The Westward Drift and Geomagnetic Secular Change

Short-term fluctuations in the length of the day on the order of a decade are discussed in relation to fluctuations in westward drift of the outer part of the Earth's core. Decreases in the Earth's rotation rate noted near 1910 and 1965 are shown to be correlated with decreases in westward drift of the Earth's magnetic eccentric dipole, which probably originates fairly deep within the Earth's core. If the outer 200-km thickness of the core moves approximately as does the eccentric dipole field, the changes in angular momentum implied for this part can explain the observed changes in the length of the astronomical second, on the basis of conservation of angular momentum. Other estimates of westward drift in the core based on higher-degree harmonic terms give a lower westward drift, and also seem to show less precision in the estimates of flow in the core. These discrepancies are unexplained.