Probing upper mantle electrical conductivity with daily magnetic variations using global-to-local transfer functions

Abstract

Summary We present new transfer functions (TFs) that can handle external electromagnetic (EM) sources of complex geometry. These TFs relate global expansion coefficients describing the source with a locally measured EM field. In this study, the new TFs concept was applied to the daily magnetic variations measured at the ground. The parameterisation of the source in terms of spherical harmonics was adopted. We used nearly 20 years of data from 125 mid-latitude observatories and explored how the results are affected by (I) solar activity conditions, (II) the choice of the prior conductivity model used for the source coefficient estimation, and (III) the presence of ocean tidal magnetic signals. We found that choosing magnetically quiet periods is beneficial due to simpler source morphology, and the choice of prior conductivity model may significantly affect the source coefficients and TFs at short periods. We further observed significant contributions by ocean tidal magnetic signals at coastal and island observatories and corrected for them. Finally, the estimated TFs were inverted for the mantle conductivity at several locations representing different geological settings.