Two thousand years of geomagnetic field direction over central Europe revealed by indirect measurements



Supplemented by 32 new directions, the Hungarian archaeomagnetic data set now consists of 217 archaeologically dated directions ranging in age from 300 BC to 1800 AD. From this data set, reference curves of secular variation of the geomagnetic field direction were computed using hierarchical modelling and curve estimation by moving average technique. Thanks to some of the new data, the gap in the earlier reference curves around 500 AD has now been filled up. For comparison's sake, directional records of comparable length from central Europe were also processed by the same curve building method. For this purpose, all dated directional data (declination and inclination with statistics) were drawn from the GEOMAGIA50 database for France, Germany, the Ukraine and Moldavia, Bulgaria and Italy and transferred via their virtual geomagnetic poles to a reference point of their respective countries. The resulting reference curves, including those for Hungary, show more or less similar temporal behaviour to the corresponding CALS7K.2 model curves (also available from the GEOMAGIA50 database), but significant deviations from the low-order CALS7K.2 predictions are also discernible owing to the likely presence of additional higher-order complications in the regional field. Therefore, the regional field and its secular variation are suggested to be approximated by the reference rather than the predicted curves. At any other location within the study area, the direction of the regional field can be obtained by spatial interpolation from the reference curves as illustrated by isogonic and isoclinic maps shown for selected times. Local time-series of interpolated directions for other central European countries lacking reference curves might serve as master curves for magnetic dating.