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Planeterella 01.Pdf The story of the terrella th Kristian Birkeland (1867–1917) was a brilliant Norwegian physicist In the 16 century… who in 1896 published his theory about auroras – that electrons The word terrella, from the Latin for little streaming out from the Sun were captured by the Earth’s magnetic Earth, first appears in a book published in field and funnelled to the magnetic poles. There they collided with 1600 by William Gilbert (1544–1603), the atoms and molecules in the upper atmosphere, ‘exciting’ them and Royal Physician to Queen Elizabeth I. He generating a glowing ring around each pole. was fascinated by magnetism and believed that Earth itself acted a giant magnet. At the time, magnetism was of great interest as worldwide trade was exploding, and the compass was one of the few tools sailors had at their disposal to avoid becoming Left: Birkeland and his terrella. The equipment was restored in 1996 and is on display in Tromsø. Right: Prediction of auroral currents from 1903, and the terrella showing similar ‘field aligned’ current flows. hopelessly lost. William Gilbert To demonstrate his theory Gilbert created a model globe from a (naturally magnetic) lodestone. Passing a small compass over the terrella, he replicated the behaviour seen when using a compass to navigate, including features such as deviations from true north. These results were published in De Magnete, Magneticisque Birkeland and his ground-breaking report on the Aurora. Conditions on the expeditions were harsh. Norwegian pride over one of their greatest scientists is evident from the design of the 200 Kroner note, featuring Corporibus, et de Magno Magnete Tellure (On the Magnet and Birkeland, his terrella, and a copy of the original Birkeland current diagram. Magnetic Bodies, and on That Great Magnet the Earth), which Birkeland personally led expeditions into the Arctic Circle to obtain remains in print today. detailed observations of auroral displays and their magnetic fields. An early attempt nearly ended in tragedy due to atrocious weather, In the 21st century… th but later expeditions were successful and monitoring outposts were In recent times terrellas have fallen out of use in research in favour In the 20 century…… established. of computer simulations, but have found a new role in educating In the late 19th and early 20th century, the discovery of radioactivity and indeed entertaining the public. and sub-atomic particles such as the electron was providing clues Responding to criticisms of his theory, Birkeland constructed his toward answering the great astrophysical questions of the age such own set of terrellas to simulate the Sun-Earth system. Metal spheres, Birkeland’s terrella was restored in 1996 and is on display in the as the source of the Sun’s power, and the cause of the Northern ranging in size from golf balls to footballs, were suspended from the Auroral Observatory in Tromsø, but it is a large, delicate instrument and Southern Lights (Aurora Borealis and Australis). top of pressure-controlled chambers. The spheres were strongly and not suitable to reproduce outside a laboratory. A team led by magnetised, and were wired to allow a high voltage to be applied Dr. Jean Lilensten, from the Institut de Planétologie et d’Astro- in France built a cheap, portable terrella- Konieczny between the sphere and a suitably placed electrode. Most of the air physique de Grenoble Rafal was pumped out of the chamber, simulating the low pressure of the type system, designed to be taken out of the laboratory and into upper atmosphere. When a high voltage was applied across the schools or for public display. This system is clearly based on the sphere and electrode a stream of electrons was created. The terrella, but is more flexible and powerful, allowing users to show sphere’s magnetic field pushed the electrons to the north and south many more planetary phenomena than Birkeland’s experiment. poles, where as expected they interacted with the low-pressure gas to form glowing auroras. They called it the planeterrella, acknowledging the terrella while Robert Ryans We now know that Birkeland’s theory was not completely correct in suggesting the new planetary accounting for all details of the aurora, though excellent for the time. features. The team graciously Interestingly, in 1903 he predicted that the aurora would contain share the plans with educational currents running along the magnetic field lines due to the motion of institutions worldwide, and at the captured electons. This theory was mostly dismissed at the Queen’s we are grateful for their time, but in 1967 satellite observations proved it correct; auroral assistance in building ours. currents of up to 1 million amps have since been observed. Similar Queen’s Dr. Chris Watson working on our planeterrella. electrical flows have also been observed in many non-auroral This poster makes use of information and figures from Lilensten’s paper describing the planeterrella: Lilensten J, Provan G, Grimald S, Brekke A, Flückiger E, Vanlommel P, Wedlund CS, Barthélémy M & Garnier P: The Planeterrella experiment: from individual initiativeto astrophysical plasmas and are now known as Birkeland currents. networking. J. Space Weather Space Clim., 2013, 3, A07. The Aurora Borealis over Norway in 2006 Find out more about the work of Queen’s Astrophysics Research Centre at http://star.pst.qub.ac.uk/.
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