Investigating the trigger for an explosive process that occurs throughout the universe

A long-standing puzzle in space science is what triggers fast magnetic reconnection, an explosive process that unfolds throughout the universe more rapidly than theory says it should. Solving the puzzle could enable scientists to better understand and anticipate the process, which ignites solar flares and magnetic space storms that can disrupt cell phone service and black out power grids on Earth.

Magnetic reconnection separates and violently reconnects the magnetic fields in plasma, the state of matter composed of free electrons and atomic nuclei that make up 99 percent of the visible universe. Researchers at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and Princeton University have recently produced a formula for tracking the development of “plasmoid-instability-mediated disruptions,” which trigger the transition from slow to fast reconnection.

The research traces the dependence of the instability on conditions ranging from the electrical conductivity of the plasma — measured by what is called the Lundquist number — to the natural noise of the system. “You give me the Lundquist number and system noise and I can fit it into the formula that will spit out the answer,” said physicist Yi-Min Huang, a Princeton University member of the PPPL Theory Department and lead author of a paper describing the process in Physics of Plasmas.

Tracking the evolution of “plasmoids”

The calculation tracks the evolution of plasmoids, bubbles that form in current-carrying sheets of plasma. When the bubbles are large enough, they trigger disruptions that cause fast reconnection. “We are interested in finding out when the plasmoids will disrupt the current sheet and the number of plasmoids when disruptions happen,” Huang said.

The formula for relating the factors that lead to disruptions is based on a complex “phenomenological” model — one deduced from a combination of physical reasoning and mathematical derivation. “As a general rule,” Huang said, “a phenomenological model must be tested through numerical simulations” against first-principle, or standard, physics models.

The versatile new formula tracks the dependence of disruptions on a broad range of high Lundquist numbers. Derived results can be compared with simulations of laboratory experiments and used to describe the development of plasmoid instabilities in natural systems. 
Previously, the dependence could only be obtained by solving the equations of the complex model, Huang said.

Co-authors of the Physics of Plasmas paper were Luca Comisso of Columbia University and Amitava Bhattacharjee, head of PPPL’s Theory Department. Support for the research comes from the National Science Foundation, the DOE Office of Science and the National Aeronautics and Space Administration.

PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit energy.gov/science.

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