A. Genevey $^{a}$ $^{b}$, Y. Gallet $^{b}$ and M. Le Goff $^{b}$
$^{a}$ Scripps Institution of Oceanography $^{b}$ Institut de Physique du Globe de Paris
Because of its rich prehistoric and historical past, Western Europe is a particularly favorable region to study the geomagnetic field variations during the last few millennia. Until now, the archeomagnetic studies were mainly focussed on the directional evolution of the geomagnetic field over the past 2000 years and the numerous data allowed the construction of a detailed curve of these variations. The number of archeomagnetic results for older periods is however much more limited. For this reason, we obtained new directional data from first millennium BC French archeological fired structures (ovens and hearths) dated by archeological constraints and/or $^{14}C$. From these results, together with a compilation of archeomagnetic data available for Western Europe, we propose a directional secular variation curve covering the entire first millennium BC and valid in Western Europe. We show that during the last three millennia, the geomagnetic field explored directions varying up by $\sim50^\circ$ in declination and $\sim15^\circ$in inclination. While the directional variations of the geomagnetic field are well defined in Western Europe for the last three millennia, our knowledge of the intensity fluctuations is much more limited and fragmentary. Until recently, no data were in particular available between $\sim$ AD 500 and $\sim$ AD 1100 (Middle Ages period in Western Europe). We therefore determined intensities for 14 groups of French ancient potsherds dated by archeological constraints and/or $^{14}C$ between $\sim$ AD 300 and $\sim$ AD 1600. Our results exhibit large intensity fluctuations during the last two millennia, marked by two maxima at $\sim$ AD 850 and $\sim$ AD 1500, and by a rapid and significant decrease in intensity between $\sim$ 850 and $\sim$1200 AD at a rate of $\sim7\muT$ per century. Taken altogether, the archeomagnetic data now provide a complete description of the secular variation (both in direction and in intensity) of the geomagnetic field in Western Europe during the past 2000 years. We underline that these results suggest a possible relationship, valid at least in Western Europe, between changes in direction and intensity.