Free Nutation Period Research Articles

On the period and quality factor of the Chandler wobble

The motion of the Earth’s geophysical pole is modeled by numerical integration of the Liouville equations. The dependence of variable parameters of these equations (the period of free nutation T and the quality factor of the shell Q) is examined as a function of time and initial data sets used for constructing geophysical perturbing functions. In particular, we used oceanic perturbing functions constructed from TOPEX/POSEIDON altimetry data. The annual and Chandler components of the pole motion were also modeled. Apart from the subtraction of quadratic trends, initial data were not filtered or averaged. The performed analysis provided model values of T = 425–440 days and Q = 20–60, and Q was found to be somewhat unstable with time.

Cen XR-3: A Neutron Star Younger than the Crab?

A model for the pulsed X-ray source Cen XR-3 has been constructed, based on studies of wobbling, young, evolving neutron stars which are generating heat in their crusts by the conversion of rotational energy through precession-induced cyclic strain. The observational features can be understood if the observed period is interpreted as the free nutation period. We predict the actual rotational period to be ∼8 ms and the slow-down rate ∼8×10−13 s s1.

The pole tide

About 10,000 mean monthly values of sea level from 11 tide stations have been analyzed by the method of Tukey to obtain the power spectra in the frequency range of 0.0125 to 6 cycles per year (cpy). The spectral density is on the order of 103 mm2/cpy and is remarkably uniform over this frequency range, with the following exceptions: (i) a sharp rise at the low-frequency tail, from 103 mm2/cpy at 0.1 cpy to 105 mm2/cpy at 0.0125 cpy (presumably associated with variations in recorded sea level arising from continental unrest); (ii) the line spectrum associated with the annual variation and its harmonics; and (iii) a weak peak of 0.84 cpy, barely above noise level, which is identified with the 14-month ‘pole tide’ corresponding to the earth's free nutation (Chandler wobble). The average pole tide for all stations gives an amplitude twice that predicted by equilibrium theory. The contribution to the peak of the pole tide is largely from three localities: Swinemunde, Marseille, and a combined set of Netherlands stations. Apparently the pole tide is not in accord with equilibrium theory. This raises some questions concerning the interpretation of Love numbers as derived from the period of free nutation.