Magnetotelluric (MT) responses are commonly interpreted assuming vertical plane-wave excitation. However, the primary electromagnetic (EM) field can be laterally inhomogeneous along the surface near auroral zones and its sources. It is suggested to use synchronous records of MT responses to an inhomogeneous source field with a dominant spatial harmonic. The applicability of this approach is checked using spatial filtering of synchronous wave data and a calculated directional pattern, which has its maximum along the wave vector of the dominant harmonic. The parameters of this harmonics are further estimated by optimization, with the EM field being presented as a sum of transverse electric (TE) and transverse magnetic (TM) modes. The TM mode, which forms in the presence of surface structures, is responsible for galvanic distortions. Inversion of the EM field components recorded synchronously at three or more sites gives the amplitudes of the two field modes and period-dependent apparent resistivities, while the static shift in the data becomes removed. The algorithm has been successfully tested on data from the BEAR project in the Fennoscandian Shield.

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