Magnetotelluric imaging under source complexity with simultaneous observations of incoherent scatter radar and magnetotelluric array in the interior of Fairbanks, Alaska

The magnetotelluric (MT) method has an assumption that the inducing source magnetic fields from ionospheric currents are spatially quasi-uniform at the Earth's surface. This assumption is necessary to obtain reliable MT impedance tensors, which reflects the ground information correctly. We examine the relationship between ionospheric currents and ground magnetic fields in the auroral zone in Alaska. Vertically-integrated ionospheric currents projected to the base of the ionosphere are estimated from analysis of data from the Poker Flat incoherent scattering radar (PFISR) array. While the PFISR array was operating, simultaneous long-period observations of the vector electric and magnetic fields at ground level were obtained at 25 MT stations deployed beneath the ionospheric footprint of the PFISR beam array north of Fairbanks, Alaska. We downward continued the magnetic fields induced by the equivalent sheet currents to ground level. Despite the temporal and spatial complexity of the equivalent sheet currents, the downward continued magnetic fields are seen to be quasi-uniform on the scale of the MT array at ground level. This is confirmed by comparing the downward continued ionospheric fields to those measured at ground level by the MT stations. Small frequency-dependent and spatially non-uniform deviations are observed between the measured ground magnetic and projected ionospheric source fields that might be attributable to secondary magnetic fields induced in the crust and mantle. The present work provides new evidence that the conventional MT method plane-wave source field assumption may be used within the auroral zone at our observation scale. We will also provide the electrical conductivity structure of the region of the interior of Alaska, Fairbanks. The inverted structure shows the conductive anomaly around Tintina-Kaltag fault in this areas.