Magnetic field data

Magnetic fields are caused by electrical currents in the centers of the planets and thus much can be learned about the planetary interiors by looking at the fi-fields. The mechanism by which planets create magnetic fields is not well understood. The centers of the planets are thought to be electrically conducting and convective. Any stray magnetic fields that might be present will modify the motion of moving electrical charges and will thus induce electrical currents, which will themselves generate their own magnetic field. What appears to happen is that some kind of positive feedback mechanism then acts to increase the strength of the magnetic field in a particular direction, which encourages the currents to flow in one particular sense. This magneto-hydrodynamic effect, called the self-exciting dynamo, is thought to be responsible for generating the magnetic fields of all the planets (Jones, 2007). Since viscous dissipation tends to reduce the field over time, internal rotational energy must be continually "tapped" by the magnetic field in order to sustain it. To do this, most models of the process require that the axis of the magnetic field is not coincident with the rotational axis (Cowling theorem) and this is observed for all the planets with the one exception of Saturn. Even when a field is "stable", as it is for the Earth, it is seen that in fact it evolves and varies over time and even undergoes complete reversals. Hence, the generation of the magnetic field can be seen to be a dynamic and ongoing process. The mean dipole moment of the giant planets' magnetic fields are listed in Table 2.5.

The internal currents generating the magnetic field are distributed, and hence the field is not a simple dipole, but has higher terms (just like the J-coefficients for gravity fields). Hence, observation of the shape of the magnetic field can be used to map the current flows in the interior.

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