Applications of the Conducting-Fluid Model
Length scales in space are so large that they are considerably greater than the skin depth for processes which are reasonably fast. Therefore, in space situations all plasmas can be assumed to have infinite conductivity and the concepts of field freezing and magnetic pressure generally apply. Using these laws we can solve a number of problems in astrophysics. For example, a plasma stream known as the solar wind is ejected from the sun and impinges on the upper atmosphere of the earth with significant effects. This stream has no magnetic field embedded in it. According to the field-free zing concept, an external magnetic field should be unable to penetrate such a plasma. There is a random interplanetary magnetic field in the solar system. The plasma stream arriving from the sun displaces this external magnetic field. It might be said that the “plasma broom” sweeps the interplanetary magnetic field from the vicinity of the sun. Magnetic belts are formed around the sun in which the magnetic field is weaker than in neighboring regions. The magnetic belts facilitate the passage to the earth of fast charged particles which are ejected from the sun (corpuscular stream). In other words, when it encounters the magnetic field of the earth, the plasma stream flows around it the way a liquid flows around a solid body. A magnetic cavity is formed around the earth inside of which a magnetic field is confined but which the plasma stream cannot enter. Many charged particles collect on the surface of this magnetic cavity. These particles are observed by satellites and rockets as the outer radiation belt. The analogy of the streaming of the plasma around the cavity and the flow of a liquid around a solid body is more than qualitative; the two phenomena can be described by similar equations. However, in the plasma case the magnetic pressure must be taken into account in addition to the kinetic pressure.
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