The differential precession of Earth’s fluid and solid cores

Abstract

The Earth’s spin axis is inclined by ∊=23.4° with respect to the ecliptic normal and precesses in space with a period of 26 kyr. The fluid core and solid inner core precess at slightly different angles. Here, we compute their differential precession angles on the basis of an elastically deforming Earth and dissipative torques inferred from nutation observations. We show that the precession angle of the fluid core is larger than that of the mantle by 1.714 arcsec and lags behind it by 0.0124 arcsec. The precession angle of the spin (and figure) axis of the inner core is smaller than that of the mantle by 1.861 arcsec and trails behind it by 3.660 arcsec. These correspond to differential velocities (Δv) at the core mantle boundary (CMB) and inner core boundary (ICB) of 2.11 and 2.21 mm/s, to associated boundary layer Reynolds (Re) numbers of 247 and 258, and to dissipations D of 4.6 and 14.5 GW. At such Re values the flow in the core should feature wavelike instabilities but should not be in a fully developed turbulent regime. Δv and Re at the CMB have remained approximately constant in the past 4 Gyr, and D has been steadily decreasing from a maximum of approximately double its present-day value. At the ICB, Δv,Re and D have all increased since inner core formation. Future projections indicate that Re and D at the ICB may reach 500 and 100 GW, respectively, in a few Gyr. Our results suggest that, for the whole of Earth’s history, dissipation from the misaligned precession of the fluid and solid cores has contributed only a small fraction of the total heat flux out of the core and has not provided an important power source for maintaining the geodynamo.