ated. The larger-scale horizontal electric fields that do map to the surface become vertical at the surface because of the high surface conductivity. Thesolar-wind/magne- tospherc field can alter the surface fields at high latitudes by 20 to 50 V/m depending on the level of geomagnetic activity and the magnitude of the dawn-to-dusk potential drop across the magnetic polar cap. Telluric Currents Telluric currents consist of both natural and man-induced electric currents flowing in the solid earth and oceans. The fundamental causes of the natural currents are elec-tromagnetic induction resulting from a time-varying, external geomagnetic field or the motion of a conducting body (such as seawater) across the Earth's internal magnetic field. These telluric currents, in turn, producc magnetic fields of their own that add to the external geomagnetic field and that produce a feedback on the ionospheric current system. The complexities assort ated with telluric currents arise from the complexities in the external current sources and the conductivity structure of the Earth (sec Chapter 16). The external inducingfield also has various scale sizes that contribute to the complex-ities in the telluric current systems. The ionospheric dynamo currents that are associ-ated with the solar diurnal and lunar tides have u plunetarv-scale size. The ionospheric current variations, however, also have smaller-scale features that are associated with auroral and equatorial electrojets. At low frequencies, the external inducing sources can be approximated by a planetary-scale field that is occasionally altered by strong spatial gradients during geomagnetically disturbed conditions. At higher frequencies (magnetic storms, substorms, or geomagnetic pulsations), the source can often be quite localized and highly time dependent. Electromagnetic induction causcd by ionospheric and magnetosphcriccurrent vari-ations has a pronounced effect on telluric currents and on man-made systems. These effects have been detected by a number of investigators, and it is now well recognized that there Ls a direct electromagnetic coupling from the ionosphere to the telluric cur-rents. The large variation in conductivity of the solid earth can give rise to various channeling effects within the Earth, thereby considerably complicating the flow pat-terns of the telluric currents. The current patterns are different for different frequen-cies of external induction. The longer the period of the time-varying field, the deeper into the Earth the induced currents are expected to flow. For example, a signal with a period of about 24 hours is generally believed Juicy Couture Outlet Online to have a skin Juicy Couture Sale depth of 600 to 800 km. The distribution of sediments, the degree of hydration, differences in porosity, and other proi>erties Juicy Couture Outlet of the Earth all have an influence on the signal resj>onse. Properly inter-preted, telluric currents can be a tool to study both shallow and deep structures within the Earth.