Natural Sources for Electromagnetic Induction Studies
Time changes of the magnetic field, regardless whether they are of internal (core) or of external (in the atmosphere, ionosphere, and magnetosphere) origin, produce secondary currents in the Earth's interior. Observations of the superposition of the primary (inducing) and secondary (induced) electromagnetic field allows for a determination of the electrical conductivity of the Earth's interior.
Mantle conductivity can be probed “from below” using signals originating from the outer core. This method requires a precise determination of the field during rapid and isolated events, like geomagnetic jerks (q.v.), and some a prioriassumptions about the kinematics of the fluid motion at the top of the core. Conductivity of mantle and crust can also be studied “from above” by the analysis of field changes of external origin, as the induced currents of these fluctuations modify the observed electromagnetic field. Both methods require good knowledge of the time‐space structure of the inducing...
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