Mathematics – Logic
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmgp51b..04k&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #GP51B-04
Mathematics
Logic
1239 Earth Rotation Variations, 1560 Time Variations: Secular And Longer
Scientific paper
We present the results of our studies about the core-surface flow over the last seven thousands years. Our analysis is based on the CALS7k.2, a time--dependent model of the Earth's magnetic field and its secular variation for the period 5000 B.C to 1950. This model relies on indirect observation of the Earth's magnetic field taken from natural archives (lake sediments) and archaeological artifacts. In order to compute the core--surface motion we invert the magnetic induction equation, which relates the magnetic field at the CMB, its temporal field variation and the fluid motion atop the core. We seek instantaneous flow solutions without considering the effect of magnetic diffusion. Invoking magnetic diffusion in the inversion scheme to gain a time dependent fluid flow requires information about the morphology and the state of the toroidal field at the CMB. These information are unavailable. However, instantaneous flow solutions may also be appropriate to evaluate processes at the CMB. These flow solutions suggest that the core-surface flow undergoes different regimes of zonal flow direction. Epochs of a mainly westward flow alternate with epochs of eastward flow. Also, the mean velocity of the flow seems to fluctuate. These changes in the flow direction seem to recur periodically, with a period of about 500 -- 800 years. While the mechanism causing these variations is unassigned, they should also reflect in the length of day variation, where due to core--mantle coupling angular momentum is transfered from the core to the mantle. Much about the exchange of angular momentum on millennial timescales is yet unknown. A comparison between the deduced length of day variation from historical eclipses and the prediction of our flow model shows a significant mismatch. We argue that this mismatch can be indirectly attributed either to millennial scale tidal forcing of the moon, or to solar radiation variability. Both mechanisms have been suggested to influence the Earth's climate with periods of 1500 to 1800 years. The long term climate variations cause fluctuations in the mass distributions at the Earth's surface, which in turn alter the length of day.
Korte Monika
Wardinski Ingo
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