The impact of LLVPs on regional secular variation of the magnetic field

Abstract

The Large Low Velocity Provinces (LLVPs) are two poorly understood features at the base of the mantle that are typically studied with seismology and explained using geodynamical models of Earth evolution. However, there are few insights from the geomagnetic perspective about how these mantle features affect main field generation, or if they perturb the motion of the Earth's outer core, without assuming their physical properties. In this study, we test three regional methodologies, namely pointwise estimate on a spatial grid, spherical harmonic analysis, and spherical Slepian functions, to separate secular variation (SV, the first time derivative of the magnetic field) in the areas beneath the LLVPs and their complement.

While all three methodologies have drawbacks and differences, our findings of the proportion of SV energy inside and outside LLVPs are robust. When inverting data from geomagnetic virtual observatories over the satellite era, the proportion of SV energy under the LLVPs is found to be between 12 % and 18 % of the total SV energy at the Earth's surface which is less than the percentage surface area of the LLVPs. However, the percentage of SV energy is larger than the corresponding surface area when separating the COV-OBS.x2 SV model, between 29 % and 37 % inside LLVPs at the Earth's surface and 33 % and 49 % at the core-mantle boundary (CMB). For both datasets the African LLVP contributes approximately 2.5 times the amount of SV energy as the Pacific LLVP at the Earth's surface but only around 1.3 times more energy at the CMB.

LLVPs show time-varying SV under their footprint on decadal timescales which, therefore, indicates that core flow varies significantly underneath them rather than being regions of stilted flow. As well as presenting a novel inversion methodology that inverts for a spherical Slepian model, rather than using spherical Slepian functions to separate an existing spherical harmonic model, we also show for the first time that the timings of geomagnetic jerks correspond with inflection points in the magnitude of spectral or spatial energy in regional SV models. We conclude that there is no evidence that SV is systematically suppressed beneath LLVPs.