|Smaller Scales: Five Physical Processes|
Magnetic reconnection is a fundamental mechanism by which magnetic energy is dissipated in the universe. The release of energy is observed to occur in bursts, rather than in a continuous manner, and is the driver for phenomena such as solar flares (a) and magnetospheric substorms. When the solar wind collides with the Earth's magnetosphere, reconnection can sometimes occur, resulting in the acceleration of energetic particles that produce spectacular auroral displays. The basic mechanism of reconnection has been understood since the late 50's. In reconnection, magnetic field lines of opposite direction in a plasma break and then cross-link or reconnect, forming an x-line magnetic topology (a-c).
Left: TRACE image of a solar flare
The topological change in the structure of the magnetic field occurs in a spatially localized boundary layer and requires some form of dissipation. The release in the magnetic stress of the newly reconnected field lines accelerates the plasma away from the x-line to velocities of the order of the Alfvén speed (d).
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Reconnection is of great interest because the highly energetic particles released can cause electrical and communications disruptions on Earth, as well as problems for satellites and spacecraft. Researchers believe that solar flares are the result of reconnection.
Three outstanding scientific issues have dominated discussion about the physics of magnetic reconnection in the last 20 years:
We now consider problems associated with reconnection. Theories of reconnection by Sweet and Parker predicted the formation of macroscopic current sheets and indicated that reconnection was too slow to explain the observations. Petschek proposed a more open x-line configuration, whose time scales were consistent with the data. Subsequent computer simulations indicated that, at the low resistivities representative of the solar corona or the magnetosphere, reconnection evolves into the Sweet-Parker, rather than the Petschek, geometry. Thus, theoretical rates of reconnection were too slow to explain observations, unless some form of "anomalous resistivity" was invoked. Anomalous resistivity can result from the inclusion of wave-particle or turbulence-particle interactions in the physical model, which provides an important source of additional transport for the particles.
A significant recent advance has been the development of kinetic models of reconnection, which include a more general form of Ohm's law for the electric field. These new models produce the open Petschek geometry and associated fast rates, consistent with observation, of reconnection. The inclusion of kinetic effects introduces smaller scales into the problem, which is important because reconnection of magnetic fields occurs at the small scales that characterize the dissipation region. Researchers are just beginning to use the latest generation of parallel computers to explore the role of "anomalous resistivity" that may arise from electric field fluctuations produced by the relative streaming of electrons and ions in the dissipation region (e).
(GIF Animation - 1.31M)
Simulation of reconnection
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