Cosmic Magnetohydrodynamics

a branch of astrophysics, developed in the 1940’s, in which the methods of magnetohydrodynamics are applied to investigations of objects in space: the sun, the stars, interstellar gas, the interplanetary medium, magnetic fields, and the matter of near-earth space containing ionized, conducting gas (plasma).

The laws of magnetohydrodynamics describe the interaction of magnetic fields with the motion of the conducting liquid or gas. Currents are induced as the conducting material moves across the magnetic field lines. The field of these currents, when added to the initial field, changes the magnetic field’s structure. When the conductivity is high or when the phenomenon is of a mass scale, the change is such that the lines of force practically follow the material and pass through the same particles (the “adherence” or “freezing-in” of the field to the material). In cases when there is a thin layer of gas between two oppositely directed fields, the lines offeree of one field rapidly pass through the gas and, interacting with the oppositely oriented lines, disappear, or undergo annihilation. The field, in turn, affects the motion of the plasma. This interaction may be described in terms of the tension and the lateral elasticity of the field lines. In this case, forces are generated that resist motions that lead to lateral compression and extension of the lines of force, which increase the magnetic energy. Low-frequency magnetohydrodynamic and magnetoacoustical waves may be propagated in the plasma. The laws of magnetohydrodynamics are applicable to cosmic phenomena, since the condition of the “adherence” of the field to the matter is fulfilled in this case to a sufficient degree owing to the large scale of the phenomena. Convective motions on the sun entrain and entangle the lines of force; the prominences are suspended above the sun’s surface, supported by the field; the field is entrained by the solar wind into interplanetary space; the magnetic field of the galaxy hinders the compression of the gaseous layer and determines its thickness; and so on. One of the most important problems of cosmic magnetohydrodynamics is the question of the origin and strengthening of the field: under certain conditions the motions of the gas may lead to strengthening of an initially weak field (dynamo-effect). This initial field may be generated, in turn, by the diffusion of electrons, which arises as a result of fluctuations in density and temperature or as a result of the friction of electrons against the photon gas of the relict radiation. The theory of the dynamo-effect serves as the basis of the modern explanation of the origin of the earth’s magnetic field.