Geomagnetic field direction in Paris since the mid-sixteenth century

Abstract

The geomagnetic field experienced at any point on the globe is essentially generated by dynamo processes in the Earth's core. This main geomagnetic field is subject to change in its direction and strength on time-scales ranging from years to centuries, known as secular variation. Studies of secular variation can reveal much about the physics of the Earth's core, but for this we must have homogeneous, high-quality measurements of the geomagnetic field covering as long a time span as possible. In this paper we present measurements of declination (for the years 1541–1883) and inclination (for the years 1660–1883) made in the vicinity of Paris. The data have been gathered from a variety of sources, and some analysis has been carried out to determine corrections for certain sub-series of measurements, so that the resulting time-series may be regarded as having been obtained from a single location—that of the Paris Observatory. To conduct this analysis we have had to consider the methods and instruments used by the early observers, and the likely errors in the measurements. Once compiled, the edited data from the Paris Observatory van be combined with those for the present-day observatory at Chambon-la-Forêt, covering the years 1883–1994. The resulting time-series records changes in direction of the geomagnetic field spanning more than 450 years for declination and more than 320 years for inclination. The similarities and differences with the only other comparable time-series, i.e. that of London (Malin and Bullard, 1981, Philos. Trans. R. Soc. London, 299: 357–422) should be the focus of promising further work.