Introduction

Furthermore, the magnitude and the sign of the compass needle deviation for a given position on the Earth’s surface is not constant in time and is subject to unpredictable changes. These changes occur on varying time scales and are usually divided into ‘daily’ (or ‘rapid’) variations and ‘secular’ variations.

The former type of variations are caused by the buffeting of the Earth’s magnetosphere by the variable solar wind which in turn influences the Earth’s ionosphere and surface magnetic field, resulting in ‘geomagnetic storms’ and often colourful displays of the aurora borealis and aurora australis. The latter type of variations are caused by slow changes in the electrical current-coupled fluid motions in the inner molten metallic core of the Earth that generate the inner geomagnetic field and its interaction with the outer semi-rigid crust that possesses its own (nearly fixed but weaker) magnetic field and appears to rotate at a slightly lower rate.

Global Magnetic Declination Maps for the Modern Period

These so-called isogonic maps were drawn with IDL software from the data generated by the FORTRAN program GEOMAG (cf. Campbell, 1997) and the associated mathematical models of the geomagnetic field distributed by the National Geophysical Data Center. The adopted models for the epochs 1600 to 1850 were prepared by the British Geological Survey (Barraclough, 1974 & 1978) and the International Geomagnetic Reference Field (IGRF) models were used for the epochs 1900, 1950 and 2000. The latter models are available at five-year intervals between 1900 and 2000 and charts depicting the main parameters of the geomagnetic field at ten-year intervals within this period (and 1995) can be viewed at the National Geophysical Data Center website. For an assessment of the reliability of these and similar models, see the IGRF Health Warning.

The British Geological Survey (BGS) models are based on historical data collected by Veinberg in the 1920’s and 1930’s (Veinberg & Shibaev, 1969). These were fitted to 4^th-degree spherical harmonic representations of the terrestrial magnetic field and are of course somewhat uncertain for the earliest epochs. The maps for the epochs 1550 and 1500 were obtained by extrapolating the 1600 BGS model backwards in time and are thus even more uncertain. They were constructed primarily for studying the course of the isogonic lines on a global level.

For a more reliable assessment of the probable location of these lines across Europe, the Mediterranean Sea, the African coast and the North Atlantic Ocean, the reader is referred to the maps by Willem van Bemmelen (1893, 1899) which are based on contemporary measurements.

Each file can either be downloaded as a zipped postscript file of about 0.95 Mb (unzips to a file of about 3.5 Mb) that can be viewed with GhostView or as a pdf file of about 1.1 Mb that can be viewed with Acrobat Reader (in order to orient the maps properly, the use of version 4.0 or later is recommended). For each epoch a set of four maps is presented: two equatorial views centred on longitudes 0º and 180º, and two polar views. On each map the isogonic lines are given at 10º intervals.

At the moment these maps are still preliminary – that is to say, they still need improvement and the problem that I am now currently addressing is how to persuade IDL to both draw and colour the inland seas (Black Sea, Caspian Sea, etc.).

At the moment maps are available for the following historical epochs:
 

Available maps

Epoch	ps file	pdf file	Adopted model
–		not yet available	Gerard Mercator (1541)
–	not yet available	not yet available	Petrus Plancius (c. 1597)
–			Simon Stevin (1599)
1744	not yet available	not yet available	Leonhard Euler (1757)
1835	not yet available	not yet available	Carl Friedrich Gauss (1838)
1500			BGS 1600 (backwards extrapolated)
1550			BGS 1600 (backwards extrapolated)
1600			BGS 1600
1650			BGS 1650
1700			BGS 1700
1750			BGS 1750
1800			BGS 1800
1850	not yet available	not yet available	BGS 1850
1900	not yet available	not yet available	DGRF 1900
1950	not yet available	not yet available	DGRF 1950
2000	not yet available	not yet available	IGRF 2000

Also available are maps depicting the global course of the isogonic lines following the theories of Gerard Mercator and Simon Stevin. Additional maps depicting the theories of Petrus Plancius, Leonhard Euler and others will follow in the future.

Magnetic Declination Maps for Earlier Epochs

Literature:

Akasofu, Syun-Ichi & Chapman, S., Solar-Terrestrial Physics: An Account of the Wave and Particle Radiations from the Quiet and the Active Sun, and of the consequent Terrestrial Phenomena (Clarendon Press, Oxford, 1972), chapter 2.

