Mass motion in the Earth's core and mantle and their influence on polar motion
Final Report Abstract
In the project 4 theoretical description for the electromagnetic (EM), topographic (TOP) and gravitational (GRAV) core-mantle coupling torques are developed and documented in a series of Scientific Technical Reports published by the GFZ, which are also electronically available. This unified description of coupling torques is used for an implementation of a numerical model, which computes the influence of the combined EM and TOP coupling torques and the angular momentum exchange at the Earth’s surface on decadal variation of Earth rotation for a rigid Earth model. The combined consideration of core-mantle coupling torques and angular momentum exchange with the atmosphere (AAM) and the ocean (OAM) is formulated in terms of equivalent excitation function for a rigid Earth model. The necessary angular momentum time series (AAM & OAM) are provided by project 2 and 10. Based on the global geomagnetic field model C3 FM at the Earth surface, the poloidal and toroidal geomagnetic field at the core-mantle boundary (CMB) are determined by a newly developed approach, which uses the non-harmonic downward continuation. Furthermore, the required radial profile of the electrical conductivity of the mantle and representation of the CMB topography are taken from published studies. The influence of EM and TOP coupling torques as well as AAM and OAM exchange at the Earth surface was investigated for a rigid Earth mantle. This study shows that polar motion is significantly influenced on decadal time scale by AAM, OAM and TOP, whereas ∆LOD is dominated by EM and TOP coupling torques. The conclusion drawn from this findings is the necessity to model decadal variation of Earth rotation with a coupled approach for core-mantle coupling and surface angular momentum exchange. Moreover, the computed decadal variations of polar motion are only in the same order of magnitude as the observed variations and show no strong correlation in time. This is a significant hint for a missing coupling process, which should contribute significantly to time variation of decadal polar motion but should only slightly influence ∆LOD. An investigation related to variation of Earth rotation for a (visco)elastic deformable Earth mantle was carried out, in which EM and TOP coupling torques and the deformation of the CMB due to dynamic pressure, caused by the fluid flow in the outer core, as well as the rotational deformation, caused by changes in the centrifugal potential, are considered. The rotational deformation contributes significantly to decadal variations of polar motion and the contribution solely of the CMB deformation is non-negligible. A comparison with observed decadal variation of polar motion leads to the finding that in this approach the short-period influence of the angular momentum exchange is clearly missing. In summary, we conclude that a combined approach for surface angular momentum exchange and core-mantle coupling torques is required, which also considers the deformation of the mantle by surface pressure changes related to the mass-redistribution causing the AAM and OAM and by dynamic pressure changes on the CMB related to the fluid-flow in the outer core. Such an approach could be build on the spectral-finite element approach developed within project 4 in the future. The implementation would require to handle large data sets, because beside AAM and OAM time series also underlying global gridded surface pressure data would be needed at any time step of the computation.
Publications
- 2007. Axial poloidal electromagnetic core-mantle coupling torque: a re-examination for different conductivity and satellite supported geomagnetic field models. Stud. Geophys. Geod., 51, 491-513
Greiner-Mai, H., Hagedoorn, J.M., Ballani, L., Stromeyer, D., Hengst, R. and Wardinski, I.
- 2008. Core-Mantle Coupling – Part I: Electromagnetic coupling torques. Sci. Techn. Rep. GFZ Potsdam, STR08/06
Hagedoorn, J. and Geiner-Mai, H.
(See online at https://doi.org/10.2312/GFZ.b103-08061) - 2008. Core-mantle coupling – Part II: Topographic coupling torques. Sci. Tech. Rep. GFZ, STR08/11
Greiner-Mai, H. and Hagedoorn, J.
(See online at https://doi.org/10.2312/GFZ.b103-08113) - 2010. Determining the time-variable part of the toroidal geomagnetic field in the core-mantle boundary zone. Phys. Earth Planet. Inter. 178, 56–67
Hagedoorn, J., Greiner-Mai, H., Ballani, L.
- 2010. The 1991 geomagnetic jerk as seen at the earth’s surface and the core-mantle boundary. Geophys. J. Int. 183, 659–680
Ballani, L., Hagedoorn, J., Wardinski, I., Stromeyer, D., Greiner-Mai, H.
- 2012. Core-Mantel Coupling – Part IV: Axial component of the core angular momentum. Sci. Tech. Rep. GFZ, STR12/10
Greiner-Mai, H., Ballani, L. and Hagedoorn, J.M.
(See online at https://doi.org/10.2312/GFZ.b103-12108) - 2012. Core-mantle coupling – Part III: Gravitational coupling torques. Sci. Tech. Rep. GFZ, STR12/01
Hagedoorn, J., Geiner-Mai, H. and Ballani, L.
(See online at https://doi.org/10.2312/GFZ.b103-12019)
