Free Inner Core Nutation (FICN) or prograde FCN (PFCN)

This motion is very similar to the FCN, but concerns the outer and inner core. It has never been observed and theoretical calculations predict a very weak resonance effect on Earth tides and nutations. The period is retrograde diurnal in the terrestial frame and prograde long-period in the celestial frame (of about 470 days). The parameters involved are the flattening of the inner core, the densities of the inner core and outer core, and the deformation of the Inner Core Boundary (ICB) due to the fluid dynamic pressure on the ICB. In a very detailed analysis of VLBI data, Mathews et al. (JGR, 2001) have determined a FICN period and damping from its resonance in the nutations. They obtain a period of 1025 days.