| Abstract: |
Large Low Shear Velocity Provinces (LLSVPs) in the lowermost mantle beneath the Pacific Ocean and Africa are key to understanding the mantle dynamics from early to the present Earth, but their origins have long been debated. To solve this issue, it is useful to clarify the relationship between the LLSVPs and the distribution of perovskite (Pv) and post-perovskite (pPv) phases that undergo a phase change in the temperature and pressure range of the D" layer at CMB(Core-Mantle Boundary). Since the conductivity jumps several orders of magnitude during the transformation from Pv to pPv, knowing the conductivity heterogeneity within the D" layer will help to achieve the above goal. However, the conventional electromagnetic sounding method using external magnetic field as a source, are difficult to detect the structure of the lowermost mantle through the highly heterogeneous Earth's surface with an accuracy that can be compared to seismic structure.
In this presentation, we present the results of a new method that uses the magnetic field at CMB just below the D" layer as a source and observes the surface magnetic field caused by the heterogeneous structure in the D" layer. The model parameters to be determined are the spherical harmonic expansion coefficients of the magnetic diffusivity in the D" layer and the expansion coefficients of the axisymmetric toroidal magnetic field at CMB, and the distribution of these parameters is sampled by the Markov Chain Monte Carlo (MCMC) method, whereas forward calculations of the magnetic induction were performed using the method formulated by Hamano (2002).
From the converged steady-state solution, it is shown that several blocks with a large amount of high conductivity pPv phase exist under the LLSVP region described above, and may play an important role in the formation of the LLSVP. The existence of pPv phase places constraints on the temperature at this location. The toroidal magnetic field at the core surface allowed us to estimate the range of average electrical conductivity of the "D" layer. Furthermore, the modeling process provide us to separate the Earth's main magnetic field observed at the surface into a magnetic field generated by mantle heterogeneities and a magnetic field of core origin. The magnetic field of core origin has solved several problems that have remained unresolved in dynamo simulations and core dynamics research, and has added new insights into the activity of the Earth's core and the process of magnetic field generation.
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