What is Aurora? How Does Aurora Happen? Information on Appearance and Causes Of The Aurora.
Aurora;a light, visible at night, that is produced in the upper atmosphere of the earth at irregular intervals. The aurora observed in the Arctic and subarctic regions is called the aurora borealis. The corresponding phenomenon in the Southern Hemisphere is called the aurora aus-tralis. In North America the most likely places for seeing an aurora are northern Alaska, the central part of Hudson Bay, and Labrador. In Europe the auroral zone passes through northern Norway and Sweden and on through the northern coast of Siberia. The auroral zone in the Southern Hemisphere lies mostly in Antarctica.
Appearance: When their light is bright enough for colors to be distinguished, aurorae usually appear yellowish green. Aurorae exceptionally high in the atmosphere are red, however, and exceptionally low aurorae may have an orange lower border. Occasionally, high aurorae seen at twilight may appear violet.
Aurorae occur in various shapes and with various degrees of activity. Thus, an aurora may take the form of a steady, homogeneous arc. This form usually extends some tens of miles vertically into the atmosphere, gradually fading out toward the higher altitudes. The arc is often less than 1 mile (1.6 km) thick in the north-south direction. However, it may stretch from east to west for several thousand miles around the earth. The lower boundary of an aurora is about 70 miles (110 km) above the surface of the earth.
Active aurorae appear as draperies, coronas, or other forms. They are made up of long, thin rays that may extend upward for several hundred miles. The rays are aligned parallel to the earth’s magnetic field. Individual rays generally are short-lived, but often disappear and reappear in the same position every few seconds. Occasionally, bright red patches are seen that last for a few minutes. Perhaps the most spectacular type of display is the so-called flaming aurora, in which wavelike patches of light start low in the north—or south—and rise high into the sky, disappearing within a second or so.
Auroral displays usually are confined to a northern and southern circular zone around the geomagnetic poles; these zones lie from 20 to 25 degrees from the poles. In the Northern Hemisphere the geomagnetic pole is found near Thule in northwest Greenland. Strong aurorae occasionally extend to middle latitudes. They are accompanied by small but important changes in the earth’s magnetism.
Cause of the Aurora: It has been found that auroral displays are more frequent during periods of intense sunspot activity. During the active periods of the 11-year solar cycle, for example, displays of the aurora borealis are frequently visible from the northern United States, where otherwise they are rarely seen.
The aurora is therefore associated with activity on the sun, as well as with the earth’s magnetism. These associations suggested to scientists that the aurora results from bombardment of the earth’s upper atmosphere by electrically charged particles from the sun. The concept was supported by the discovery by Norwegian scientist Lars Vegard in 1939 that a small portion of the auroral radiation is produced by hydrogen. Hydrogen is a rare element in the earth’s atmosphere but is the major constituent of the sun. In 1950 more precise studies of auroral radiation (conducted in the United States by Aden B. Meinel and Carl W. Gartlein) showed that the hydrogen producing some of the radiation was impinging on the atmosphere at speeds of several thousand miles per second. This finding reinforced the view that the aurora is caused by charged particles—protons and electrons—coming from beyond the atmosphere.
Later studies by Anders Omholt in Norway and others demonstrated that the most important source of auroral energy is the bombardment of the atmosphere by electrons. Various theories have been proposed but not proved to account for the propagation of solar particles through interplanetary space, the capture of some of these particles by the earth’s magnetic field, and their subsequent bombardment of the earth’s atmosphere.