Revised Magnetospheric Model Reveals Signatures of Field-Aligned Current Systems at Mercury

Mercury is the smallest and innermost planet of our solar system and has a dipole-dominated internal magnetic field that is relatively weak, very axisymmetric and significantly offset toward north. Through the interaction with the solar wind, a magnetosphere is created. Compared to the magnetosphere of Earth, Mercury's magnetosphere is smaller and more dynamic. To understand the magnetospheric structures and processes we use in situ MESSENGER data to develop further a semi-empiric model of the magnetospheric magnetic field, which can explain the observations and help to improve the mission planning for the BepiColombo mission en-route to Mercury. We present this semi-empiric KTH22-model, a modular model to calculate the magnetic field inside the Hermean magnetosphere. Korth et al. (2015, , 2017, ) published a model, which is the basis for the KTH22-model. In this new version, the representation of the neutral sheet current magnetic field is more realistic, because it is now based on observations rather than ad-hoc assumptions. Furthermore, a new module is added to depict the eastward ring shaped current magnetic field. These enhancements offer the possibility to improve the main field determination. In addition, analyzing the magnetic field residuals allows us to investigate the field-aligned currents and their possible dependencies on external drivers. We see increasing currents under more disturbed conditions inside the magnetosphere, but no clear dependence on the z-component of the interplanetary magnetic field nor on the magnetosheath plasma beta. We present a revised model of Mercury's magnetospheric magnetic field The model now includes an eastward ring shaped current and the neutral sheet current is calculated more precisely with Biot Savart's law The strength of the field-aligned currents increases with higher magnetic activity