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

Abstract

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 Øersted 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.