FWF Archeomagnetism

The geomagnetic field undergoes significant temporal variations in morphology and strength. Such variations of the magnetic field occur on timescales from below minutes to millions of years. This project focuses on centennial changes of the earth’s magnetic field. The main goal is improving our knowledge on the underlying physical processes of secular variation, which originate in the geodynamo of the earth’s core, as well as its consequences and implications. Several peculiarities of secular variation have been identified since magnetic observations started back in the middle of the 19th century. Most striking is a significant reduction of the geomagnetic field strength throughout the observational period. The dipole moment of the field lost more than 10% of its initial strength in only 150 years. Additionally, the direction of the field changed considerably, the declination in Austria varied by about 20°. An expansion of the observational period further back in time is essential to investigate the temporal dependence of geomagnetic variations in detail and to study the causes and consequences of such secular variation. Within the framework of this project sources for geomagnetic measurements will be located and investigated. All data will be used for geomagnetic field reconstruction. Magnetic determinations in Europe started in the early medieval times. Focus will be set on magnetic measurements for mining activities, historic maps, orientation of sundials, church constructions, for various scientific interests, e.g. in monasteries, in astronomic observatories and during expeditions as well as measurements of the Imperial and Royal War Navy. These results will be accompanied by indirect results from archeomagnetic and paleomagnetic sources. Such indirect data are indispensable for two reasons: Firstly, these data allow expanding the observational time scale farther back in time. Secondly, indirect data are the only source of geomagnetic field strength information before the eighteen-forties. The reliability of such indirect data, in dependency of material and measurement technique, is however strongly debated. Therefore a comparison of temporal and spatial closely related direct and indirect field records is projected. This provides an independent test on accuracy and the eventual presence of biasing mechanisms. Both, direct and indirect data will be summarized within an openly accessible geomagnetic database. For modeling, a Bayesian inversion approach will be applied to reconstruct the spherical-harmonic evolution of the Earth’s magnetic field. This provides a link between surface geomagnetic field variations and dynamo processes. Thus, a better understanding of the dynamics and implications of field variations will be achieved. An adequate reconstruction of the past geomagnetic field variations allocates an additional age dating method for archeologists. The obtained field model will further contribute to the current discussion of the coherence between climate and geomagnetism.