Fig. 4

Inertial core–mantle coupling and excitation of free core nutation (FCN). Pressure at the core–mantle boundary (CMB) (red arrows) does not exert a torque if the rotation axis of the mantle and the core are coincident (a). Deviation from this state let the pressure at CMB make opposite torques to the mantle (blue arrows) and to the core (orange arrows) (b). This causes FCN, i.e., rotational movements of the spin axes of the mantle and the core relative to stars in the opposite directions