Joint Time‐Domain Modeling Magnetic Field Variations of Ionospheric and Magnetospheric Origin: Conceptual Framework and Practical Implementation

Abstract This study presents a time‐domain methodology for jointly modeling magnetic field variations resulting from ionospheric and magnetospheric currents, with the ultimate goal of improving imaging of Earth's electrical conductivity at mantle depths. Three ionospheric current systems–the equatorial electrojet (EEJ), the polar electrojet (PEJ), and the mid‐latitude current system (MLCS; called Sq in quiet times)–produce quasi‐periodic diurnal variations (DV). The longer‐period (LP) signals are primarily due to irregular fluctuations in magnetospheric currents. Traditionally, DV and LP signals are treated separately. For example, the analysis of LP signals is often based on nighttime data to reduce the effects of ionospheric sources. However, because of EM induction in the Earth, signals of ionospheric origin persist even at night. As for DV, its analysis is usually performed in the frequency domain. However, the morphology of all ionospheric sources varies from day‐to‐day, depending on the solar activity and the Earth's orbital position, which suggests the analysis of DV in the time domain. Moreover, the analysis of EEJ and MLCS signals is usually based on non‐polar data to reduce the effects of PEJ. In this study, we present a methodology to simultaneously model magnetic fields from all sources directly in the time domain using day and night as well as non‐polar and polar data. The approach uses two types (data‐based and physics‐based) of source parameterizations and accounts for 3‐D electromagnetic induction effects. Using observatory data, we obtain continuous spatio‐temporal models of multi‐source external and induced magnetic fields for 1998–2021.