One of the longstanding mysteries of astronomy—how stars and galaxies acquire their magnetic fields—may now be one step closer to being solved, thanks to the efforts of researchers from the US Department of Energy’s Princeton Plasma Physics Laboratory (PPPL).
In a recent edition of the journal Physical Review Letters, PPPL researchers Jonathan Squire and Amitava Bhattacharjee reported that they found that small magnetic disturbances can combine to form larger-scale magnetic fields similar to those found around objects across the universe.
Squire and Bhattacharjee analyzed the behavior of dynamos, which occur when an electrically-charged fluid such as plasma swirls so that a magnetic field is created and then amplified. Experts knew that plasma turbulences could create multiple small magnetic fields, but how those fields combined to produce a larger one remained unknown.
“We can observe magnetic fields all over the universe, but we currently lack a sound theoretical explanation for how they are generated,” said Squire. At the heart of the puzzle was the unlikely concept of smaller disturbances combining to form something larger and more organized.
Simulations suggest that small magnetic fields can combine
The study authors explained that the phenomenon is like a tornado, which forms when several atmospheric disturbances occurring during a storm combine to form one giant vortex. In a like manner, large-scale magnetic fields around galaxies and stars seem to form from a multitude of smaller disturbances, but unlike tornados, they persist instead of disappearing.
“Something is holding up the universe’s magnetic fields for billions of years,” Bhattacharjee, head of PPPL’s Theory Department and co-author of the study, said in a statement. “But how exactly does the universe get these persistent magnetic properties?” To find out, he and Squire conducted a series of statistical and numerical simulations using computers at PPPL.
They found that, under certain conditions, small magnetic fields can combine into one larger one. Specifically, when this occurs, there is a large amount of velocity shear (which occurs when two areas of a fluid move at different speeds). Their simulations indicate that the larger fields are able to persist, but to confirm their findings, they would need to run simulations for very low levels of dissipation (a measure of energy loss).
“It is impossible to run simulations for dissipation as low as those of real astrophysical plasmas,” said Bhattacharjee, “but our analytical and computational results, in the range in which they are done, strongly suggest that such dynamo action is possible.”
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Feature Image: NASA/Solar Dynamics Observatory
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