Chaotic plasmas & orderly magnetic fields

Print edition : April 19, 2013

ONE of the enduring mysteries of astrophysics is how highly organised structures such as vast magnetic fields stretching out for millions of light years can emerge from the frenetic motion of the superhot ions and electrons that constitute a plasma. Scientists at Lawrence Livermore National Laboratory (LLNL) have discovered that streaming plasmas created by powerful lasers appear to give rise to “self-organised” electromagnetic fields similar to those found throughout the universe, such as those that emanate from young stars or supernovae.

Using the OMEGA Extended Performance laser at the University of Rochester’s Laboratory for Laser Energetics, the team discovered that supersonic plasmas colliding head-on generated large, stable “structures” of electric or magnetic fields by a mechanism yet to be explained. As revealed in proton radiography images, these structures are oriented perpendicular to the direction of the two plasma flows, have detailed features, are much larger and persist much longer than would be predicted from the chaotic motions of the plasma ions and electrons.

“What we observed was completely unexpected,” says Hye-Sook Park, a physicist and the leader of the team. “The plasmas we created moved so quickly that we expected them to freely stream past each other without causing the formation of any regular or long-lasting electric or magnetic fields.” Scientists use the term “self-organisation” to describe the process of large-scale structures arising from chaotic, random activity, including the motion of plasma ions and electrons. Self-organisation is evident in the formation of sand dunes, snowflakes and even leopard’s spots. The phenomenon is believed to play an important role in a wide range of astronomical phenomena. For example, it may contribute to the creation of the organised magnetic fields that are critical to star formation and galaxy evolution. The LLNL experiments show that self-organisation also seems to occur when two supersonic plasma streams meet. The brief interactions between charged particles cause energy to be transported from smaller to larger scales. Throughout the universe, the self-organisation that is evident is similar to what the experiments have seen in this simulated magnetic field in a cluster of galaxies. Collisionless shock waves can occur in the near vacuum of outer space when plasma constituents pass by each other at high velocities, largely without colliding. These can give rise to magnetic fields stretching for hundreds of light years and appear in a wide array of exotic astronomical settings such as violent solar flares. Closer to the earth, a collisionless shock exists where the solar wind encounters the earth’s magnetic field.

“Most astrophysical collisionless shocks can’t be directly measured,” says Nathan Kugland, a member of the research team. “We look to laboratory experiments to better understand these objects. We believe we are seeing order rise from disorder, where microscopic processes lead to macroscopic structures... [which] emerge from turbulence throughout the universe, and it appears that self-organisation can arise from microscopic plasma instabilities on a vastly smaller scale.”

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