What a PhD on Core-Mantle Interaction looks like

I’m interested in how we can separate regions of the Earth’s main magnetic field into local regions to better understand how the mantle and core interact. It is important to remember that the main field is the most dominant contribution (>90%) to the Earth's magnetic field at the Earth's surface and changes over time due to the movement of conductive liquid in the outer core. This liquid is mostly composed of iron and is swirling in a complex current system due to the release of heat from the centre of the Earth, the turning motion of the planet, and the magnetic field perturbing the conductive liquid. Core flow and magnetic field models at the CMB tend to be described by spherical harmonics, which are not suitable for separation into individual regions due to large leakage being generated during the separation (Backus, 1968; Wieczorek and Simons, 2005). Spherical Slepian functions can spatially and spectrally separate bandlimited potential fields by transforming the spherical harmonic coefficients into the Slepian basis and sorting the functions by contribution to the patch (Simons and Plattner, 2015). 

We wished to make geophysical interpretations of the impact of the Large Low Velocity Provinces (LLVPs) on the core surface flow over time. LLVPs are two antipodal regions of anomalously low seismic velocity cover ~25% of the CMB surface (Koelemeijer, 2021). Long-lived features in the Earth’s magnetic field have been speculated to be linked to the LLVP structures as evidence for top-down control on the geodynamo (Tarduno et al., 2015). Whether these features apply a thermal forcing, a chemical exchange, dynamic topography or other effect to the core remains to be explored (McNamara, 2019; Zhao et al., 2015; Rhodri Davies et al., 2012).
The decomposition of SV at the Earth’s surface achieved from 5 biannual snapshots from May 2008 to May 2016 using 69 altitude-cognizant Slepian eigenfunctions to describe the Inside LLVPs. The blue circles in the global spherical harmonic plot show the data variability over the time period due to the satellite coverage.
In my PhD, we successfully incorporated spherical Slepian functions into regional SV inversions from satellite data for 2006–2021 and separated 150 years of COV-OBS.x2 SV model coefficients to investigate how LLVPs may be affecting core surface flow over time (Hammer et al., 2021; Huder et al, 2020). We identify that the energy within the region is incrementally changing over time. The spectral energy within the LLVPs at the Earth’s surface are changing over time and there is good correlation between periods of known acceleration change (from Mandea et al., 2010; and Duan and Huang, 2020) and inflection points in the spectra at l = 2 and l = 4 which reflect changes in signal due to antipodal structures. Inversions of satellite energy within the LLVPs have been relatively constant over the last 20 years and is roughly proportional to the surface area of the LLVPs but the longer time series shows a reduction in spectral energy within the LLVPs over time which is slowing over time. This work requires further investigations about the best applications of spherical Slepian functions, the cause of this SV change and extending the time period (e.g. using GGF100k, Panovska et al., 2019).

Hannah Rogers has just submitted her PhD thesis at the University of Edinburgh and is a member of the IAGA Social Media team. Her specialism is in investigating regional magnetic fields of Earth at the surface and the core-mantle boundary using mathematical methodologies. You can follow her on Twitter at @Hannah_Rogers94.


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