Using SOLA for investigating regional dynamics of flow at the top of the outer core

Investigating the dynamics of liquid iron at the top of the core requires a difficult inversion process from magnetic field measurements, where traditional techniques do not allow for regional investigations or studies of Secular Variation (first time derivative of magnetic field, SV) on short-time periods. Improved data coverage and quality from satellite missions (particularly Swarm) have provided insight into short-period dynamics (such as jerks and waves), and improved understanding of localised features (such as jets near the tangent cylinder). However, there continues to be a need for new techniques for better spatio-temporal models of flow motions at the top of the core, especially for regional investigations.

The aim of this work is to investigate regional dynamics at the core-mantle boundary (CMB) using the SOLA (Subtractive Optimally Localised Averages) methodology. This method allows us to create point estimates of radial field SV at the CMB by considering global satellite measurements. A time window is selected that allows a suitable trade-off between the desired spatial and temporal resolution of the local estimate.  The output is a localised average of radial field SV at a particular location on the CMB, along with an error estimate and a spatial resolution kernel. The SOLA methodology allows for a local estimate at any desired location directly at the CMB, which opens the way to investigations of regional dynamics, waves, and jerks. Producing localised-average estimates at the CMB bypasses many of the problems encountered when using only a subset of magnetic satellite data, downward continuing data, and models based on spherical harmonics.

Finally, we discuss how the SOLA methodology can be incorporated into the PyGeodyn code to produce stochastic core flow models. This approach can provide additional information on wave-like flow motions at the top of the Earth’s core where these are most prominent, and permits new investigations of shorter period phenomenon. We present our on-going work and highlight considerations of the SOLA method for investigating SV and core surface flow modelling.