Free Core Nutation and its relation to the Spin-Over Mode

TL;DR

This paper investigates the relationship between Free Core Nutation and the Spin-Over Mode in planetary bodies, showing how their proximity affects nutation damping and resonance characteristics, with implications for Earth's and other planets' rotational dynamics.

Contribution

It provides a detailed analysis linking Free Core Nutation to the Spin-Over Mode through a two-layer planetary model, highlighting the effects of dissipation on nutation resonance.

Findings

Free Core Nutation frequency is offset by about -1 day for Earth.

Proximity of modes causes a minimum in the transfer function for nutations.

Dissipation reduces the Quality factor of resonance.

Abstract

The time-varying response of the Earth's and other planets' rotation to external gravitational torques depends strongly on their internal structure. In particular, the existence of the mode known as the Free Core Nutation in the fluid core, is known to amplify the forced nutations in the near-diurnal retrograde frequency band (as measured in the planetary frame of reference). Due to their proximity in shape and frequency, this mode is sometimes equated with the so-called Spin-Over Mode which denotes the free oscillation of a steadily rotating ellipsoidal fluid core. Through a careful study of the freely rotating two-layer planetary model with a rigid mantle and an inviscid fluid core, we show that the Spin-Over Mode frequency corresponds to that where the sum of the external and internal torques on the mantle are balanced, causing it to rotate steadily. The presence of dissipation at the Core-Mantle Boundary causes the Free Core Nutation to become damped and slightly offset its resonance frequency. We show that this offset, which is $\approx-1$ day for the Earth, can be interpreted as the result of the proximity of the Free Core Nutation frequency to that of the Spin-Over Mode, which now corresponds to a minimum in the magnitude of the transfer function for nutations. We also show how this proximity leads to a slightly lower Quality factor for the resonance than that compute from the usual formula. We conclude by discussing possible implications of this mechanism for Mars, the Moon, and the long-term evolution of the Earth.