  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 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, and the bias is large,
especially over the altitudes explored in the MT data analysis (i.e., 100-150
km), where the E layer exists. 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.
