Terrella - Kristian Birkeland's Terrella

Kristian Birkeland's Terrella

Kristian Birkeland was a Norwegian physicist who, around 1895, tried to explain why the lights of the polar aurora appeared only in regions centered at the magnetic poles.

He simulated the effect using a "terrella," a sphere in a vacuum tank to which he directed beams of cathode rays, later identified as electrons, and found they indeed produced a glow in regions around the poles of the terrella. Because of residual gas in the chamber, the glow also outlined the path of the particles. Neither he nor his associate Carl Størmer (who calculated such paths) could understand why the actual aurora avoided the area around the poles themselves. We now know this relates to the origin of the auroral electrons, which is actually inside the Earth's magnetosphere, the region of space controlled by the Earth's magnetism. Birkeland believed the electrons came from the Sun, since large auroral outbursts were associated with sunspot activity.

Birkeland constructed several terrellas. One large terrella experiment was reconstructed in Tromsø, Norway.