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Magnetic ReconnectionIn a plasma, sharp changes in magnetic field direction are unstable. Magnetic Reconnection is the process which removes the sharp change in field direction. Reconnection events are seen in the sun's photosphere, where they can cause solar flares, in the Earth's magnetosphere, and in laboratory experiments. According to magnetohydrodynamics (MHD), this happens because the plasma's electrical resistivity near the boundary layer opposes the Current (electricity) necessary to sustain the change in the magnetic field. The need for such a current can be see from one of Maxwell's equations, -
The resistivity of the current layer allows magnetic flux from either side to diffuse through the current layer, cancelling out flux from the other side of the boundary. When this happens, the plasma is pulled out by magnetic tension along the direction of the magnetic field lines. The resulting drop in pressure pulls more plasma and magnetic flux into the central region, yielding a self-sustaining process. A current problem in plasma physics is that observed reconnection happens much faster than predicted by MHD. There are two competing theories to explain the discrepancy. One posits that the electromagnetic turbulence in the boundary layer is sufficiently strong to scatter electrons, raising the plasma's local resistivity. This would allow the magnetic flux to diffuse faster. A second explanation, from Hall MHD states that the ions decouple from that magnetic field at a distance comparable to the ion skin depth, . The electrons are then accelerated to very high speeds by Whistler waves. Because the ions can move through a wider "bottleneck" near the current layer and because the electrons are moving much faster in Hall MHD than in standard MHD, reconnection may proceed more quickly.
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