(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
The term "aurora borealis" was used in 1621 by the French scientist and philosopher Pierre Gassendi, but George Siscoe has given reasons to believe it was introduced by Galileo Galilei in 1619 (p. 51 in "Majestic Lights", cited below).
Elias Loomis of Yale University compiled, in 1860, a map marking how many times in an average year were auroras observed in various locations (click here to see his map). A more accurate map was compiled in 1881 by Hermann Fritz (1830-1883) (click here to see Fritz'es map).
It was long suspected that the aurora was caused by electrons arriving from the outside and hitting the high atmosphere.
The Norwegian physicist Kristian Birkeland (1867-1917), for instance, placed a magnetized sphere, a "terrella" representing the Earth, inside a vacuum chamber, and aimed a beam of electrons towards it. He was gratified to see that the electrons were steered by the magnetic field to the vicinity of the terrella's magnetic poles.
However, it was only in 1954 that auroral electrons were actually observed, by detectors aboard a rocket launched into the aurora by Meredith, Gottlieb and Van Allen, of Van Allen's team at the University of Iowa. Carl McIlwain, another member of that team, used a 1959 rocket experiment to identify the particles as electrons of an average energy corresponding to acceleration by 6000 volts (see high energy particles).
Nowadays scientific satellites regularly cross streams of auroral electrons and measure their properties, and aurora is also observed from the ground with video cameras and special radars.
Birkeland constructed more than one terrella experiment, including (in 1913) a much larger one in a bigger chamber, shown next to his picture on the current Norwegian 200-kroner bill. That terrella was restored in 1995 by Terje Brundtland and is now demonstrated to visitors
at the Auroral Observatory in Tromsø, Norway. For an article about that restoration, see here.
Birkeland constructed more than one terrella experiment, including (in 1913) a much larger one in a bigger chamber, shown next to his picture on the current Norwegian 200-kroner bill. That terrella was restored in 1995 by Terje Brundtland and is now demonstrated to visitors at the Auroral Observatory in Tromsø, Norway. For an article about that restoration, see here.
Questions from Users:
*** Experimental simulation of the polar aurora
*** Building a Terrella experiment like Birkeland's
Next Stop: #4. Electrons
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