The Westward Drift and Geomagnetic Secular Change

Abstract

Summary 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. Since quite early times it has been known that the Earth undergoes several small and little-understood motions additional to its motion about the Sun, the precessional motion, and the daily rotation (Munk & MacDonald 1960). Astronomical observations on the Moon and stars during the past century have shown that the Earth’s rotation is non-uniform (Brouwer 1952). Since the Earth’s total angular momentum changes very slowly during centuries of time, effects noted during only a few decades suggest that compensatory motions within the Earth’s interior would seem required when fluctuations in surface rotation are observed (Munk & Revelle 1952; Vestine 1952). The main possibility for such motions, which could yield internal angular momentum changes within decades, would seem to be in the Earth’s fluid metallic core. Accordingly it is of interest to estimate possible changes in the rotation of the core expected to compensate for fluctuations in angular momentum of the mantle indicated by changes in the length of day. Since the Earth’s field is thought to originate in a hydromagnetic core, the geomagnetic field lines may be firmly attached to the surface flow in the core. Therefore, field lines moving about on the Earth’s surface may show whether such compensatory motion in the core takes place. Previous work establishing such connection has unhappily been less certain than desired (Kalinen 1949; Vestine 1952; Smirnov 1965). Vestine (1952) showed that if the Earth’s eccentric dipole motion is monitored at five-year intervals since 1840, it undergoes a westward drift of about 0-30”/yr7 except for several five-year intervals centred near 1910, for which the westward drift is perceptibly reduced (to less than half), corresponding to an increase in the angular velocity of the core. This occurred at a time when the mantle and crust slowed down, so that the day became longer.