Russian Journal of Earth Sciences
Vol. 6, No. 6, December 2004
On the resonance excitation of nearly diurnal free nutation 280-230 Ma
S. M. Molodensky
Schmidt Institute of Physics of the Earth, Russian Academy of Sciences,
Moscow, Russia
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
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.