Four centuries of geomagnetic secular variation from historical records

Abstract

We present a new model of the magnetic eld at the core{mantle boundary for the interval 1590{1990. The model, called gufm1, is based on a massive new compilation of historical observations of the magnetic eld. The greater part of the new dataset originates from unpublished observations taken by mariners engaged in merchant and naval shipping. Considerable attention is given to both correction of data for possible mislocation (originating from poor knowledge of longitude) and to proper allocation of error in the data. We adopt a stochastic model for uncorrected positional errors that properly accounts for the nature of the noise process based on a Brownian motion model. The variability of navigational errors as a function of the duration of the voyages that we have analysed is consistent with this model. For the period before 1800, more than 83 000 individual observations of magnetic declination were recorded at more than 64 000 locations; more than 8000 new observations are for the 17th century alone. The time-dependent eld model that we construct from the dataset is parametrized spatially in terms of spherical harmonics and temporally in B-splines, using a total of 36 512 parameters. The model has improved the resolution of the core eld, and represents the longest continuous model of the eld available. However, full exploitation of the database may demand a new modelling methodology.

Will the magnetic North Pole move to Siberia?

“…If the flux concentra tion in the Siberian lobe decreases, the dip pole changes its direction of movement, probably quite abruptly, as has happened already around 1630,1730, and 1860, accord ing to the GUFM model of Jackson et al [2000].The velocity of the pole would remain high if the pole is located in a region of strong reversed flux, and one may expect decreasing velocity once it has crossed the reversed-flux area. However, linking the dip poles at the Earth's surface to the magnetic field at the core-mantle boundary must be taken with caution, since the present pole positions are very sensitive to small changes in field morphology…”

A New View of Long-Term Geomagnetic Field Secular Variation

“…The fact that geomagnetic field measurements at the Earth's surface have changed over time (secular variation) was first noted by the Chinese by the thirteenth Century AD and later by Henry Gellibrand in Europe in 1634 (Merrill et al, 1998). We now have a good sense of historical secular variation (HSV) of the geomagnetic field for the last 400 years or so (e.g., Bullard et al, 1950;Yukutake, 1967;Yukutake and Tachinaka, 1968;Bloxham and Gubbins, 1985;Jackson et al, 2000). We also understand, to some degree, the source of that magnetic field through dynamo activity in the Earth's outer core (see Merrill et al, 1998 for overview).…”

Correlation of Earth’s magnetic field with lower mantle thermal and seismic structure

“…They are centered away from the poles and are near regions of high seismic velocity in the adjacent mantle (Figures 1,2b). They have moved comparatively little during the last 400 years of direct observation (Jackson et al, 2000) and show up in the time average of paleomagnetic data from the last few million years (Gubbins & Kelly, 1993;Johnson & Constable, 1995;Carlut & Courtillot, 1998;Johnson et al, 2003). Geomagnetic field geometry is dictated to a large extent by Earth's rotation, which explains symmetry about the equator, and the tangent cylinder, which explains the location of the main flux concentrations away from the poles.…”