ON THE RESONANCE EXCITATION OF NEARLY DIURNAL FREE NUTATION 280-230 MA

Abstract

Recent paleomagnetic data have shown that the history of the Earth's magnetic field includes a unique, fairly long (280 Ma to 230 Ma) interval during which the orientation of the Earth's magnetic dipole remained stable relative to the Earth's rotation axis. As noted in [Molodensky, 1981], this geologic epoch coincided with the time when the retrograde annual nutation excited by the tidal wave <font face="Symbol">Y1 coincided in frequency with the nearly diurnal free resonance of the Earth's liquid core. The resonance excitation of the nearly diurnal free nutation could lead to an abrupt increase in the amplitude of nearly diurnal oscillations of the liquid core, and this could result in a drastic change in the geomagnetic dynamo regime (particularly in the turbulent boundary layer at the liquid core/mantle boundary, where the velocity gradient of the nearly diurnal oscillations is highest). To estimate the effectiveness of this mechanism, one should evidently take into account effects of tidal energy dissipation; due to these effects, the resonance amplitude tends toward a certain finite limit. For this purpose, below I address the resonance effects of the tidal energy absorption in (i) oceans, (ii) inelastic mantle, and (iii) viscous core with due regard for the electromagnetic coupling of the liquid core with the mantle. In order to estimate the inelasticity of the mantle in the range of tidal frequencies, the viscosity of the liquid core, and the core-mantle electromagnetic coupling, I used results derived from analysis of new data on amplitudes and phases of forced nutation. The amplitude of resonance oscillations of the liquid core was shown to exceed their contemporary amplitude by about an order of magnitude, so that the hypothesis under consideration appears to be rather plausible.