Overview

Earth's magnetic field originates from dynamo processes within the core. It produces a protective shield against cosmic and solar radiation. The geomagnetic field, however, is not constant but experiences continuous variations in strength and direction. These variations are measured at geomagnetic observatories all around the world. In Austria these observations started as early as the 1850's after the foundation of the "k.k. Centralanstalt für Meteorlogie und Erdmagnetismus", which is known as ZAMG.

In the future measurements of magnetic field variations of yet unmatched precision will be continued at the Conrad Observatory. On basis of geomagnetic observations one can identify peculiarities of Earth's magnetic field. A striking feature is a drop in magnetic field strength over the entire observation period, which is approximately two hundred years. The magnetic dipole moment dropped by more than 10%. Additionally, the direction of the magnetic field changed considerably by more than 20° of declination in Europe. In order to examine temporal dependencies across long time-scales and identify sources of long-term magnetic field variations it is necessary to extend observation periods far into the past.

Along side observatory observations, other early direct measurements of Earth's magnetic field exist. Components of the magnetic field were measured for various reasons. Amongst others to identify orientation of sundials, to determine bearing of a compass for mining and navy, for historical maps, as well as for scientific interest, e.g. astronomical observatories and expeditions.

Project aims

This project aims to collect information on direct observation from historical sources. Moreover, data of indirect measurements (archaeomagnetic and palaeomagnetic) will be added to the results.

Indirect data is vital on two grounds:

On one hand, these data permit a significant extension of the time series into the past. On the other hand, only these data provide information on magnetic field strength for the time before the forties of the 19th century.

However, the reliability of indirect data, which depends on analytical method and material, is much-debated. Comparison of direct and indirect data within a narrow time and local frame, starting from the late Middle Ages, provides an independent validation on reliability and precision.

By means of a Bayesian inversion model of Leonhardt and Fabian (2007), the Earth's magnetic field can be described by spherical harmonics and therefore a correlation between surface observations and dynamo processes in the Earth's core can be made. This provides a deeper understanding of sources as well as effects of field variations. Accordingly, this can challenge the hotly debated relationship between climate and magnetic field.



Furthermore, precise reconstruction of magnetic field variations in the past provides an additional tool of dating in archeology.

Results
This project is currently in the initial phase.
Results will be available soon.

Start January 2013
End December 2015

Project supervisor
Dr. Roman Leonhardt (Direktor des Conrad Observatorium, ZAMG)

Project staff
Mag. Patrick Arneitz (ZAMG)

Project partners

Lehrstuhl für Geophysik, Montanuniversität Leoben (Austria)
Ludwig Boltzmann Institute for Archaeological Prospection and Virtual Archaeology (Germany)
Observatorium Tihany (Hungary)
Akademie der Wissenschaften der Tschechischen Republik (Czech Republic)
NGU Trondheim (Norway)
Observatorium Hurbanovo (Slovakia)

Funding
Fonds zur Förderung der wissenschaftlichen Forschung - FWF

References
*) Leonhardt, R., Saleh, A. and Ferk, A. (2010): Archaeomagnetic field intensity during the Roman period at Siwa and Bahryn oasis, Egypt: Implications for the fidelity of Egyptian archaeomagnetic data. Archaeometry,52, 502-516.
*) Stark, F., Leonhardt, R., Faßbinder, J. and Reindel, M. (2009): The Field of Sherds: Reconstructing Geomagnetic Field Variations from Peruvian Potsherds. In: New Technologies for Archaeology. Eds: Reindel, M. Wagner, G.A., Springer, Berlin, Heidelberg, pp. 103-116.
*) Leonhardt, R., and Fabian, K. (2007): Paleomagnetic reconstruction of the global geomagnetic field evolution during the Matuyama/Brunhes transition: Iterative Bayesian inversion and independent verification. Earth Planet. Sci. Lett., 253, 172-195.

This is an ongoing project.