At the Earth's surface, magnetic field sensors (,magnetometers) not only respond to the Earth's main field but also to the summation of the external ionospheric current fields and to the fields of induced internal currents that flow within the conducting Earth (Figure 3.19). Geomagneticians use the separated, surface field measurements of the quiet-day external and internal Sq current signals to measure the upper-mantle electrical conductivity of the Earth. Once they establish the ionospheric source current size and patterns, special mathematical techniques allow them to compute how the Earth's conductivity must change with depth to allow the observed induced currents to flow there and contribute to the observed surface field behavior. The time variation and scale of the ionospheric quiet currents limit the penetration depth, for the conductivity determinations, from the surface to the start of the Earth's lower mantle (Figure 2.24).

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