Everything about Magnetic Reconnection totally explained
Magnetic reconnection is the process whereby magnetic field lines from different magnetic domains are spliced to one another, changing their patterns of connectivity with respect to the sources. It is a violation of an approximate conservation law in plasma physics, and can concentrate mechanical or magnetic energy in both space and time.
Solar flares, the largest explosions in the
solar system, may involve the reconnection of large systems of magnetic flux on the
Sun, releasing in minutes energy that's stored in the magnetic field over a period of hours to days. Magnetic reconnection in
Earth's
magnetosphere is one of the mechanisms responsible for the
aurora, and it's important to the science of controlled
nuclear fusion because it's one mechanism preventing
magnetic confinement of the fusion fuel.
In an electrically conductive
plasma, magnetic field lines are grouped into 'domains' – bundles of field lines that connect from a particular place to another particular place, and that are topologically distinct from other field lines nearby. This topology is approximately preserved even when the magnetic field itself is strongly distorted by the presence of variable currents or motion of magnetic sources, because effects that might otherwise change the magnetic topology instead induce
eddy currents in the plasma; the eddy currents have the effect of canceling out the topological change.
The most common type of magnetic reconnection is
separator reconnection, in which four separate magnetic domains exchange magnetic field lines. Domains in a magnetic plasma are separated by
separatrix surfaces: curved surfaces in space that divide different bundles of flux. A separatrix surface may be compared to the
fascia that separate
muscles in an organism: field lines on one side of the separatrix all terminate at a particular magnetic pole, while field lines on the other side all terminate at a different pole of similar sign. Since each field line generally begins at a north magnetic pole and ends at a south magnetic pole, the most general way of dividing simple flux systems involves four domains separated by two separatrices: one separatrix surface divides the flux into two bundles, each of which shares a south pole, and the other separatrix surface divides the flux into two bundles, each of which shares a north pole. The intersection of the separatrices forms a
separator, a single line that's at the boundary of the four separate domains. In separator reconnection, field lines enter the separator from two of the domains, and are spliced one to the other, exiting the separator in the other two domains (see the figure).
Separatrices often (but not always) coincide with
current sheets that mark a sudden change in the direction of the magnetic field, but a current sheet isn't necessary to the formation of a separatrix, or to magnetic reconnection. As a limiting case,
refrigerator magnets moved near one another in Earth's atmosphere cause nearly continuous magnetic reconnection, although no electrical current flows through the
air (it is an
insulator).
According to simple resistive
magnetohydrodynamics (MHD) theory, reconnection happens because the plasma's
electrical resistivity near the boundary layer opposes the
currents necessary to sustain the change in the magnetic field. The need for such a current can be seen from one of
Maxwell's equations,
» . 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.
New measurements from the Cluster mission
for the first time now can determine unambiguously the scale sizes of magnetic reconnection in the earth's
magnetosphere, both on the dayside
magnetopause and in the
magnetotail.
Cluster is a four-spacecraft mission, with the four spacecraft in a tetrahedron arrangement, to separate spatial from temporal changes as the suite flies through space. Cluster has now also unambiguously discovered 'reverse reconnection' near the polar cusps. 'Dayside reconnection' allows interconnection of the earth's magnetic field with that of the Sun (the
Interplanetary Magnetic Field), allowing particle and energy entry into the Earth's vicinity. Tail reconnection allows release of energy stored in the Earth's magnetic tail, injecting particles deep into the magnetosphere, causing auroral substorms. 'Reverse reconnection' is reconnection of Earth's tail magnetic fields with northward Interplanetary Magnetic Fields, causing sunward convection in the Earth's
ionosphere. The upcoming
Magnetospheric Multiscale Mission will improve on Cluster results by having a tighter constellation of spacecraft, allowing finer spatial measurements and finer time detail. In this way the behavior of the electrical currents in the electron diffusion region will be understood.
Further Information
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