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.

中文翻译：

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局部源电流对大地电磁响应的海拔影响

文献中已经报道了局部源电流对地球大地电磁 (MT) 响应的影响，包括周期和地下结构的变化。本研究的重点是由源电流的垂直和水平距离变化引起的 MT 响应中的偏差。在距源电流不同距离处计算场对齐谐振周期（20 和 200 秒）的 MT 响应。源距离的微小变化会导致 MT 响应发生变化。20 秒和 200 秒的视电阻率变化分别超过原始值的 10% 和 50%。尤其是在电离层 E 层的高度范围内（即 100-150 公里），MT 源电流经常流动，这种特征尤其明显。由于电离层和控制电离层电过程的区域的电导率随高度的分布随时间而变化，因此源场的垂直距离会发生变化。因此，在评估 MT 响应的时间变化时，我们应该通过检查电离层环境来仔细对待它们。