The Northern Lights
 
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The Northern Lights 
(Aurora Borealis)

Aurora Borealis, better known as The Northern Lights, is the celestial phenomenon of bands, curtains or streamers of colored light that appear in the sky in the Arctic and Antarctic regions of the earth. In the Antarctic, the lights are called the Aurora Australis, or Southern Lights. They are less frequent then the Northern Lights. In the far north, the number of auroral displays can be as high as 200 a year. In the south the number is usually fewer than 20.

The sun gives off high-energy charged particles (also called ions) that travel out into space. A cloud of such particles is called plasma. The stream of plasma coming from the sun is known as the solar wind. As the solar wind interacts with the fringes, of the earth's magnetic field, some of the particles are trapped by it, and they follow the lines of magnetic force down 60 to 100 kilometers above the earth's surface. When the particles strike the gases in this area, they start to glow, producing the spectacle that we know as the auroras, northern and southern. The variety of colors: red, green, blue and violet that appear in the sky correspond to the different gases.

The auroras appear over the Earth’s polar regions in what are known as the auroral ovals; in the northern hemisphere the auroral oval bulges that much further to the south, the stronger the solar wind is at any given moment. The oval normally extends over northern Finland and Scandinavia, the whole of Canada and the northern USA, Alaska and Siberia. In the event of a solar storm, it may reach as far south as the skies over central Europe.

Totally-red auroras occur infrequently, not more than a few have been seen in this
part of the world during the last twenty years. All-red auroras are seen near the beginnings of large magnetic storms, and only during some of these. At such times, large numbers of electrons originating on the sun stream into the high atmosphere. There they strike oxygen atoms resident at altitudes 200 to 500 km and cause them to emit quanta of pure red light. Normal green auroras and green auroras tinged with reddish lower borders occur at much lower altitude, typically near 100 km. The electrons that produce these auroras are more energetic than those creating blood-red auroras, so they penetrate more deeply into the atmosphere. During their passage through the upper reaches, these electrons also cause red emissions from oxygen atoms but the red color usually is so much weaker than the green that it is detected only with special instruments.