Evolution of Galaxy-Spanning Magnetic Fields Explained

by Jonathan Sherwood for the University of Rochester

Researchers at the University of Rochester have uncovered how giant magnetic fields up to a billion, billion miles across, such as the one that envelopes our galaxy, are able to take shape despite a mystery that suggested they should collapse almost before they'd begun to form. Astrophysicists have long believed that as these large magnetic fields grow, opposing small-scale fields should grow more quickly, thwarting the evolution of any giant magnetic field. The team discovered instead that the simple motion of gas can fight against those small-scale fields long enough for the large fields to form. The results are published in a recent issue of Physical Review Letters.

"Understanding exactly how these large-scale fields form has been a problem for astrophysicists for a long time," says Eric Blackman, assistant professor of physics and astronomy. "For almost 50 years the standard approaches have been plagued by a fundamental mystery that we have now resolved."

The mechanism, called a dynamo, that creates the large-scale field twists up the magnetic field lines as if they were elastic ribbons embedded in the sun, galaxy or other celestial object. Turbulence kicked up by shifting gas, supernovae, or nearly any kind of random movement of matter, combined with the fact that the star or galaxy is spinning carries these ribbons outward toward the edges. As they expand outward they slow like a spinning skater extending her arms and the resulting speed difference causes the ribbons to twist up into a large helix, creating the overall orderly structure of the field.

The turbulence that creates the large-scale field, however, also creates opposing small-scale fields due to the principle of conservation of magnetic helicity. As both large and small fields get stronger, they start to suppress the turbulence that gave rise to them. This is called a "backreaction," and researchers have long suspected that it might halt the growth of the large field long before it reached the strength we see in the universe today. Blackman and George Field, the Robert Wheeler Wilson Professor of Applied Astronomy at Harvard University, found that in the early stages the backreaction was weak allowing the large field to grow quickly to full strength. Once the large field comes to a certain strength, however, conservation of magnetic helicity will have made the backreaction strong enough to overcome the turbulence and stop further growth of the large field. The large-scale field and the backreaction then keep to a steady equilibrium.

To tease out the exact nature of the backreaction, the team took a new approach to the problem. "Most computer simulations use brute force," Blackman explains. "They take every known variable and crank through them. Such simulations are important because they yield results, but like experiments, you don't know what variables were responsible for giving you those results without further investigation."

Blackman and Field simplified the problem to pinpoint which variables affected the outcome. They found that only the helical component of the small field contributes to the backreaction, twisting in the opposite direction to that of the large field. Scientists were not sure how strong the backreaction had to be to start influencing the turbulence, but the team has shown that the backreaction is weak when the large-scale field is weak, having little effect on the turbulence. It's not until the large field grows quite strong that the backreaction grows strong as well and begins to suppress the motions of matter, stopping the further growth of the overarching magnetic field.

The simple theory will likely be able to explain how magnetic fields evolve in stars like our sun, whole galaxies, and even gamma-ray bursts-the most powerful bursts of energy ever seen in the universe. Scientists suspect that gamma-ray bursts use powerful magnetic fields to catapult intense outflows into space. In addition, the theory explains the ordered magnetic structures that emerge in advanced "brute force" computational experiments by Axel Brandenburg, professor at the Nordic Institute for Theoretical Astrophysics, and by Jason Maron, postdoctoral fellow at the University of Rochester. Blackman is now collaborating with both scientists to further explore the consequences of the theory.

This research was funded by the U.S. Department of Energy.