Reconnection of Magnetic Fields: Magnetohydrodynamics and Collisionless Theory and Observations

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Research Letter Free Access. Walker, E-mail address: rwalker igpp. Melvyn L. Read the full text. Tools Request permission Export citation Add to favorites Track citation. Share Give access Share full text access. Share full text access. Please review our Terms and Conditions of Use and check box below to share full-text version of article.

Magnetic reconnection: MHD theory and applications

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Identifying the electron diffusion region in a realistic simulation of Earth's magnetotail

Email or Customer ID. In three dimensions, the geometry of the field lines become more complicated than the two-dimensional case and it is possible for reconnection to occur in regions where a separator does not exist, but with the field lines connected by steep gradients.

The first theoretical framework of magnetic reconnection was established by Peter Sweet and Eugene Parker at a conference in Sweet pointed out that by pushing two plasmas with oppositely directed magnetic fields together, resistive diffusion is able to occur on a length scale much shorter than a typical equilibrium length scale. The Sweet-Parker model describes time-independent magnetic reconnection in the resistive MHD framework when the reconnecting magnetic fields are antiparallel oppositely directed and effects related to viscosity and compressibility are unimportant.

When the inflow density is comparable to the outflow density, conservation of mass yields the relationship. The left and right hand sides of the above relation represent the mass flux into the layer and out of the layer, respectively. Equating the upstream magnetic pressure with the downstream dynamic pressure gives.

Solving for the outflow velocity then gives. Sweet-Parker reconnection allows for reconnection rates much faster than global diffusion, but is not able to explain the fast reconnection rates observed in solar flares, the Earth's magnetosphere, and laboratory plasmas. Additionally, Sweet-Parker reconnection neglects three-dimensional effects, collisionless physics, time-dependent effects, viscosity, compressibility, and downstream pressure.

Numerical simulations of two-dimensional magnetic reconnection typically show agreement with this model. One of the reasons why Sweet-Parker reconnection is slow is that the aspect ratio of the reconnection layer is very large in high Lundquist number plasmas. The inflow velocity, and thus the reconnection rate, must then be very small. In , Harry Petschek proposed a mechanism where the inflow and outflow regions are separated by stationary slow mode shocks. Simulations of resistive MHD reconnection with uniform resistivity showed the development of elongated current sheets in agreement with the Sweet-Parker model rather than the Petschek model.

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When a localized anomalously large resistivity is used, however, Petschek reconnection can be realized in resistive MHD simulations. Because the use of an anomalous resistivity is only appropriate when the particle mean free path is large compared to the reconnection layer, it is likely that other collisionless effects become important before Petschek reconnection can be realized. In the Sweet-Parker model, the common assumption is that the magnetic diffusivity is constant.

Nevertheless, if the drift velocity of electrons exceeds the thermal velocity of plasma, a steady state cannot be achieved and magnetic diffusivity should be much larger than what is given in the above.

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Another proposed mechanism is known as the Bohm diffusion across the magnetic field. Lazarian and Vishniac considered the magnetic reconnection in the presence of a random component of magnetic field in a totally ionized and inviscid plasma assuming that the resistive effects could be described with an Ohmic resistivity. Lazarian and Vishniac showed that, in general, this cannot affect the final result.

In fact, their model is independent of small scale physics which determines the local reconnection rate. This model has been successfully tested by numerical simulations. On these scales, the Hall effect becomes important. Two-fluid simulations show the formation of an X-point geometry rather than the double Y-point geometry characteristic of resistive reconnection.

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.

Magnetic reconnection occurs during solar flares , coronal mass ejections , and many other events in the solar atmosphere. In the past, observations of the solar atmosphere were done using remote imaging; consequently, the magnetic fields were inferred or extrapolated rather than observed directly. However, the first direct observations of solar magnetic reconnection were gathered in and released in by the High Resolution Coronal Imager.

Magnetic reconnection events that occur in the Earth's magnetosphere in the dayside magnetopause and in the magnetotail were observed by spacecrafts such as Cluster II [18] and the Magnetospheric Multiscale Mission. It has observed numerous reconnection events in which the Earth's magnetic field reconnects with that of the Sun i. These include 'reverse reconnection' that causes sunward convection in the Earth's ionosphere near the polar cusps; 'dayside reconnection', which allows the transmission of particles and energy into the Earth's vicinity and 'tail reconnection', which causes auroral substorms by injecting particles deep into the magnetosphere and releasing the energy stored in the Earth's magnetotail.

The Magnetospheric Multiscale Mission , launched on 13 March , improved the spatial and temporal resolution of the Cluster II results by having a tighter constellation of spacecraft. This led to a better understanding of the behavior of the electrical currents in the electron diffusion region.

Vassilis Angelopoulos of the University of California, Los Angeles, who is the principal investigator for the THEMIS mission, claimed, "Our data show clearly and for the first time that magnetic reconnection is the trigger. Magnetic reconnection has also been observed in numerous laboratory experiments. The confinement of plasma in devices such as tokamaks , spherical tokamaks , and reversed field pinches requires the presence of closed magnetic flux surfaces. By changing the magnetic topology, magnetic reconnection degrades confinement by disrupting these closed flux surfaces, allowing the hot central plasma to mix with cooler plasma closer to the wall.

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Bibcode : JGR Journal of Geophysical Research: Space Physics. Bibcode : JGRA.. September Solar Physics.

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