10 - Magnetohydrodynamics

Summary

Magnetic fields are often given a secondary position of importance in the study of dense plasma. Indeed, many aspects of plasma may be adequately described without including magnetic fields. There are some physical processes, however, that require consideration of the effects of magnetic fields, and in certain circumstances their effects may dominate.

Magnetic fields are found nearly everywhere there is plasma, from the farthest regions of deep space to that of our closer terrestrial environment. The magnetosphere surrounding the Earth has an induction of order 10^–5 G, and the stream of charged particles from the Sun (the solar wind) interacts with it to create the Van Allen radiation belts. As the charged particles spiral down the magnetic field lines toward the Earth's surface, their collisions with the atoms and molecules of the upper atmosphere create the polar aurorae.

A bit farther from Earth, the magnetic fields are responsible for the existence of solar flares. Flares occur around sunspots where intense magnetic fields penetrate the photosphere and link the corona to the solar interior. These flares are powered by the sudden release of magnetic energy stored in the corona and can extend far from the surface of the Sun. One also observes coronal mass ejections, in the form of charged particles, which occur during magnetic reconnection events.