Coronal loops on the sun that are linked to magnetic fields. Credit: NASA/Solar Dynamics Observatory Read more at: http://phys.org/news/2015-11-physicists-clue-formation-magnetic-fields.html#jCp
Small magnetic perturbations can combine to form large-scale magnetic fields just like those found throughout the universe. Squire and Bhattacharjee at PPPL analyzed the behavior of dynamos, which occur when an electrically charged fluid like plasma swirls in a way that creates and then amplifies a magnetic field. Scientists have known that plasma turbulence can create lots of small magnetic fields, but the mechanism by which those fields could produce a single large field is elusive.
The puzzle consists of the seeming unlikelihood of small disturbances coming together to form something large and organized. Throughout nature, order tends to dissolve into chaos, not the other way around. Like tornados, the large-scale magnetic fields throughout the universe seem to be produced by lots of small disturbances. But unlike tornadoes, these magnetic fields persist. “Something is holding up the universe’s magnetic fields for billions of years,” said Amitava Bhattacharjee, head of PPPL’s Theory Department. “But how exactly does the universe get these persistent magnetic properties?”
After running computer simulations on a PPPL computer named “Dawson,” the scientists found that small disturbances can combine to form one large disturbance when there is a lot of velocity shear—when 2 areas of a fluid are moving at different speeds. “We used a variety of computational and analytic methods to approach the problem from a few different angles,” said Squire.
The team first used statistical simulations, which create a kind of average of the behavior of the entire system. “Statistical simulations capture the properties of hundreds of simulations without actually running them all,” said Bhattacharjee. The scientists also used numerical simulations that begin with initial conditions that are allowed to progress in lengthier runs.
“The results indicate that small-scale magnetic fluctuations can create large-scale magnetic fields that persist,” Bhattacharjee said. “But in order to be conclusive about persistence for long times, we must run simulations for very low dissipation,” a measure of energy loss. “It is impossible to run simulations for dissipation as low as those of real astrophysical plasmas, but our analytical and computational results, in the range in which they are done, strongly suggest that such dynamo action is possible.”
These findings might lead to greater understanding of the behavior of many kinds of astronomical phenomena, including the disks of material that form around black holes and the 11-year solar cycle of our own sun. Computer programs cannot yet simulate these vast astronomical phenomena, so learning how to create simplified models that capture the workings of these large turbulent systems would be helpful.
http://phys.org/news/2015-11-physicists-clue-formation-magnetic-fields.htmljCp
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