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Magnetic Reconnection

Drake, Swisdak, TenBarge

Magnetic Reconnection
The figure shows results from a large-scale three-dimensional simulation of reconnection made using thousands of processors doing parallel computations. At the x-line in the magnetic field lines (orange-red) topological changes can occur, leading to the development of turbulence (planar region).

In many plasmas an embedded magnetic field is a significant reservoir of energy. The sudden release of this energy underlies many large-scale explosive events including solar flares, substorms in the terrestrial magnetosphere (the source of the auroras), disruptions in laboratory fusion experiments, and perhaps even astrophysical events such as gamma ray bursts. This typically occurs where the magnetic field reverses direction so that it can self-annihilate and transfer its energy to plasma flows and intense, high energy beams of particles.

Called magnetic reconnection, this process leads to the cross-linking of magnetic field lines, the formation of a topological x-line and a breakdown of the the ideal "frozen-in" flux condition. Because it occurs in so many varied environments and because it intimately links large-scale phenomenon to small-scale kinetic behavior, reconnection has been considered one of the most important topics in plasma physics over the past forty years.

Work at Maryland has played a part in many key discoveries about magnetic reconnection. Traditionally it was believed that Alfven waves played the key role in driving the process, but at small scales, the motions of electrons and ions decouple and the dynamics is, in fact, controlled by whistler waves. Whistlers fundamentally alter the reconnection process, causing the release rate of magnetic energy to be insensitive to the mechanism which breaks the frozen-in condition. A second example begins with observations of reconnecting plasmas that often reveal large numbers of non-thermal particles with power-law distributions in energy. Generating such distributions with one x-line is difficult, but when many x-lines form (which is often the case), they form seas of what are termed magnetic islands. The interactions of these islands as they evolve can lead to efficient and significant particle acceleration through a process known as Fermi acceleration.

The work is performed with a mixture of theory and large-scale computer simulations, primarily using a massively-parallel particle-in-cell code that tracks the self-consistent evolution of billions of particles. The results have been applied to systems as diverse as the outer heliosphere (Voyager 1 passing the heliopause), the solar wind at 1 AU, flares in the solar corona, the formation of dipolarization fronts in the magnetotail, and sawtooth crashes in tokamaks.

This research is supported by the National Science Foundation, the Department of Energy, and the National Aeronautics and Space Administration.