d. Geomag. Geoelectr., 25, , 1973 Magnetic Secular Variation in Denmark,

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1 Research Note d. Geomag. Geoelectr., 25, , 1973 Magnetic Secular Variation in Denmark, Niels ABRAHAMSEN Laboratory o f Geophysics, University of Aarhus, Denmark (Received October 6, 1972; Revised December 18, 1972) 1. Compilation of D and I in Denmark Values of declination (D) and inclination (I) have been observed since 1907 at the Magnetic Observatory at Rude Skov 20 km north of Copenhagen by researchers of the Danish meteorological institute (Magnetic Yearbook, Denmark 1956). Between 1892 and 1900 geomagnetic measurements were made at an older observatory in Copenhagen (BocK and SCHUMANN, 1948). Before 1892 fewer regular measurements were made and most of the values known were collected in the works of HANSTEEN (1819, 1855, 1860), D'ARREST (1859) and PAULSEN (1892). In Table 1, such values for Copenhagen are cited, the oldest value in D being from the year 1649 and in I from Some values of D and I from southern Scandinavia are further given in Table 2, and under the supposition that the earth's magnetic field is a simple dipole field, these values are reduced at the site of the Rude Skov Observatory as DRS and IRS. Finally, in Table 3 readings from maps by Hansteen (partly based on the values cited in Tables 1 and 2), van Bemmelen and Halley are given for Copenhagen, the error indicated being the estimated visual error from the reading of the maps. In Fig. 1 the values D and I from Tables 1, 2 and 3 are plotted with smooth curves subjectively drawn through them, giving most weight to the values of Table 1 and those of Table 2, which are situated within a distance of 30 km from Rude Skov. These values are indicated by dots, while other values in Table 2 are indicated by crosses. Values of D and I, read from the curves at intervals of 10 years between 1500 and 1760 and intervals of 5 years after the mean values of a modern observatory are usually given to 0.1'. This is because accuracy in determining the direction of the magnetic field in the past by means of measuring the thermoremanent magnetization in baked clay (which at 105

2 106N. ABRAHAMSEN

3 Magnetic Secular Variation in Denmark, Comparisons and Conclusions To compare the Danish secular variation with other European curves, the two curves in Fig. 1 have been combined and redrawn in Fig. 2 in the tangential projection described by BAUER (1895). As the direction is a function of the geographical position, the curves in Fig. 2 have been recomputed by means of the central dipole hypothesis to the geographical site of Copenhagen (Rude Skov) and are shown in Fig. 3. Although the curves were made more similar, discrepancies are still present, probably due partly to the westward drift (BULLARD Fig. 1. Secular variation of declination and inclination in Copenhagen (Rude Skov), based on the direct measurements of Tables 1, 2 and 3.

4 108N. ABRAHAMSEN Table 2. Southern Scandinavia Table 3. Copenhagen (from maps) et at., 1950; BURLATSKAYA et al., 1970) and partly more local, but unknown effects (BARRACLOUGH, 1971). Measurements of the remanent magnetization in baked clay from a fireplace of the remanent magnetization in archaeologically dated samples will have to be made before this part of the Danish curve is well determined. The extension of the Danish curves further back in time must be based on pure archaeomagnetic data as older direct measurements alone are too few to

5 Magnetic Secular Variation in Denmark, Table 4. Secular Variation, Complied for Rude Skov Sources: : Fig. 1(Tables 1, 2 & 3) : BOCK and SCHUMANN, : Magnetic Yearbook, : L.AURIDSEN, give reliable results by spherical harmonic analysis (YUKUTAKE, 1971), and simple dipole reductions, e.g., from the known curves for London (AITKEN, 1970), obviously do not give the required accuracy, as revealed in Fig. 3 where

6 110N. ABRAHAMSEN Fig. 2. Secular variation of some European sites, not dipole corrected; legend as in Fig. 3. Fig. 3. Secular variation of some European sites, recomputed to the site of Copenhagen by central dipole hypothesis. The non conform curves reveales as well the westward drift as local peculiarities. C: Copenhagen (Rude Skov) L: London P: Paris R: Rome : Oslo PO: Potsdam RI: Riga I0: Inclination at C for central axial dipole field.

7 Magnetic Secular Variation in Denmark, REFERENCES AITKEN, M,J., Dating by archaeomagnetic and thermoluminescent methods, Phil. Trans. Roy. Soc. Lond. A, 269, 77-88, Selskabs Forh., 74-84, BARRACLOUGH, D.R., Secular variation in Scandinavia, Nature, 233, 94-98, BAUER, L.A., Beitrage zur kenntniss des Wesens der Sacular-variation des Erdmagnetismus, 54p, Berlin, BOOK, R. and W. SCHUMANN, Katalog der Jahresmittel, Geophysikal. Inst. Potsdam, nr. 8, BULLARD, E.C., C. FREEDMAN, H. GELLMAN and J. NIXON, The westward drift of the earth's magnetic field, Phil. Trans. Roy. Soc. London, 243, 67-92, BURLATSKAYA, S., T. NECHAEVA and G. PETROVA, Some archaeomagnetic data indicative of the westward drift of the geomagnetic field, Archaeometry,11, , EKEBERG, K.G., Forsok til en Magnetisk Inclinations-Charta, Kungl. Svenska Vetenskaps Acad. Handl., XXIX, , HALLEY, E., Geomagnetism, edited by S. Chapmann and J. Bartels, plate 38, Oxford Univ. Press London, HAHSTEEN, C., Untersuchungen fiber den Magnetismus der Erde, pp, Christiania (Oslo), HANSTEEN, C., Den magnetiske Inclinations Forandring i den nordlige tempererede Zone, Vid. Selsk. Skr. 5. Raekke, 4. Bind, & , HANSTEEN, C., Den magnetiske Inclinations og Intensitets Forandringer i Kjobenhavn, Oversigt over Vidsk. Selsk. Forh., 32-36, HELLMANN, G., Die Anfange der magnetischen Beobachtungen, Z.d. Gesellschaft f. Erdkunde, XXXII, , LAURIDSEN, E. Kring, Personal communication, Magnetic yearbook, Denmark 1956, Danish Meteorological Institute, MAY, W.E., "True"-reading magnetic compasses, Terr. magn. atm. Electr., 53, , NEUMAYER, G., Atlas, des Erdmagnetismus, Justus Perthes, Gotha, PAULSEN, A., Communications de l'observatoire Magnetique de Copenhague, 1-71, SLAUCITAJS, L., On the geomagnetic secularvariation in the past centuries at Riga, Contributions of Baltic University No. 7, 6pp, Hamburg, VAN BEMMELEN, W., Die Abweichung der Magnetnadel, Beobachtungen Sacular-Variation, West- and Isogonensusteme bis zur Mitte des XVIIT"ten Jahrhunderts,109 pp., Batavia,1899. WASSERFALL, K.F., A study on the secular variation of magnetic elements based on data for D, land H for Oslo, , J. Geophys. Res., 55, , VESTINE, E.H., L. LAPORTE, C. COOPER, I. LANGE and W.C. HENDRIX, Description of the earth's main magnetic field and its secular change , Carnegie Inst. Wash. Publ., 578, 1-532, WLEUGEL, P.J., Formodning om at Magnetnalen her i Kj benhavn har naet sit Maximum at vestlig Afvigning, Vid. Selsk. Phys. Skr. I. Del, I Hrefte, ,1821. YUKUTAKE, T., Spherical harmonic analysis of the earth's magnetic field for the 17th and the 18th centuries, J. Geomag. Geoelectr., 23, 11-31, 1971.