(Files in red–history)
2. Magnetic Field
2H. Oersted, 1820
3H. Birkeland 1895
3a. Loomis & Aurora
3b. Fritz & Aurora
3c. The Terrella
4H. Thomson, 1896
4a. Electric Fluid
In Alaska, Canada, Norway, Finland or northern Russia, on a clear night, a greenish glow is often seen in the sky, known as the "Northern Lights."
During magnetic storms, the glow may move southwards, and on occasion it can be seen in much of the US. It often appears as a glow on the horizon, like the glow preceding sunrise, and has therefore become known among scientists as "aurora borealis" ("aurora" for short), Latin for "northern dawn." A similar phenomenon is also seen in southern polar regions.
Auroral light is produced at a height of about 100 km (60 miles) when fast electrons, arriving from space, slam into atoms and molecules of the atmosphere. The computer screen displaying these words may be lit up in a similar way, by a beam of fast electrons accelerated electrically towards it, then steered and modulated so as to form letters and pictures.
The magnetic connection is also demonstrated by the fact that the rays of the aurora lie along magnetic field lines, and that the Earth's magnetic field observed beneath a bright and active aurora tends to be disturbed.
ColorThe green light of the aurora has a precisely defined color in the spectrum ("narrow spectral line"). Such precise colors are usually the signatures of the atoms which emit them: for instance, street lights (depending on the metal vapor they contain) usually emit either the yellow-orange light of sodium or the bluish light of mercury.
The green light of the aurora puzzled scientists for many years, since it fit no known element. It turned out to be produced by oxygen atoms, but under conditions that in our atmosphere only exist in the very rarefied upper levels. A red aurora, occasionally seen, arises at even greater heights and is also produced by electrons hitting oxygen.
Next Stop: #3H. History of Early Auroral Studies