Forced nutations of the earth: influence of inner core dynamics 2. Numerical results and comparisons

Mathews, P. M. ; Buffett, B. A. ; Herring, T. A. ; Shapiro, I. I. (1991) Forced nutations of the earth: influence of inner core dynamics 2. Numerical results and comparisons Journal of Geophysical Research, 96 (B5). pp. 8243-8257. ISSN 0148-0227

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We apply the theory developed in Paper 1 (Mathews et al., this issue), which includes the solid inner core explicitly in the dynamical equations, to obtain the eigenfrequencies and other characteristics of the Earth's nutational normal modes as well as the amplitudes of forced nutations at various tidal frequencies, for two commonly used Earth models, 1066A and the Preliminary Reference Earth Model (PREM). We also make an evaluation of various procedures for taking account of known deviations of the Earth from models, notably in the dynamical ellipticity e for which the two models yield values which are over 1% smaller than the value e deduced from the precession constant. On adopting the procedure of simply replacing e by e in the equations of our theory, the values obtained for some of the nutation amplitudes for model 1066A differ significantly from the corresponding results of Wahr (1981b). The largest of the differences, which occur in the prograde semiannual, retrograde 18.6 year, and retrograde annual nutation terms, amount to -0.59, 0.35, and -0.25 milliarcseconds (mas), respectively, while the standard errors in the very long baseline interferometry (VLBI) determinations are now only about 0.04 mas except in the long period terms. The difference in the procedures used to take account of the discrepancy between e and e contributes -0.56, 0.81, and -0.17 mas, respectively, to the above-noted differences. For the purpose of comparison with VLBI-observed data, we use the results for a "modified PREM," defined by a set of Earth model parameters which differ from those of PREM only in having e for the dynamical ellipticity of the Earth as a whole and a modified value for the dynamical ellipticity ef of the fluid outer core. The amplitudes computed for this model, with corrections applied for the effects of ocean tides and mantle anelasticity, are in generally satisfactory agreement with observed values, when the modified ef is determined by matching the theoretical and observed values for the retrograde annual term. (The modified ef is 0.002665, about 4.6% higher than in PREM, equivalent to an increase, relative to PREM, of about 430 meters in the difference between the equatorial and the polar radii of the core-mantle boundary. We find that contributions from inner-core dynamics to the prograde semiannual and annual, and the retrograde 18.6 year and annual terms, recomputed for modified PREM, amount to -0.09, 0.03, -0.36, and -0.09 mas, respectively.) The largest residual remaining, other than in the long-period terms which still have an uncertainty of about 1 mas, is -0.25 mas in the prograde fortnightly amplitude. Consideration of possible sources of the discrepancies is facilitated by a resonance expansion of the amplitude of forced nutations, as a function of frequency, normalized relative to that for a rigid Earth model. We also provide tables which exhibit the sensitivities of various relevant quantities (the eigenfrequencies and the coefficients which appear in the resonance expansion, as well as the nutation amplitudes at important tidal frequencies) to possible errors in the Earth parameters which enter our theory. Reconciliation of theoretical and experimental values for the prograde fortnightly term, for instance, could be accomplished, without affecting significantly the comparison for other nutation terms, by a decrease of about 10% in the value of the compliance parameter k that represents, in effect, the deformability of the Earth as a whole in response to perturbations of its rotation; but this change in k would have to be produced by some mechanism which does not affect the values of the other compliances.

Item Type:Article
Source:Copyright of this article belongs to American Geophysical Union.
ID Code:20528
Deposited On:20 Nov 2010 14:22
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