On the plane wave approximation of the external geomagnetic field in regional induction studies

In interpreting geomagnetic and magnetotelluric sounding results, it is usually assumed that the external field inducing currents within the Earth can be approximated at middle geomagnetic latitudes by a plane wave vertically incident on the Earth's surface. Using data from eight European observatories, we analyzed the correctness of this assumption depending on the magnetic activity and geomagnetic latitude. Our analysis shows that, in many cases, this assumption is invalid. Therefore, it seems appropriate to preliminarily select data for their subsequent analysis.