Press Release

Evolution of Galaxy-Spanning Magnetic Fields Explained

By SpaceRef Editor
January 8, 2003
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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.

SpaceRef staff editor.