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(Files in red–history)
Index 25. Auroral Currents 25H. Birkeland Currents 25a. Triad 25b. Io Dynamo 25c. Space tether 26. Polar Caps 26H. Birkeland, 1895 27. Aurora from Space 28. Aurora Origin 28a. Plus and Minus 29. Low Polar Orbit 30. Magnetic Storms 30a. Chicago Aurora |
In experiments begun around 1900, he put in a glass tank a small spherical magnet that modeled the Earth, which he named, like William Gilbert before him, terrella or "little Earth." Elsewhere in the tank he mounted an "electron gun" like the one found in TV picture tubes--a hot wire emitting electrons, and plates charged to positive voltages to pull them out and speed them on their way.
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There remained one puzzle. Polar explorers had reported that the aurora was
extremely rare near the magnetic poles themselves. Why? A few of Birkeland's experiments indeed produced a ring of light with a dark center, but in general he only got a polar patch of light, covering the magnetic poles of the terrella. Birkeland's younger friend, the mathematician Carl Stoermer, analyzed the motion of the electrons mathematically and even computed many of their orbits, a tough task in the pre-computer days around 1907-10. He found no compelling reason why electrons entering the field from far away would avoid the poles. Stoermer died in 1957, still frustrated by this problem. The answer only came when satellites began probing the distant magnetosphere. They showed conclusively that the aurora did not in general come from the Sun or from distant space, but originated in the Earth's own magnetosphere. On the other hand, the electrons of the polar rain, which apparently did come from the Sun, were indeed found in patches surrounding the magnetic poles, exactly like the patches of light Birkeland observed on his terrella.
Further Reading:"A Brief History of Magnetospheric Physics before the Spaceflight Era," Reviews of Geophysics, 27, 103-114, 1989.
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Next Stop: #27. Auroral Imaging
Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: education("at" symbol)phy6.org
Co-author: Dr. Mauricio Peredo
Spanish translation by J. Méndez