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Geomagnetically induced currents (GIC) produced by the geoelectric fields at the earth's surface may affect the normal operation of power systems. These electric fields are produced by magnetospheric-ionospheric currents and are affected by currents induced within the ground. To investigate the effects of the geoelectric field due to different source currents, we examine the ratio of the geoelectric field to the geomagnetic field (surface impedance) as a function of the horizontal distance from the source, and of the height and frequency of the source. The small-scale and large-scale surface impedance (ZS and ZL) are given by the “line current” and “plane wave” sources, respectively. We show that ZS may sometimes be significantly different from ZL, so the use of the latter in connection with studies of GIC will lead to erroneous results. However, when the distance from the source increases or the frequency considered increases, ZS approaches ZL.