Altitude effects of localized source currents on magnetotelluric responses

The effects of localized source currents on Earth’s magnetotelluric (MT) responses have been reported in the literature in terms of the changes in period and subsurface structure. The focus in this study is on the bias within the MT responses arising from variations in the vertical and horizontal distances of the source current. The MT responses at the periods of field aligned resonance (20 and 200 s) were calculated at various distances from the source current. A slight change in source distance causes a shift in the MT responses. The shift of apparent resistivity at 20 and 200 s exceeds 10% and 50% of the original values, respectively. Such features are found especially over the altitude range of the ionospheric E layer (i.e., 100–150 km), where MT source currents often flow. The vertical distance of the source field varies because the distribution of conductivity with altitude in the ionosphere and the region controlling the ionospheric electrical process change temporally. Thus, in assessing the temporal changes in MT responses, we should treat them carefully by checking the ionospheric environment.[graphic not available: see fulltext]