Abstract
The presence of stable stratification has broad implications for the thermal and compositional state of the outer core, the evolution of Earth's deep interior, and the energetics of the geodynamo. Yet the origin, strength, and depth extent of stratification in the region below the core-mantle boundary remain open questions. Here we compare magnetic fields produced by numerical dynamos that include heterogeneous stable thermal stratification below their outer boundary with models of the geomagnetic field on the core-mantle boundary, focusing on high latitude structures. We demonstrate that the combination of high magnetic field intensity regions and reversed magnetic flux spots, especially at high latitudes, constrains outer core stratification below the core-mantle boundary. In particular, we find that the negative contribution to the axial dipole from reversed flux spots is a strong inverse function of the stratification. Comparison of our numerical dynamo results to the structure of the historical geomagnetic field suggests up to 400 km of permeable, laterally heterogeneous thermal stratification below the core-mantle boundary.
Highlights
Stable stratification at the top the outer core has been inferred using both seismic and geomagnetic data, typically with divergent results
Is the outer core stratification inferred by recent seismic studies compatible with the geomagnetic field and its secular variation? Core flow inversions based on the geomagnetic secular variation are best accommodated by including upwelling and downwelling motions extending very close to the core-mantle boundary (Gubbins, 2007; Amit, 2014; Lesur et al, 2015; Huguet et al, 2016)
The stratification analyzed in this study is due to thermal gradients that deviate from adiabatic conditions and are maintained by the heat flux imposed at the outer boundary
Summary
Stable stratification at the top the outer core has been inferred using both seismic and geomagnetic data, typically with divergent results. Stratification effects have been extensively studied in the context of the solar dynamo (e.g., Browning et al, 2006, 2007; Käpylä et al, 2008; Tobias et al, 2008; Brummell et al, 2010; Masada et al, 2013), Jupiter (Zhang and Schubert, 2000), Saturn (Christensen and Wicht, 2008; Stanley, 2010), and Mercury (Christensen, 2006; Manglik et al, 2010) All these investigations found that the presence of a stratified layer affects the morphology of the magnetic field. These comparisons favor the existence of stratification below the CMB and indicate that substantial radial motions are present there, implying that the stratification is rather weak and permeable to outer core convection
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