar particles occur in a cycle that rises and falls in activity. Auroras are associated with a high number of sunspots. This relationship has been known since 1852, when Edward Sabine first identified it using sunspot and aurora spotting data that went back 100 years. This relationship seems to also correlate with climate patterns.
There is speculation that fluctuations in climate are closely related to the presence of aurora activity in Earth's atmosphere. The Maunder minimum is the term that was first used to identify the period 1645-1715. This was a time in which aurora activity was rarely seen, and sunspots were not observed with any frequency. In 1976, it was demonstrated that the Maunder minimum was associated with abnormal amounts of C14 in tree rings. When the Edda Period, a period of high sunspot activity in the 12th and 13th centuries was examined, numerous references to aurora activity were also found.
Some researchers have theorized that global warming may be associated with sunspot activity, or global cooling with its absence. Research is being conducted into the possibility that global warming is caused by solar emissions, rather than by the increasing presence of higher levels of carbon dioxide in the atmosphere due to pollution emissions since the Industrial Revolution.
SEE ALSO: Antarctic Circumpolar Current; Arctic Ocean; Atmospheric Vertical Structure; Solar Wind; Sunlight.
BIBLIOGRAPHY. Syun-Ichi Akasofu, Exploring the Secret of the Aurora (Kluwer Academic Publishers, 2002); Neil Bone, Aurora: Observing and Recording Nature's Spectacular Light Show (Springer-Verlag, 2007); D.A. Bryant, ed., Electron Acceleration in the Aurora and Beyond (Taylor & Francis, 1999); Neil Davis, The Aurora Watcher's Handbook (University of Alaska Press, 2000); Alister Vallance Jones, Aurora (Springer-Verlag, 1974); Tod Salat, Alaska's Spectacular Aurora (Todd Communications, 2003);
Andrew J. Waskey Dalton State College
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