The Aurora occurs most often in regions known as the "Auroral Ovals". These are rings of a radius roughly 1500mi (2400 km) around the Earth's magnetic Poles. The Aurora occurs at a height of about 60 miles (96 km) above the Earth's surface. The word Aurora Australis refers to the Aurora that occurs in the southern hemisphere (southern lights), whereas Aurora Borealis refers to the Aurora that occurs in the northern hemisphere (northern lights). On rare occasions, Aurora can occur at lower latitudes, and even at the equator. For example, a majestic Aurora was seen above Paris, France, on April 15, 1869. For two days, on March 13-14, 1989, the Aurora was seen almost over the entire United States. At lower latitudes the Aurora is less pronounced, and often obscured by intense air and light pollution of more inhabited areas of Earth today. In the past, it was much easier to observe this phenomenon at lower latitudes (see the Auroral History section of this homepage). The Aurora also occurs on other planets. For example, recent pictures taken by the "Voyager" space probe revealed extensive Auroral activity on Jupiter.

Once through this "polar funnel" the Solar Wind begins to descend into the Earth's upper atmosphere. When these high speed charged particles enter the Earth's upper atmosphere, they collide with the atoms of the atmospheric gases (mostly oxygen and nitrogen). During such collisions electrons in atoms can be excited to higher energy levels within an atom, or even completely "kicked out" of an atom. The excited electrons emit energy in the form of light when they return to their original positions in their parent atoms. This emitted light forms the Aurora. The color of the Aurora depends on the type of gas interacting with the Solar Wind, and how strongly the electrons were excited. For example, if the collision of an atom occurred with a highly energetic (fast) particle of the Solar Wind, the amount of energy released when electrons return to their original positions (levels) will be large. Then the Aurora would take on a bluish color. If, on the other hand, the particle of the Solar Wind was not very fast, the final energy released will be small, and consequently the Aurora would take on a reddish color. All of this occurs in the region of the Earth's atmosphere called the ionosphere. It stretches between 80 and 500 kilometers above the Earth's surface.

Man-made neon lamps are analogies of the Aurora. Also here, a plasma of excited gases emits light. However the excitation is caused in neon lamps not by the Solar Wind, but rather by an artificial "wind" of charged particles produced by an electric field created inside the neon lamps.

For more details consult the references quoted below.

As in the case of comets, the rarity of Aurora (around centers of civilization) made people usually fearful of this phenomenon in the ancient times, and in the middle ages. Even people who were accustomed to Aurora (for example in Norway) believed that the Aurora represented "old maids dancing and waving their white gloved hands". The North American Indians considered the Aurora to be the Gods dancing across the night sky, and the Eskimos in Greenland and the Hudson Bay area thought that the northern light was the kingdom of the dead.

With the arrival of the Renaissance, a systematic study of the Aurora began. Between the years 1500 and 1640 there was a great number of reported Auroral occurrences below the Arctic circle. During this period it was quite usual to see an Aurora occur somewhere in Europe, about twice every year. However, it was at the beginning of the seventeenth century that truly scientific study of the Aurora began. The first half of this century witnessed a great deal of Auroral reports, and in 1619 the term Aurora borealis was introduced by Galileo. For the other half of this century, and into the eighteenth century there was a period of an almost complete absence of solar activity, and thus only a few Auroras were reported. This period is known today as the Maunder Minimum, which lasted 70 years from 1645-1715. In 1716, the famous English astronomer, Edmund Halley made an observation that the Auroral form called the corona, was merely an effect of perspective. The eighteenth century witnessed a large growth in Auroral physics. During this century the first textbook devoted entirely to the subject of the Aurora was published by Jean Jacques d` Ortous de Mairan, in 1754. During the first half of this century a heated debate between Euler, Mairan, and Halley took place over whose theory most accurately explained the physical nature of the Aurora.

The most significant discoveries towards understanding the Aurora were made during the last decade of the nineteenth century, and in this century. The most important of these was made by the great Norwegian physicist, Kristian Birkeland. After developing a passionate interest in the Aurora, he devoted almost his whole life to studying it. Between 1897 and 1903 Birkeland made a series of Auroral expeditions to Northern Norway. His first expedition was a failure, on his second, and third expeditions he managed to obtain the first Auroral photographs, gathered important data on the Aurora, and was able to establish four Arctic stations, all within Auroral occurrence zones. Birkeland's greatest contribution, however, were his famous experiments. They were stimulated by the discovery of X rays (by Roentgen), the research done on cathode rays, and the demonstration (by Crookes) that cathode rays were deflected by magnetic fields. With these developments, Birkeland, proposed in 1896 that Auroras were caused by a beam of cathode rays emitted by the sun. These rays would reach the vicinity of the Earth and be effected by the Earth's magnetic field, which in turn would guide them to the high-latitude regions of the Earth, creating the Aurora. We know today that this the correct explanation. To support his theory Birkeland set-up a series of remarkable model experiments (around1910) in his laboratory, that were able to reproduce many of the characteristics of the Aurora. Birkeland placed a magnetized sphere inside a vacuum chamber and projected a beam of electrons at the sphere. This sphere was coated with fluorescent paint to enable Birkeland to view the streaming electrons' trajectories. Birkeland was able to accurately reproduce how Solar Wind would make its way into the Earth's magnetic poles, and was able to simulate the Auroral ovals near the Earth's magnetic poles. Some of the minor problems with Birkeland's theory were resolved by Svante Arrhenius, a Swedish physicist who supported Birkeland's theory.