Paris Observatory Geodetic VLBI Center

Free core nutation from IERS data

This page displays the information relevant to the empirical modeling of the free nutation of the Earth's figure axis associated with the free rotational mode of the fluid core inside the viscoelastic mantle (free core nutation or FCN). The empirical model is adjusted to the complex-valued nutation dX + i dY measured by VLBI using a least-square fit of a retrograde circular term of period -430.21 days and constant offsets, let

where A is the searched complex amplitude of the FCN. Yearly values of A are obtained by fitting over a sliding window of width 7 years and displaced by steps of one year. The fit is applied to various nutation series submitted to or computed by the IERS Earth Orientation Center. The models are updated regularly, typically every week as series are updated by the analysis centers. A Fortran program including a subroutine is provided to obtain FCN values at any epoch from a given empirical model. For each series, we make available a table of the yearly amplitudes and a time series of daily FCN values at the epochs of the data. Series ivq and ivr refer to the IVS quarterly and rapid combinations, respectively.

Amplitude and phase comparison

Individual data, models, and figures