#26H.     Polar Cap -- History

    The aurora is mainly seen in polar or near-polar regions. Why? Starting about 1895, the physicist Kristian Birkeland, in Norway, tried to answer this question experimentally.

    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.





    Birkeland believed that auroral electrons came from the Sun, and the electron gun, aimed at the terrella, represented that source. The air in the tank was then pumped out (as well as one could do so 100 years ago) and Birkeland was gratified to see that the electron beam indeed converged towards the polar regions of the terrella and avoided the equator. One of those experiments was recently restored to operating conditions and may be seen at the Auroral Observatory in Tromsø, Norway. For an article about that restoration, see here.

    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.

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