Geodynamic constraints on the steady and time-dependent inner core axial rotation

Details Geodynamic constraints on the steady and time-dependent inner core axial rotation Geodynamic constraints on the steady and time-dependent inner core axial rotation

Aug 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007geoji.170..886d&link_type=abstract

Geophysical Journal International, Volume 170, Issue 4, pp. 886-895.

Astronomy and Astrophysics

Astronomy

8

Earth'S Magnetic Field, Earth'S Rotation, Inner Core, Inner Core Rotation, Inner Core Viscosity

Scientific paper

In this work, constraints on the steady and time-dependent rates of axial inner core rotation are established based on the angular momentum balance between the inner core, fluid core and mantle. It is shown that the rate of a steadily rotating inner core is limited by the torque on the mantle from surface forces at the core-mantle boundary (CMB). The rate of rotation of an oscillating inner core is constrained by the changes in mantle rotation induced by gravitational coupling, which must not exceed the observed changes in length of day. Assuming that the largest torque at the CMB is from electromagnetic forces, our results suggest that the maximum amplitude of an inner core oscillating at a period of 60 yr is ~0.03° yr-1, while the maximum amplitude of a steadily rotating inner core is ~0.3° yr-1. This implies that the largest inner core rotation rate compatible with the ensemble of seismic studies, 0.2° yr-1, may be explained by a steady rotation but cannot be explained by a decadal oscillation. Though a steady rotation of 0.2° yr-1 is possible, it constrains the conductance of the lower mantle to be in the range of 1.6 -2.5 × 109 S and the viscous relaxation timescale of the inner core to be in the range of 5-8 yr (i.e. a bulk inner core viscosity of 2.5 -4 × 1017 Pa s). Larger steady and oscillating rotation rates may be possible if the inner core can deform on a timescale shorter than ~5 yr or if the conductance of the lower mantle is larger than ~2.5 × 109 S, though this would then contradict constraints on these two quantities derived from other observations. Finally, we note that a steadily rotating inner core is compatible with the westward motions of magnetic field features at the CMB, as the latter provide the electromagnetic torque on the mantle that balances the gravitational torque from the rotating inner core. In addition, these westward motions must be predominantly due to advection by flow rather than to the propagation of hydromagnetic waves, as the electromagnetic torque at the CMB is otherwise too small to allow an inner core rotation rate as large as 0.2° yr-1.

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