A sophisticated theoretical model of the auroral electrojet current system is applied to estimate the source effect distortion in magnetotelluric apparent resistivity values at auroral latitudes. Clear differences from the correct Cagniard plane wave value can occur at periods larger than about 1 min. It is further indicated that changing values of the parameters included in the electrojet model may essentially affect the source effect distortion, which complicates an estimation of the distortion. Thus it is very difficult to extract information about the Earth's conductivity structure from magnetotelluric measurements influenced by electrojet currents. In order not to overestimate the source effects, models in which the electromagnetic field, which is produced by an electrojet system, is superposed on a background field are also discussed. As expected, the source effect distortion then becomes clearly smaller in the apparent resistivity, but it does not vanish.
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