OF LIBERAL ARTS & SCIENCES
Department of Physics and Astronomy
A planeterrella is a contraption used to simulate space physics phenomena involving plasma and celestial bodies. The device consists of a bell jar inside which the air pressure can be pumped to approximately one ten-thousandth atmospheric pressure. Two metallic spheres containing permanent magnets are inside the bell jar and a power source is used to create an electric field between the spheres, or between either sphere and any of 4 metallic needles in the device. This electric field causes electrons emitted from the surface of the metal to be energized to high enough energy to strip electrons off (ionize) of some of the neutral atoms present in the residual air, creating a plasma. This plasma interacts with the magnetic field from the magnets inside the spheres, in a similar manner to how plasma emitted from the sun (called the solar wind) interacts with Earth's magnetic field. Through modifying the arrangements of the magnets and the course of the electrons, we are able to reproduce such occurrences as the Aurora Borealis or Aurora Australis, the Van Allen radiation belts, stellar jets, coronal holes, and stellar ring currents.
The planeterrella at the University of Iowa is based off of a design conceived by Dr. Jean Lilensten of the Planetary and Astrophysical Institute of Grenoble in France: Planeterrella web page. In the Iowa Planeterrella, three modifications were made so that it could be incorporated into the Hawkeye's in Space museum exhibit. The first is an electronics switchboard that allows users to switch between different configurations by turning a knob (currently these are the radiation belt, aurora and ring current demonstrations). The second is an automated pressure control that maintains the pressure inside the device between a set range of values. Third, a clear Plexiglas box covers the bell jar so users cannot disturb it during use. These modifications have enabled the device to be used by anyone without supervision.
The University of Iowa has long played a pioneering role in the exploration of space. It was here that James Van Allen worked to discover Earth’s radiation belts. Frontier research exploring space continues to be carried out by University of Iowa scientists using instruments built at the University of Iowa. To learn more about this ongoing research, please visit the Department of Physics and Astronomy faculty web pages.
The Iowa Planeterrella was constructed entirely by students. Master’s student Mr. Richard Bleckley and PhD student Mr. Ryan Hood constructed the base device in 2015. Undergraduate student Mr. Zachary Luppen made the modifications described above, to adapt it to the needs of the museum exhibit, in 2016. Construction was aided by the Department of Physics and Astronomy Electronics and Machine Shops.
Kristian Birkeland operating the first terrella experiment. Image in the public domain, from wikipedia.
A century ago, Norwegian scientist Kristian Birkeland (1867-1917) hypothesized that the aurora on Earth are caused by plasma streaming out from the Sun that gets trapped in the Earth’s magnetic field, causing currents in the polar regions of the upper atmosphere. To test this hypothesis, Birkeland led the “Norwegian Polar Expedition” from 1899 to 1900, which provided the first evidence (using ground based magnetic field measurements) of the current patterns in the polar region. Today, these are knows as the Birkeland currents. The existence of Birkeland currents was debated scientifically until a direct measurement was made in 1963 from the Navy satellite 1963-38C.
Upon returning from his expedition, Birkeland designed the “Terrella” experiment to study the interaction of plasma and magnetic fields in and effort to better understand the physics associated with the Earth-Sun interaction. Birkeland also made several other major contributions to science and technology. He invented the coil gun, and was the first to develop an industrial-scale method for nitrogen fixation from air (the Birkeland-Eyde process). Nitrogen fixation is the essential process needed to produce ammonia for farm fertilizer. Birkeland was nominated for a Nobel prize seven times. Learn more about Kristian Birkeland here.
The Iowa Planeterrella is currently on display at the: Hawkeye's In Space exhibit in the Old Capitol building on the University of Iowa campus.