New method for determining free core nutation parameters, considering geophysical effects

Context. In addition to the torques exerted by the Moon, Sun, and planets, changes of precession-nutation are known to be caused also by geophysical excitations. Recently studies suggest that geomagnetic jerks (GMJ) might be associated with sudden changes of phase and amplitude of free core nutation. We showed that using atmospheric and oceanic excitations with those by GMJ improves substantially the agreement with observed celestial pole offsets. Aims. Traditionally, the period T-f and quality factor Q(f) of the free core nutation (FCN) are derived from VLBI-based celestial pole offsets (CPO). Either direct analysis of the observed CPO, or indirect method using resonant effects of nutation terms with frequencies close to FCN, are used. The latter method is usually preferred, since it yields more accurate results. Our aim is to combine both approaches to better derive FCN parameters. Methods. We numerically integrated the part of CPO that is due to geophysical excitations for different combinations of T-f, Q(f), using Brzezinski's broadband Liouville equations (Brzezinski 1994, Manuscripta geodaetica, 19, 157), and compared the results with the observed values of CPO. The values yielding the best fit were then estimated. The observed CPO, however, must be corrected for the change of nutation that is caused by the T-f, Q(f) values different from those used to calculate IAU 2000 model of nutation. To this end, we have used the Mathews-Herring-Buffet transfer function and applied it to the five most affected terms of nutation (with periods 365.26, 182.62, 121.75, 27.55 and 13.66 days). Results. The results, based on the CPO data in the interval 1986.0-2016.0 and excitations with three different models, are presented. We demonstrate that better results are obtained if the influence of additional excitations at GMJ epochs is added to excitations by the atmosphere and oceans. Our preferred values are T-f = 430.28 +/- 0.04 mean solar days and Q(f) = 19 500 +/- 200.