An enduring astronomical mystery is how stars and galaxies acquire their magnetic fields. Physicists Jonathan Squire and Amitava Bhattacharjee at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) have found a clue to the answer in the collective behavior of small magnetic disturbances. In a paper published in October in Physical Review Letters, the scientists report that small magnetic perturbations can combine to form large-scale magnetic fields just like those found throughout the universe. This research was funded by the DOE Office of Science.
Squire and Bhattacharjee 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. "We can observe magnetic fields all over the universe," said Squire. "But we currently lack a sound theoretical explanation for how they are generated."
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. For instance, if you add a glob of milk to coffee, the glob will dissolve into a collection of tendrils and continue to dissipate until the milk has mixed with all of the coffee. Though not impossible, it's highly unlikely that the dissipated milk would spontaneously gather and reform the original glob.
This kind of natural organization occurs occasionally in the natural world, however. When a tornado forms, the myriad disturbances in the atmosphere during a severe storm coalesce into one giant vortex. But the tornado eventually collapses and the order disappears.
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 and co-author of the paper. "But how exactly does the universe get these persistent magnetic properties?"
In the paper, Squire and Bhattacharjee show that under certain conditions small magnetic fields -- instead of small velocity fields, which have been studied more often -- can combine to form one large field. 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 two 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 so-called 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.
PPPL, on Princeton University's Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas -- ultra-hot, charged gases -- and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy's Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.
Raphael Rosen | EurekAlert!
Physicists made crystal lattice from polaritons
20.03.2018 | ITMO University
Mars' oceans formed early, possibly aided by massive volcanic eruptions
20.03.2018 | University of California - Berkeley
A new scenario seeking to explain how Mars' putative oceans came and went over the last 4 billion years implies that the oceans formed several hundred million...
For the first time, an interdisciplinary team from the University of Basel has succeeded in integrating artificial organelles into the cells of live zebrafish embryos. This innovative approach using artificial organelles as cellular implants offers new potential in treating a range of diseases, as the authors report in an article published in Nature Communications.
In the cells of higher organisms, organelles such as the nucleus or mitochondria perform a range of complex functions necessary for life. In the networks of...
Animal photoreceptors capture light with photopigments. Researchers from the University of Göttingen have now discovered that these photopigments fulfill an...
On 15 March, the AWI research aeroplane Polar 5 will depart for Greenland. Concentrating on the furthest northeast region of the island, an international team...
The world’s second-largest ice shelf was the destination for a Polarstern expedition that ended in Punta Arenas, Chile on 14th March 2018. Oceanographers from...
19.03.2018 | Event News
16.03.2018 | Event News
13.03.2018 | Event News
20.03.2018 | Physics and Astronomy
20.03.2018 | Physics and Astronomy
20.03.2018 | Earth Sciences