Kristian Birkeland (1867-1917) was born in Oslo and wrote his first scientific paper at the age of 18. He organized several expeditions to Norway's high-latitude regions where he established a network of observatories under the auroral regions to collect magnetic field data. The results of the Norwegian Polar Expedition conducted from 1899 to 1900 contained the first determination of the global pattern of electric currents in the polar region from ground magnetic field measurements. The discovery of X-rays inspired Birkeland to develop vacuum chambers to study the influence of cathode rays on magnets. An example of one of his experiments is depicted on the left front of the bank note. It shows a magnetized terrella, simulating the Earth, suspended in an evacuated box. Birkeland noticed that an electron beam directed toward the terrella was guided toward the magnetic poles and produced rings of light around the poles and concluded that the aurora could be produced in a similar way. He developed a theory in which energetic electrons were ejected from sunspots on the solar surface, directed to the Earth, and guided to the Earth's polar regions by the geomagnetic field where they produced the visible aurora.
The scale of Birkeland's research enterprises was such that the time-honored matter of funding became an overwhelming obstacle. Recognizing that technological invention could bring wealth, he developed an electromagnetic cannon. In 1900, he obtained patents on what we now call an electromagnetic-rail-gun and, with some investors, formed a firearms company. The rail-gun worked, except the high muzzle velocities he predicted (600 m/s) were not produced. The most he could get from his largest machine was 100 m/s, corresponding to a disappointing projectile range of only 1 km. So he renamed the device an aerial torpedo and arranged a demonstration with the express aim of selling the company. At the demonstration, one of the coils in the rail-gun shorted and produced a sensational inductive arc complete with noise, flame, and smoke. This was the first failure of any of the launchers that Birkeland had built. It could easily have been repaired and another demonstration organized. However, fate intervened in the form of an engineer named Sam Eyde. At a dinner party only one week later, Eyde told Birkeland that there was an industrial need for the biggest flash of lightning that can be brought down to Earth in order to make artificial fertilizer. Birkeland's climactic reply was I have it! There were no more attempts to sell the firearms company, and he worked with Eyde only long enough to build a plasma arc device for the nitrogen fixation process. Birkeland then enjoyed adequate funding for his only real interest: research.
Vision of Field-Aligned Currents Stirs Controversy
Birkeland suggested that polar electric currents-today referred to as auroral electrojets-were connected to a system of currents that flowed along geomagnetic field lines into and away from the polar region. He provided a diagram of field-aligned currents in his famous book, The Norwegian Aurora Polaris Expedition 1902-1903. This diagram is reproduced on the back of the bank note in the lower right corner. This book contains chapters on magnetic storms on the Earth and their relationship to the Sun, the origin of the Sun itself, Halley's comet, and the rings of Saturn. Birkeland's vision of field-aligned currents became the source of a controversy that continued for a quarter of a century, because their existence could not be confirmed from ground-based measurements alone.
The absolute proof of Birkeland's field-aligned currents could only come from observations made above the ionosphere with satellites. A magnetometer onboard a U.S. Navy navigation satellite launched in 1963 observed magnetic disturbances on nearly every pass over the high-latitude regions of the Earth.
The magnetic disturbances were originally interpreted as hydromagnetic waves, but it was soon realized that they were due to field-aligned or Birkeland currents. The first complete map of the statistical location of Birkeland currents in the Earth's polar region was developed in 1974 by A.J. Zmuda and J.C. Armstrong and refined in 1976 by T. Iijima and T.A. Potemra from satellite-borne magnetic field observations. The ring encircling the magnetic pole depicted on the back of the bank note is similar to the patterns of Birkeland and current derived from satellites.
Birkeland's face appears a second time in a watermark in the blank space above the drawing of the Terrella, and his rudimentary magnetosphere appears on the back but is only visible under ultraviolet light.
The Norwegian 200 krones note is a handsome example of modern currency, and it is a fine tribute to one of the world's greatest space physicists.
