Constraining the axial dipole moment variations during the historical period from new archeointensity results obtained in Bukhara (Uzbekistan, Central Asia)

Since the middle of the 19th century, direct intensity and directional measurements of the Earth's magnetic field are available which enable an accurate determination of its space and time variations. Before that time, between ca. 1600 and 1840 AD, only direct directional measurements are available. The construction of global field models over that time window thus requires either a specific treatment of the axial dipole component of the field or the use of the available archeomagnetic dataset. In this study, we propose an approach based upon the construction of a regional archeointensity variation curve in Central Asia. We analyze baked brick fragments sampled in Bukhara (Uzbekistan), dated between the end of the 16th and the beginning of the 19th century. This city is particularly interesting because of the preservation of old buildings precisely dated thanks to archives. We obtain a series of archeointensity results using the Triaxe experimental protocol, allowing the recovery of the geomagnetic field intensity variations in Central Asia during the historical period. We compare that evolution with the one predicted by a number of global geomagnetic field models. In good agreement with archeointensity data previously obtained in Western Europe, the new central Asian results show a wider range of axial dipole variations between 1590 and 1840 than previously proposed.