Variations of the Earth´s magnetic field: dynamics and implications

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.

The reconstruction of the Earth's magnetic field is based on different data types, which are, on the one hand, characterized by highly variable measurement and age uncertainties and, on the other hand, a strongly heterogeneous spatio-temporal distribution. The main focus of the project was the integration of these different records within a single database along with a thorough analysis of associated uncertainties, which were adequately considered for the final modelling approach.In the initial phase of the project new geomagnetic records were acquired. Historical (direct) observations were located and recorded, inter alia, in archives, monasteries and on historical maps. A study on medieval churches from Lower Austria and northern Germany revealed that their axes were not oriented in eastern direction by means of the compass. Further records providing information on the geomagnetic field far beyond the historical epoch can be gained from archeo- and paleomagnetic (indirect) measurements. Archeomagnetic campaigns were conducted, which contributed to the extension of the data collection or were used for archeomagnetic purposes.The newly acquired direct and indirect records as well as already published collections were compiled in the online-database HISTMAG (http://www.conrad-observatory.at/zamg/index.php/data-en/histmag-database), which formed the basis for detailed evaluation of the different datasets. Systematic deviations between indirect and direct records were investigated in combination with newly developed methods for the handling of these extremely inhomogeneous datasets. A user-comment-function within HISTMAG database allows for a scientific discussion and, therfore beside the available meta information , for an additional quality assessment of single records.The existing modelling approach was adopted based on the database and studies on data quality. Unreliable records were filtered and the heterogeneous data distribution, which is characterized by a strong increase in number within the historical epoch, was homogenized. The final model allows for field predictions at any location on the Earth's surface with corresponding uncertainties for the last 3000 years. Additionally, studies on the field behavior at the core-mantle-boundary as well as on field features (e.g., so-called jerks) can be performed.Beside general statements on the temporal and spatial evolution of the geomagnetic field, the model can be used for archeomagnetic dating purposes as already demonstrated for an archeological study. A further application example is given by the possible connection of strong field and climatic variations.