Eastward and westward drift of the Earth's magnetic field for the last three millennia

Abstract

We analyse the secular variation captured by the archaeomagnetic field model CALS7K.2 in an effort to determine episodes of eastward and westward motions of Earth's magnetic field at the core–mantle boundary (CMB) over the past 3000 yr. The direction, amplitude and geographical distribution of these motions are described. We find that the clearest azimuthal motions are observed at mid- to high latitudes in the Northern hemisphere, where both eastward and westward motions occur. These azimuthal motions correspond to displacements and distortions of the two main, quasi-stationary, high-latitude magnetic flux patches. Similar motions are not observed in the Southern hemisphere, although this may be a consequence of the poorer data coverage there. The globally averaged drift for the past 1000 yr has been westward since 1400 AD, but eastward between 1000 AD and 1400 AD. In the broad region of the CMB under Europe, the times of transition in the direction of the mean azimuthal motion coincide with the times at which “archaeomagnetic jerks” have been reported. Our results suggest that these are caused by a relatively rapid (<100 yr) change in the direction of the underlying azimuthal flow near the core surface. We find indications that equatorial westward motions of field features at the CMB, similar to those observed during historical times, may have been present for much of the past 3000 yr. When observed, these low-latitude motions are most prominent in the Atlantic hemisphere, which we interpret as a signature of core–mantle thermal coupling.