Current Status of Geophysical Models of Nutation

Abstract

Geophysical modeling of nutations involves (a) the formulation of a general theory of the forced nutations of an idealized Earth (spheroidal, oceanless, elastic), supplemented by separate theoretical treatments of the effects of the oceans, anelasticity, and other deviations from the ideal, and (b) fitting of the theoretical expressions to VLBI-estimated amplitudes of the various spectral components of the nutations. The aim is to obtain best-fit estimates of Earth parameters which influence the nutations; and the degree of success of the modeling is judged by the precision of the estimates obtained, as well as the quality of the fit as measured by χ2red, the chisquared per degree of freedom.We present here our findings from two types of fits using, for the observational input, the nutation amplitude estimates for seven important pairs (retrograde and prograde) of circular nutations, together with their standard errors and mutual correlations, which were kindly furnished by T.A. Herring (private communication, 1993). The theoretical framework employed was that of Mathews et al. (1991), hereinafter referred to as MBHS, together with the Zhu and Groten (1989) rigid Earth nutation series modified to take account of the recently-estimated correction of −0.3mas/cyto the precession constant. Estimates of ocean tide effects, and of anelasticity effects based on Wahr and Bergen’s (1986) results for model QMU with α = 0.15 (hereafter abbreviated to WB), were taken from Herring et al. (1991). Alternative estimates of anelasticity effects (DZ, due to Dehant (1990) based on the Zschau model), were also considered.