Our Beautiful Moon and its Mysterious Magnetism by Mike Fuller

Author:Mike Fuller
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

6.2 Magnetic Fields of the Cosmos

Is our geomagnetic field unique, or do we see magnetic fields elsewhere in the cosmos? Actually the cosmos abounds with magnetic fields. From the scale of the atom, to the galaxy itself, magnetic fields are everywhere. Indeed the very thoughts in our brains and the beating of our hearts have magnetic fields associated with them. The Sun has a magnetic field, which also reverses. It gives rise to an interplanetary field, which permeates the solar system. The giant outer planets—Jupiter, Saturn, Uranus, and Neptune—all have magnetic fields. Among the inner planets, the Earth, and probably Mercury have magnetic fields. Mars almost certainly had one, but does not now. In the Sun, the giant outer planets, and the Earth, the fields are maintained by magnetohydrodynamic processes taking place in regions of high electrical conductivity. The Sun is not alone among the stars in having a magnetic field—far stronger fields are common.

What causes these universal magnetic fields? The key mechanism is the motion of electric charges, as has been known since the time of Hans Christian Oersted, who noticed, apparently during a lecture he was giving, that when he brought a compass near to a wire carrying an electric current the compass was deflected. However, the generation of the ubiquitous magnetic fields of the cosmos is not a matter of electric wires threading the universe, but a more subtle mechanism whereby magnetic fields are amplified from seed fields. On the cosmic scale these seed fields may take the form of turbulent flow of ionized material with their charged particles, although discussions of their origin tend to be a bit vague.

Let us assume that seed fields are available, then next we need to recognize that most of the universe is a very good electrical conductor, unlike the atmosphere of our planet, which is a very poor electrical conductor. Given a large scale electrically conducting medium with internal motions, magnetic fields will follow, pretty much as night follows day. The field lines are distorted and stirred up by the motion of the conductor in which they are trapped. As a result of the work of a number of theoreticians at the middle of the last century led by Walter Elsasser and Sir Edward Bullard, mechanisms were proposed whereby the geomagnetic field can be maintained by processes going on in the electrically conducting fluid of the outer core. The details of this process get complicated and so are addressed in Appendix 5. We just need to remember that the generation of magnetic fields in highly conducting media by the mechanical energy of their internal motions is universal.

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