An investigation into the axial dipole field and mantle-induced heterogeneity in Earth

Abstract

The Earth’s axial dipole field is generated by dynamo action in the planet’s outer core. It is understood from observations that the convection in the outer core is influenced by the lateral heterogeneity in the overlying viscous mantle. The high-latitude magnetic flux lobes symmetrically placed on either side of the equator point to lower-mantle control of the geodynamo. The first part of this thesis studies the formation of a large-scale axial dipole magnetic field in rapidly rotating dynamos. It is shown that the growth of the magnetic field from a small seed state is accompanied by the generation of slow magnetostrophic waves in the energy- containing scales. The slow waves make the dominant contribution to the kinetic helicity, which supports the axial dipole field. The analysis of dipole formation in nonlinear dynamos shows a poloidal-poloidal field conversion through wave motion in contrast to the conventional toroidal-poloidal conversion. The second part of the thesis studies both experimentally and numerically the effect of a large lower-mantle thermal in- homogeneity on the core. The numerical model considers a dynamo driven by double-diffusive convection, wherein compositional buoyancy is dominant and thermal buoyancy is close to the onset condition. Apart from the response of the magnetic field, the mantle-induced heterogeneity of the inner core is examined. Following a switch in coherent (long-lived) convection from the Atlantic to Asia under increased forcing, a switch is noted in the peak heat flux at the inner core boundary from the western to the eastern hemisphere. A large thermal heterogeneity at the core-mantle boundary (CMB) may also result in a reversal of heat flux at the inner core boundary (ICB) beneath the Pacific and Africa, indicating possible melting in these regions. The peak heat flux on the inner core boundary occurs at longitudes 80◦E- 100◦E. The results also suggest that the peak-to-peak variation of heat flux at Earth’s CMB should be O(10) times greater than the mean heat flux for the East-West difference in seismic P-wave velocity to approach the lower bound of the variation proposed by previous observational studies mapping the inner core heterogeneity.