A terrella is a small magnetised model ball representing the Earth, that is thought to have been invented by the English physicianWilliam Gilbert while investigating magnetism, and further developed 300 years later by the Norwegian scientist and explorer Kristian Birkeland, while investigating the aurora. Terrellas had been used until the late 20th century to attempt to simulate the Earth's magnetosphere, but have now been replaced by computer simulations.
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 by directing cathode rays at a terella in a vacuum tank, 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 could understand why the actual aurora avoided the area directly above the poles themselves. 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.
Other terrellas
The GermanBaron Carl Reichenbach also experimented with a terrella. He used an electromagnet, placed within a large hollow iron sphere, and this was examined in the darkroom under varying degrees of electrification. Brunberg and Dattner in Sweden, around 1950, used a terrella to simulate trajectories of particles in the Earth's field. Podgorny in the Soviet Union, around 1972, built terrellas at which a flow of plasma was directed, simulating the solar wind. Hafiz-Ur Rahman at the University of California, Riverside conducted more realistic experiments around 1990. All such experiments are difficult to interpret, and are never able to scale all the parameters needed to properly simulate the Earth's magnetosphere, which is why such experiments have now been completely replaced by computer simulations. Recently the Terrella has been further developed by a team of physicists at the Institute of Planetology and Astrophysics in Grenoble, France to create the Planeterrella which uses two magnetised spheres which can be manipulated to recreate several different auroral phenomena.