Barraclough, D.R., “Spherical Harmonic Analyses of the Geomagnetic Field for Eight Epochs between 1600 and 1910”, Geophysical Journal of the Royal Astronomical Society, 36 (1974), 497-513.

Barraclough, D.R., Spherical Harmonic Models of the Geomagnetic Field (HMSO, London, 1978 [= Geomagnetic Bulletin of the Institute of Geological Sciences, no. 8]).

Barraclough, D.R., “Historical Observations of the Geomagnetic Field”, Philosophical Transactions of the Royal Society of London, ser. A, 306 (1982), 71-78.

Bartels, J., “The Eccentric Dipole approximating the Earth’s Internal Magnetic Field”, Terrestrial Magnetism and Atmospheric Electricity, 41 (1936), 225-??? [not seen].

Bloxham, J., “Models of the Magnetic Field at the Core-Mantle Boundary for 1715, 1777, and 1842”, Journal of Geophysical Research, ser. B (Solid Earth), 91 (1986), 13954-13966.

Bloxham, J. & Barraclough, D.R., “The Complication of Historical Geomagnetic Observations for use in Studies of the Earth’s Core”, in: W. Schröder (ed.), Historical Events and People in Geosciences: Selected Papers from the Symposia of the Interdivisional Commission on History of the IAGA during the IUGG General Assembly, held in Hamburg, 1983 (Peter Lang, Frankfurt am Main/Bern/New York, 1985), pp. 147-162.

Bloxham, J., Gubbins, D. & Jackson, A., “Geomagnetic Secular Variation”, Philosophical Transactions of the Royal Society of London, ser. A, 329 (1989), 415-502.

Bloxham, J. & Jackson, A., “Time-Dependent Mapping of the Magnetic Field at the Core-Mantle Boundary”, Journal of Geophysical Research, ser. B (Solid Earth), 97 (1992), 19537-19563.

Campbell, W.H., Introduction to Geomagnetic Fields (Cambridge University Press, Cambridge, 1997).

Chapman, S. & Bartels, J., Geomagnetism, 2 vols. (Oxford University Press, London, 1940).

Hutcheson, K.A. & Gubbins, D., “Earth’s Magnetic Field in the Seventeenth Century”, Journal of Geophysical Research, ser. B (Solid Earth), 95 (1990), 10769-10781.

James, R.W., “???”, Geophysical Journal of the Royal Astronomical Society, 17 (1969), 305-??? [not seen].

James, R.W., “Eccentric Dipole Coefficients and Magnetic Centres”, Planetary and Space Science, 18 (1970), 441-443 [not seen].

Langel, R.A., “Main Field”, in: J.A. Jacobs (ed.), Geomagnetism (Academic Press, London, 1987), vol. I, pp. 249-512.

Merrill, R., McElhinny, M.W. & McFadden, P.L., The Magnetic Field of the Earth: Paleomagnetism, the Core, and the Deep Mantle (Academic Press, San Diego, 1996).

Schmidt, A., “Die magnetische Mittelpunkt der Erde und seine Bedeutung”, Beiträge der Geophysik, 56 (1934), 346-??? [not seen].

Thompson, R. & Barraclough, D.R., “Geomagnetic Secular Variation Based on Spherical Harmonic and Cross Validation Analyses of Historical and Archaeomagnetic Data”, Journal of Geomagnetism and Geoelectricity, 34 (1982), 245-263 – this paper provides revised coefficients for the models of Barraclough (1974, 1978).

Veinberg, B.P. & Shibaev, V.P., Catalogue: The Results of Magnetic Determinations at the Equidistant Points and Epochs 1500-1940 (Institute of Terrestrial Magnetism, Ionosphere and Radiowave Transmission, Moscow, 1969) – English translation published by the Canadian Department of the Secretary of State, no. 0031 (1970). Veinberg’s detailed bibliographical notes to his catalogue were never published as they were destroyed during WW II.

Winch, D.E., “Eccentric Geomagnetic Dipole Potential”, Planetary and Space Science, 16 (1968), 1341-1347 [not seen].