Accounting for quiet-time magnetospheric field contributions in geomagnetic field modelling

Accurate modelling of Earth’s core and lithospheric magnetic field requires minimising the influence of contributions from ionospheric and magnetospheric currents. This study examines different approaches for accounting for quiet-time magnetospheric contributions in geomagnetic field modelling, with a focus on the magnetospheric ring current. Traditional indices, such as the $D_{\mathrm{st}}$ or the RC indices, have been widely used for this but exhibit limitations. The study explores variations of the RC index, incorporating ground-based observatory data from different latitude ranges, and evaluates their effect on geomagnetic field modelling. Additionally, approaches based on a combined analysis of ground and satellite data are assessed for their ability to provide a more accurate representation of magnetospheric contributions, going beyond its usual approximation by a uniform field (the so-called $P_1^0$-approximation). Results indicate that including mid-latitude ground observatory data and utilising higher spherical harmonic for describing magnetospheric contributions lead to improved geomagnetic field models and reduced data misfit.