The main purpose of this paper is to draw attention to the important but long neglected problem of electrical coupling between the ionosphere and the lower atmosphere. A numerical technique is used to calculate electric fields and currents between 0- and 150-km altitude produced by large-scale horizontal electric fields known to exist in the polar cap and auroral ionosphere. A two-dimensional model assuming a flat earth and a vertical geomagnetic field is used. The results show that horizontal electric fields in the ionosphere map down to ~10 km with little attenuation, in agreement with previous authors' results. In addition, these horizontal ionospheric electric fields should cause significant modulations of vertical electric fields down to the earth's surface. The effects of conductivity irregularities in the ionosphere, stratosphere, and troposphere are also examined. Localized conductivity enhancements associated with aurora are expected to produce a horizontal electric field of up to a few millivolts per meter at ~30-km altitude in the absence of any horizontal field in the ionosphere. These conductivity enhancements have less than a 1% effect on the vertical electric field on the ground. The effects of stratospheric and tropospheric irregularities are negligible at large heights but become important below ~20 km. Their magnitude depends on how the ratio between the local resistivity and the height-integrated columnar resistance is altered.