The magnetohydrodynamics of current sheets

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Abstract
A universal phenomenon in cosmic plasmas is the creation of sheets of intense current near X-type neutral points (where the magnetic field vanishes). These sheets are important as sites where the magnetic-field energy is converted efficiently into heat and bulk kinetic energy and where particles can be accelerated to high energies. Examples include disruptions in laboratory Tokamaks, substorms in the Earth's magnetosphere and flares on the Sun. During those phenomena the antiparallel magnetic-field lines on both sides of a current sheet can be cut and reconnect with each other, so changing the global topology of the field. The author summarises the examples of current sheets and describes how they are formed. Then the basic behaviour of a one-dimensional sheet is presented, together with an account of the linear tearing-mode instability that can cause the field lines in such a sheet to reconnect. Such reconnection may develop in different ways: it may arise from a spontaneous instability or it may be driven, either from outside by motions or locally by a resistivity enhancement. The last three sections describe the various processes that may occur during the non-linear development of tearing as well as the many numerical and laboratory experiments that are aiding the understanding of this intriguing cosmical process.