Few sights are more awesome than images of the huge flares that erupt from the surface of the Sun, spewing arching streams of glowing gas hundreds of thousands of miles into space. These exploding "solar prominences" also have an important impact here on Earth; the storms of charged particles they emit can disrupt communications and even interfere with the flow of electricity through transmission lines.
Measurements and data collected from spacecraft and terrestrial observatories have provided many insights into the forces that cause these enormous events on our restless and inaccessible star. But some scientists, working in a nascent discipline called "laboratory astrophysics," won't be satisfied until they can duplicate the phenomena in their laboratories--from solar flares to the birth of stars and galaxies.
As a case in point, the flaming outburst at the left took place not on the surface of the Sun but in a laboratory at the California Institute of Technology. The events shown occurred in a 1.4- meter diameter, 2-meter long vacuum chamber. The plasma reaches a height of a mere 20 centimeters over 10 microseconds, expanding at the rate of 70 kilometers per second. The solar microcosm is photographed with CCD cameras with 10-100 nanosecond shutter speeds. But for its small size, the researchers, Paul M. Bellan and graduate student, J. Freddy Hansen, believe that their sun in a bottle is the most accurate simulation yet of the formation of a solar prominence.
The Cal Tech team, which is scheduled to reveal details of the research on November 17 in a paper at the 1998 American Physical Society Division of Plasma Physics Meeting in New Orleans, based their conclusions on the investigation of events known to plasma physicists as spheromaks. Numerical simulations of spheromaks show that they form when superheated ions, known as a plasma, become trapped in twisting tubes created by powerful magnetic fields. When two of these tubes intertwine, they become unstable--when the twisting reaches a certain point, they rapidly expand outward, just like a solar prominence.
In the lab, spheromaks are created by injecting ions through a specially designed plasma gun into a vacuum chamber. A plate carrying a very high charge simulates the surface of the sun. The currents, at hundreds of kiloamperes and voltages in the kilovolt range, are created by high energy capacitors. Microsecond switching between two electrodes causes twisting to occur in a tube of plasma that becomes trapped between the two electrodes.
The researchers report at the APS meeting that they have refined their apparatus to create the prominences and have added a two camera system which allows them to make 3-dimensional movies of the event as it takes place. Just as in the computer models, these images show development of twisted filaments that thread each other.
This tiny simulation is helping astrophysicists confirm theories and is providing important new insights into the real events that take place on the enormous scale of the sun. And if you can't get to the sun to watch the show, this is next best thing.
