Sensitivity analysis of the lithospheric magnetic field at satellite altitude: the effects of the inducing field and the shape of the magnetic lithosphere

As a means of quantitative interpretation, forward calculations of the global lithospheric magnetic field in the Spherical Harmonic (SH) domain have been widely used to reveal geophysical, lithological, and geothermal variations in the lithosphere. Traditional approaches either do not consider the non-axial dipolar terms of the inducing field and its radial variation or do so by means of complicated formulae. Moreover, existing methods treat the magnetic lithosphere either as an infinitesimally thin layer or as a radially uniform spherical shell of constant thickness. Here, we present alternative forward formulae that account for an arbitrarily high maximum degree of the inducing field and for a magnetic lithosphere of variable thickness. Our simulations based on these formulae suggest that the satellite magnetic anomaly field is sensitive to the non-axial dipolar terms of the inducing field but not to its radial variation. Therefore, in forward and inverse calculations of satellite magnetic anomaly data, the non-axial dipolar terms of the inducing field should not be ignored. Furthermore, our results show that the satellite magnetic anomaly field is sensitive to variability in the lateral thickness of the magnetized shell. In particular, we show that for a given vertically integrated susceptibility distribution, underestimating the thickness of the magnetic layer overestimates the induced magnetic field. This discovery bridges the greatest part of the alleged gap between the susceptibility values measured from rock samples and the susceptibility values required to match the observed magnetic field signal. We expect the formulae and conclusions of this study to be a valuable tool for the quantitative interpretation of the Earth’s global lithospheric magnetic field, through an inverse or forward modelling approach.