Geomagnetic dipole tilt changes induced by core flow
Résumé
The tilt of the geomagnetic dipole decreased from about 11.7◦ in 1960 to 10.5◦ in 2005, following more than a century when it remained nearly constant. The recent poleward motion of the dipole axis is primarily due to a rapid decrease in the equatorial component of the dipole moment vector. Using maps of the equatorial dipole moment density and its secular change derived from core field models, we identify regions on the core–mantle boundary where the present-day tilt decrease is concentrated. Among the possible causes of equatorial dipole moment change on the core–mantle boundary, tangential magnetic diffusion is negligible on these time scales, and although radial magnetic diffusion is potentially significant, the rapid changes in equatorial moment density indicate it is not the dominant mechanism. We show that magnetic flux transport can account for most of the observed equatorial dipole moment change. Frozen-flux core flow models derived from geomagnetic secular variation reveal a nearly balanced pattern of advective sources and sinks for the equatorial dipole moment below the core–mantle boundary. The recent tilt decrease originates from two advective sinks, one beneath Africa where positive radial magnetic field is transported westward away from the equatorial dipole axis, the other beneath North America where negative radial magnetic field is transported northward away from the equatorial dipole axis. Each of these sinks is related to a prominent gyre that has evolved significantly over the past few decades, indicating the strong variability of the large-scale circulation in the outer core on this time scale.