Characterizing the magnetic signal generated in the magnetosphere from 1996 to 2024 using ground geomagnetic data

The geomagnetic field, as observed at the Earth's surface or LEO satellite altitudes (between 300 km and 800 km), is the combination of signals generated by various internal and external sources. The internal sources are mainly associated to the liquid outer core flow, magnetized rocks in the lithosphere and induced electric currents in the crust and mantle. External sources are electric currents flowing in the ionosphere and magnetosphere. We focus on the contributions from the magnetospheric fields and describe a modeling approach in Spherical Harmonics (SH) based on magnetic observatory vector field measurements. The aim of this study is to model the magnetospheric field contributions observed during geomagnetically quiet time up to SH degree 6, with a one-hour temporal resolution for the period covering years 1996.0 to 2024.8. The adopted modeling approach is based on the Kalman filter and the correlation-based technique, which leads to series of hourly snapshot models together with robust error estimates. The series of models in time compare well with the global magnetospheric Ring Current index (RC) We observed and described various magnetospheric field structures, including local time asymmetries and contributions from ring and magnetotail currents. We also examined annual, semi-annual, monthly and daily variations in magnetospheric field Gauss coefficients in the Fourier domain.