  The magnetotelluric (MT) responses of the Earth are biased by spatially
heterogeneous source fields. Recently, such biases have been reported even in
mid-latitude areas, where localized source currents rarely flow. This study
focuses on shifts in the MT responses arising from variations in the focus
latitude of the solar quiet (Sq) current. The MT responses at 60 s were
calculated by changing the center of the Sq current. Slight variations in the
focus latitude cause large shifts in the apparent resistivity and phase. During
periods of quiet geomagnetic activity, the center varies within a wide range of
20-45 N, whereas this range narrows when there are disruptions. Even though
this study considers only a limited case, the results demonstrate the
instability of the impedances in periods of quiet geomagnetic activity and a
correlation of the MT responses with the magnitude of the geomagnetic activity.
As a consequence, even when a station is located at mid-latitudes, the
impedances need to be treated carefully for time-lapse MT soundings, for
example, by checking the ionospheric current conditions and their dependence on
the geomagnetic activity.
