Free core nutation: direct observations and resonance effects

The motion of Earth's spin axis in space is monitored by Very Long-Baseline Interferometry (VLBI), and since 1994 also its rate is measured by Global Positioning System (GPS). The method of “combined smoothing”, developed recently at the Astronomical Institute in Prague enables one to combine both series. The solution, based on observations in 1979.6–2004.7, is compared with the recently adopted IAU2000 model of precession-nutation. The analysis reveals substantial residual offsets from the model. The dominant peak in frequency domain can be identified with Retrograde Free Core Nutation (RFCN), and we find also statistically significant deviations of several forced nutation terms. From the direct analysis of VLBI observations of celestial pole offsets from IAU2000 model of precession-nutation, it follows that the dominant period apparently grew from the average 435 days (VLBI observations in 1983.7–2004.7) to 460 days (VLBI/GPS combination in 1994.3–2004.6). A study of indirect determination of the RFCN period from the observed forced nutation terms through the resonance effects is presented, and the difference from the value found by direct analysis is discussed. It is demonstrated that the resonance approach does not confirm the apparent change of the period obtained from the direct analysis of observed celestial pole offsets; the resonance value is close to 430 solar days, and it seems to be very stable in time. The different value found from the direct analysis of observations can be probably ascribed to a small additional excitation by the atmosphere and/or ocean with retrograde terrestrial period around 23h 53min mean solar time.