Astrophysicists settle cosmic debate on magnetism of planets and stars

3-D radiation magneto-hydrodynamic FLASH simulation of the
experiment, performed on the Mira supercomputer at Argonne
National Laboratory. The values demonstrate strong
amplification of the seed magnetic fields by turbulent
dynamo. Credit: Petros Tzeferacos/University of Chicago

The universe is highly magnetic, with everything from stars
to planets to galaxies producing their own magnetic fields.
Astrophysicists have long puzzled over these surprisingly
strong and long-lived fields, with theories and simulations
seeking a mechanism that explains their generation.


Using one of the world’s most powerful laser facilities, a team
led by University of Chicago scientists experimentally
confirmed one of the most popular theories for cosmic generation: the turbulent dynamo.
By creating a hot turbulent plasma the size of a penny, that
lasts a few billionths of a second, the researchers recorded
how the turbulent motions can amplify a weak magnetic to the strengths of those observed in our
sun, distant stars, and galaxies.

The paper, published this week in Nature Communications,
is the first laboratory demonstration of a theory, explaining
the magnetic field of numerous cosmic bodies, debated by
physicists for nearly a century. Using the FLASH physics
simulation code, developed by the Flash Center for
Computational Science at UChicago, the researchers designed an
experiment conducted at the OMEGA Laser Facility in Rochester,
NY to recreate turbulent dynamo conditions.

Confirming decades of numerical simulations, the experiment
revealed that turbulent plasma could dramatically boost a weak
magnetic field up to the magnitude observed by astronomers in
stars and galaxies.

“We now know for sure that turbulent dynamo exists, and that
it’s one of the mechanisms that can actually explain
magnetization of the universe,” said Petros Tzeferacos,
research assistant professor of astronomy and astrophysics and
associate director of the Flash Center. “This is something that
we hoped we knew, but now we do.”

A mechanical dynamo produces an electric current by rotating
coils through a magnetic field. In astrophysics, dynamo theory
proposes the reverse: the motion of electrically-conducting
fluid creates and maintains a magnetic field. In the early 20th
century, physicist Joseph Larmor proposed that such a mechanism
could explain the magnetism of the Earth and Sun, inspiring
decades of scientific debate and inquiry.

While
demonstrated that turbulent plasma can generate magnetic fields
at the scale of those observed in stars, planets, and galaxies,
creating a turbulent dynamo in the laboratory was far more
difficult. Confirming the theory requires producing plasma at
extremely high temperature and volatility to produce the
sufficient turbulence to fold, stretch and amplify the magnetic
field.

To design an experiment that creates those conditions,
Tzeferacos and colleagues at UChicago and the University of
Oxford ran hundreds of two- and three-dimensional simulations
with FLASH on the Mira supercomputer at Argonne National
Laboratory. The final setup involved blasting two penny-sized
pieces of foil with powerful lasers, propelling two jets of
plasma through grids and into collision with each other,
creating turbulent fluid motion.

“People have dreamed of doing this experiment with lasers for a
long time, but it really took the ingenuity of this team to
make this happen,” said Donald Lamb, the Robert A. Millikan
Distinguished Service Professor Emeritus in Astronomy &
Astrophysics and director of the Flash Center. “This is a huge
breakthrough.”

The team also used FLASH simulations to develop two independent
methods for measuring the magnetic field produced by the
plasma: proton radiography, the subject of a recent paper from
the FLASH group, and polarized light, based on how astronomers
measure the magnetic fields of distant objects. Both
measurements tracked the growth in mere nanoseconds of the
magnetic field from its weak initial state to over 100
kiloGauss—stronger than a high-resolution MRI scanner and a
million times stronger than the magnetic field of the Earth.

“This work opens up the opportunity to verify experimentally
ideas and concepts about the origin of magnetic fields in the
universe that have been proposed and studied theoretically over
the better part of a century,” said Fausto Cattaneo, Professor
of Astronomy and Astrophysics at the University of Chicago and
a co-author of the paper.

Now that a turbulent dynamo can be created in a laboratory,
scientists can explore deeper questions about its function: how
quickly does the magnetic field increase in strength? How
strong can the field get? How does the magnetic field alter the
turbulence that amplified it?

“It’s one thing to have well-developed theories, but it’s
another thing to really demonstrate it in a controlled
laboratory setting where you can make all these kinds of
measurements about what’s going on,” Lamb said. “Now that we
can do it, we can poke it and probe it.”

Explore further:

Computational astrophysics team uncloaks magnetic fields of
cosmic events

More information: P. Tzeferacos et al, Laboratory
evidence of dynamo amplification of magnetic fields in a
turbulent plasma, Nature Communications (2018).
DOI: 10.1038/s41467-018-02953-2