9:00 AM - 9:15 AM
[SEM16-01] Role on the geodynamo simulation in paleaomagnetic data analysis: Preliminary experiments on polarity reversals
Keywords:Geodynamo, Magnetic Reynold number, Paleomagnetic analysis, Polarity reversals
Geodynamo simulation is a useful tool for extracting direct relationships between magnetic field variations and the dynamics of the Earth's outer core, as revealed by palaeomagnetic data analysis. Glatzmaier et al. (1999) and Olson et al. (2015) focused on geomagnetic polarity reversals and compared them with palaeomagnetic analytical data, showing that the presence of heterogeneities at the core-mantle boundary (CMB) has a significant effect on pole shifts and their trajectories. However, in order to discuss the details of the relationship between the paleomagnetic data and the dynamics of the Earth’s outer core, a simulation model that fully reproduces the physical properties of the Earth's outer core is required, but such a modelling study is difficult to perform with current computational power. In order to resolve such an issue, the path theory towards the Earth's outer core condition has been proposed by Aubert et al. (2017) to satisfy the constraint of the magnetic Reynolds number (~1000) from geomagnetic data analysis. Nakagawa and Davies (2022) has extended this theory and checked the applicability of long-term events used in palaeomagnetic data analysis. That study also succeeded in finding magnetic pole reversals without altering the expected force balance in the dynamics of the Earth's outer core. However, it has not been applied to the results of palaeomagnetic data analysis as in Glatzmaier et al. (1999) and Olson et al. (2015). Therefore, in this study, preliminary experiments are conducted to explore the direct applicability of geomagnetic pole reversals to palaeomagnetic data analysis using a geodynamo simulation model that satisfies the magnetic Reynolds number constraint. In this preliminary experiment, the geocentric axial dipole (GAD) approximation in magnetic pole shift analysis is checked for its validity.
For the geodynamo simulation, a case of LEDT039 (magnetic Reynolds number = 1046) in Nakagawa and Davies (2022) is mainly used. In this case, thermal and compositional convection are the driving mechanism of core convection, with a compositional convection fraction of 86%. The seismic heterogeneity in the lowermost mantle is converted into a thermo-chemical flux based on Aubert et al. (2013) with an isentropic heat flux on the CMB of 13 TW and used as a boundary condition at the core-mantle boundary. The force balance obtained in that case can be explained by the magnetic field-buoyancy-Coriolis force balance in a quasi-geostrophic balance as a leading order (QG-MAC) with finding magnetic pole reversals. As mentioned above, the force balance does not change during a magnetic pole reversal, but the effect of inertial forces increases during the magnetic pole reversal. In order to use the workflow of the paleomagnetic analysis, the magnetic field on the CMB obtained by the simulation is converted into Gaussian coefficients up to degree of 32.
To investigate the validity of the GAD approximation, the magnetic pole shift and its speed computed by geodynamo simulations are used to compare between the GAD approximation using only the Gauss coefficients related to the geomagnetic dipole and the Gauss coefficients for all orders. The analytical method obtains the three geomagnetic components (foreshortening, declination and total magnetic force) from the simulated Gauss coefficients, from which they are transformed into the mean virtual geomagnetic poles (VGPs). Diagnostic quantities that can be compared with the analytical values, such as the rate of pole variation, are also computed. The results comfirm that the GAD approximation is sufficient for the analysis of the magnetic pole reversal when comparing the pole shift paths and the shift speed between the GAD approximation and the Gaussian coefficients of all orders. The pole shift paths are also affected by the heterogeneous structure on the CMB. The validity on the magnetic Reynolds number constraints in dynamo simulations is discussed by comparison with time series of magnetic pole inversions from palaeomagnetic databases.
For the geodynamo simulation, a case of LEDT039 (magnetic Reynolds number = 1046) in Nakagawa and Davies (2022) is mainly used. In this case, thermal and compositional convection are the driving mechanism of core convection, with a compositional convection fraction of 86%. The seismic heterogeneity in the lowermost mantle is converted into a thermo-chemical flux based on Aubert et al. (2013) with an isentropic heat flux on the CMB of 13 TW and used as a boundary condition at the core-mantle boundary. The force balance obtained in that case can be explained by the magnetic field-buoyancy-Coriolis force balance in a quasi-geostrophic balance as a leading order (QG-MAC) with finding magnetic pole reversals. As mentioned above, the force balance does not change during a magnetic pole reversal, but the effect of inertial forces increases during the magnetic pole reversal. In order to use the workflow of the paleomagnetic analysis, the magnetic field on the CMB obtained by the simulation is converted into Gaussian coefficients up to degree of 32.
To investigate the validity of the GAD approximation, the magnetic pole shift and its speed computed by geodynamo simulations are used to compare between the GAD approximation using only the Gauss coefficients related to the geomagnetic dipole and the Gauss coefficients for all orders. The analytical method obtains the three geomagnetic components (foreshortening, declination and total magnetic force) from the simulated Gauss coefficients, from which they are transformed into the mean virtual geomagnetic poles (VGPs). Diagnostic quantities that can be compared with the analytical values, such as the rate of pole variation, are also computed. The results comfirm that the GAD approximation is sufficient for the analysis of the magnetic pole reversal when comparing the pole shift paths and the shift speed between the GAD approximation and the Gaussian coefficients of all orders. The pole shift paths are also affected by the heterogeneous structure on the CMB. The validity on the magnetic Reynolds number constraints in dynamo simulations is discussed by comparison with time series of magnetic pole inversions from palaeomagnetic databases.