The internal magnetic field of Earth or any planet is normally considered to have three components at most: A large, globally coherent field is attributed to a dynamo operating deep within the planet; a spatially complex, usually much smaller field is attributed to permanent magnetism of near‐surface rocks, and a small time‐varying magnetotelluric field is induced by the external time‐varying field. For Earth at least, the separation of these three is not difficult because of their very different spatial and temporal characteristics. There are some bodies for which the last two fields dominate. For example, the Martian field appears to be dominated by permanent magnetism in the crust, and the change in field near Europe is dominated by an induced field, presumably caused by electrical currents induced in a salty ocean beneath an icy shell. see Dynamos, Planetary and Satellitefor a discussion of the conventional explanations of observed fields. This section is concerned with other...
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