A Model of Hourly Variations of the Near-Earth Magnetic Field Generated in the Inner Magnetosphere and Its Induced Counterpart

We present a new model of the near-Earth magnetospheric field produced by electric currents in the inner magnetosphere and the associated induced magnetic field. The model is designed to track hourly variations of these fields and accounts for their local time asymmetries. It is built by applying spherical harmonic analysis to vector measurements from the ground observatory network at low and mid-latitudes. The primary and induced fields are separated with an approach in the time domain that uses a a priori radially-symmetric electric conductivity model of the Earth. The model coefficients are computed at one-hour time steps between 1997 and 2022. This model is shown to be consistent to within a few nT with previously developed indices which track the magnetospheric ring current. It is also validated against data from the Swarm, CHAMP and oersted satellites. The fit to satellite data is comparable to that of the CHAOS-7.15 model for geomagnetically quiet times, and improved by up to 20% on some components for geomagnetically moderate and active times. We attribute these differences mostly to a better representation of local time asymmetries, both on average and during individual geomagnetic storms. This model can be used in various applications, such as investigating the properties of the magnetospheric field and its sources and separating the magnetospheric field from the fields of other sources in geomagnetic field modeling. Geomagnetic field modeling aims at building data-based mathematical representations, or models, of the various contributions to the total Earth's magnetic field measured at or near the Earth's surface. One of such contributions is the magnetic field generated by electric currents in the inner magnetosphere, including the so-called ring current. In this paper, we present a new model of the near-Earth magnetic field generated in the inner magnetosphere based on data collected in ground magnetic observatories. The model covers the 1997-2022 time span, includes improved representations of the local time asymmetries of the field and of the effect of electrical induction in the Earth's mantle, and is validated against data collected in low Earth orbits by the Swarm, CHAMP and Oersted satellites. We find that the model provides an improved representation of the magnetic field generated in the inner magnetosphere during periods of moderate and high geomagnetic activity, including magnetic storms. A new ground-data based model of hourly variations of the primary inner near-Earth magnetospheric and associated induced field is presentedComparison with satellite data shows up to 20% performance improvement compared to the CHAOS-7 model for moderate and active timesThe model accounts for variations of the field with local time and can be used to study geomagnetic storms