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.
