Magnetic reconnection occurs in three dimensions when the electric current is so strong locally that the magnetic field lines slip through the plasma, so that there is a change of magnetic connectivity of plasma elements. A three-dimensional null point possesses two special classes of field lines that link to the null, namely an isolated spine curve and a fan surface of field lines.
Fig 5.1. A 3D Null Point.
We have discovered four distinct types of magnetic reconnection in three dimensions, the first two at null points, the third at a separator and the fourth in the absence of null points (Priest and Titov, 1996, Phil. Trans. Roy. Soc. Lond A 354, 2951; Priest and Demoulin, 1995, J. Geophys. Res. 100, 23443). A null point, where the magnetic field vanishes, possesses an isolated field line called a spine that approaches (or recedes from) the null and a whole surface of field lines called a fan that recede from (or approach) the null.
Fig 5.2. Spine Reconnection.
Spine Reconnection may be driven by continuous plasma motions across the fan and is accompanied by a concentration of current along or near the spine and an unfurling of a flux surface from the spine as it is cut and rejoined.
Fig 5.3. Fan Reconnection.
Fan Reconnection occurs in response to plasma motions across the spine and is accompanied by a concentration of current in the fan and a swirling motion of field lines in opposite directions above and below the fan.
Fig 5.4. Separator Reconnection.
Separator Reconnection occurs with a concentration of current along a separator, the special magnetic field line that joins two nulls.
Fig 5.5. Quasi-Separatrix Layer Reconnection.
Quasi-Separatrix Layer Reconnection takes place at singular surfaces (quasi-separatrix layers) where the gradients of the mapping of field lines from one boundary to another are large enough that the field lines can flip rapidly through the plasma.
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