a b s t r a c tNineteen new archeointensity results were obtained from the analysis of groups of French pottery fragments dated to the Early Middle Ages (6th to 10th centuries AD). They are from several medieval ceramic production sites, excavated mainly in Saran (Central France), and their precise dating was established based on typo-chronological characteristics. Intensity measurements were performed using the Triaxe protocol, which takes into account the effects on the intensity determinations of both thermoremanent magnetization anisotropy and cooling rate. Intensity analyses were also carried out on modern pottery produced at Saran during an experimental firing. The results show very good agreement with the geomagnetic field intensity directly measured inside and around the kiln, thus reasserting the reliability of the Triaxe protocol and the relevance of the quality criteria used. They further demonstrate the potential of the Saran pottery production for archeomagnetism. The new archeointensity results allow a precise and coherent description of the geomagnetic field intensity variations in Western Europe during the Early Medieval period, which was until now poorly documented. They show a significant increase in intensity during the 6th century AD, high intensity values from the 7th to the 9th century, with a minimum of small amplitude at the transition between the 7th and the 8th centuries and finally an important decrease until the beginning of the 11th century. Together with published intensity results available within a radius of 700 km around Paris, the new data were used to compute a master curve of the Western European geomagnetic intensity variations over the past 1500 years. This curve clearly exhibits five intensity maxima: at the transition between the 6th and 7th century AD, at the middle of the 9th century, during the 12th century, in the second part of the 14th century and at the very beginning of the 17th century AD. Some of these peaks are smoothed, or nearly absent when the selection of the data is extended to a 1250 km radius around Paris. The apparent regularity in the occurrence of intensity maxima, with a recurrence of $250 years, is particularly intriguing and might reflect a new characteristic of the secular variation, at least in Western Europe. It clearly requires further investigation and in particular the acquisition of new data from older periods.

[1] Ten archeointensity results have been obtained from brick and ceramic fragments collected in France and precisely dated to between the tenth and eighteenth centuries. Intensity experiments were performed using the Triaxe protocol taking into account cooling rate and thermoremanent magnetization anisotropy effects. Together with our previous results from France and Belgium, we computed a geomagnetic field intensity variation curve for Western Europe covering the past 1100 years. This curve is characterized by a general decreasing trend at the millennial timescale punctuated by three short intensity peaks, during the twelfth century, around 1350-1400 AD and $1600 AD. A similar evolution but with smoother variations due to data scatter is also observed in Western Europe and to a lesser extent in Eastern Europe when all available archeointensity data fulfilling quality criteria are used. Comparison of our archeointensity variation curve with the climatic record derived from fluctuations in length of the Swiss glaciers shows a good temporal concordance between all geomagnetic field intensity maxima detected in Western Europe over the past millennium and colder episodes. A comparison is further discussed between these intensity maxima and episodes of low rates of 14 C production. A common pattern of variations between both records is recognized between the middle of the tenth and of the beginning of eighteenth centuries. If significant, such coincidences suggest a dual geomagnetic and solar origin for the century-scale climate and radionuclide production variations during at least the past millennium.

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