Investigating geomagnetic jerks with Swarm: Using the spatial gradient tensor for flow modelling

The ESA Swarm mission has, with along- and across-track differences of the magnetic field measurements, made it possible to generate spatial gradients data of the geomagnetic field and its secular variation (SV). Inverting the SV data for core-surface flows allows us to investigate the Earth’s outer core with higher resolution than using vector component data.

We invert for core-surface flow models directly from the spatial gradient tensor SV data, without the use of stochastic or numerical models, imposing flow equatorial symmetry, quasi- or tangential geostrophy, or band-pass filtering. We develop three different types of model, all damped to minimise spatial complexity and minimise acceleration between epochs. The first set is otherwise unconstrained, and different spatial regularisations are used. In the second set, we allow for torsional oscillations by relaxing the temporal damping on certain flow coefficients. The third set has differential damping on the equatorially symmetric and asymmetric flow components, in order to investigate the extent to which asymmetric flow is required to fit the data. We predict the intradecadal variation in length-of-day (LOD) from each model, and find that only the model which allows for torsional oscillations shows a good fit to the LOD data.

The azimuthal acceleration of all three model types shows evidence of fast westward low-latitude waves at the core-surface.  During the 2017 geomagnetic jerk, there is an abrupt westward shift in these wave-features in all our models. Previous literature suggests that geomagnetic jerks may originate from Alfvén wave packets emitted from the inner-outer core boundary, propagating outwards. We suggest that the observed westward shift at the jerk epoch may occur when these wave-packets interfere with the waves at the core surface.

Finally, we consider the use of spatial gradients from the CHAMP mission. Spatial gradients can be derived from along-track differences of the magnetic field from CHAMP, which allows us to compare the quality of core surface flow models from the CHAMP and Swarm missions. Our analysis suggests it is unlikely that CHAMP yields data of sufficient resolution to observe this proposed wave-interaction, showcasing the success of the Swarm mission.