SECULAR VARIATION IN ITALY FROM HISTORICAL GEOMAGNETIC-FIELD MEASUREMENTS

Geomagnetic field measurements have been made in Italy at various times and at several locations during the last few hundred years. Some historical measurements that date back to the sixteenth century, including observations by Hartmann, are presented together with many other recently found measurements. Information has been obtained from careful searches in the libraries of old institutes and universities. Magnetic networks and field measurements have been collected to complete a geomagnetic catalogue and, if possible, to draw national maps for epochs in the past. Absolute magnetic data from Italian observatories, starting in the last century, have also been included in the catalogue. Analysis of these data can give detailed information about the secular variation which is not only relevant for Italy but for the whole Mediterranean area. Simple space-time mathematical functions used to fit the original data are compared with more complex global models. Finally, a spectral analysis allows us to infer possible periodicities in the data set.