A new approach to modeling the time-space variations of ionospheric electric currents and magnetic fields during the September 2017 geomagnetic storm

The study and modeling of Earths ionospheric electric currents and of the associated magnetic fields is fundamental for geomagnetic field modeling, and for the study of ionosphere coupling with the neutral atmosphere and the magnetosphere. Ionospheric electric currents, which exist during both quiet and disturbed geomagnetic activity periods, can be studied using magnetic field measurements from ground magnetic observatories and satellites. Modeling these currents during geomagnetic storms is particularly challenging due to the limited data available combined with high time-space variations during such events. In this study, we propose a new approach to modeling the storm-time ionospheric electric currents and magnetic fields. The approach relies on a joint utilization of magnetic data and the physics-based Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM). The TIEGCM time-space variations are first analyzed using a toroidal-poloidal decomposition of the magnetic field. To extract a priori information on the 3D spatial structure of the ionospheric magnetic field, principal component analysis is next applied to the spherical harmonics coefficients to obtain a small number of spatial modes that represent a substantial amount of magnetic field spatial variations. Temporal variations are represented by temporal modes computed with ground observatory data following Egbert et al. (2021). The entire procedure can be carried out in the frequency domain to account for induced fields. Spatial and temporal modes can be combined to parametrized the magnetic field measured by the Swarm satellites and the model coefficients estimated by solving an inverse problem. We present preliminary results obtained with this approach for the geomagnetic storm of September 2017.