Estimating ocean tide model uncertainties for electromagnetic inversion studies

Over a decade ago the semidiurnal lunar M2 ocean tide was identified in CHAMP satellite magnetometer data. Since then and especially since the launch of the satellite magnetometer mission Swarm, electromagnetic tidal observations from satellites are used increasingly to infer electric properties of the upper lithosphere. In most of these inversions, numerical ocean tidal models are used to generate oceanic tidal electromagnetic signals via electromagnetic induction. The modelled signals are subsequently compared to the satellite observations. During the inversion, since the tidal models are considered error free, discrepancies between forward models and observations are projected only onto the induction part of the modelling, e.g., Earth's resistivity distribution. Our study analyses uncertainties in oceanic tidal models from an electromagnetic point of view. Velocities from hydrodynamic and assimilative tidal models are converted into tidal electromagnetic signals and compared. Respective uncertainties are estimated. The studies main goal is to provide errors for electromagnetic inversion studies. At satellite height, the differences between the hydrodynamic tidal models are found to reach up to 2 nT, i.e., over 100 % of the M2 signal. Assimilative tidal models show smaller differences of up to 0.1 nT, i.e., over 30 % of the M2 signal.