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The Magnetic Poles of the Earthy

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The Magnetic Poles of the Earthy J. Geomag. Geoelectr., 34, 225-240, 1982 The Magnetic Poles of the Earthy E. DAWSON and L. R. NEWITT Division of Geomagnetism, Earth Physics Branch, Energy, Mines and Resources, Ottawa, Ontario, Canada (Received November 26, 1981) The motions of the Earth's magnetic dip poles and geomagnetic poles over the past 400 years are investigated. In addition, relations are sought between geomagnetic and virtual geomagnetic poles over historic times. Since 1750, the secular motion of the dip poles along their paths has been counterclock- wise. During this century this secular motion has been directed in a northwest direction with an average velocity of 11km per year for the north dip pole and 10km per year for the south dip pole. In 1980, the north dip pole was located at 77.30N and 101.80W in the Canadian Archipelago, and the south dip pole at 65.60S and 139.40E just off mainland Antarctica. In addition to its secular motion, each dip pole undergoes a diurnal motion caused by ionospheric current systems. This motion, which is clockwise in the north and anticlockwise in the south, may result in a displacement from the unperturbed position of 65km and 30km respectively, on an average disturbed day. The average velocity of the north geomagnetic pole during this century is N 1km per year in a northwest direction. It is about a magnitude less for the virtual geomagnetic pole. Both the geomagnetic and virtual geomagnetic pole paths display a clockwise rotation. There is a great deal of uncertainty in our attempt to use virtual geomagnetic poles to extend the geomagnetic pole path back in time to 7500 B. C. Much of this uncertainty is due to the sparse distribution of archeomagnetic measurements at a given epoch. 1. Introduction Commander James Clark Ross was the first to take magnetic measurements specifically to determine the position of the magnetic dip poles. It is just over 150 years since he first reached the north magnetic pole on Cape Adelaide, Boothia Peninsula on June 1, 1831. To mark the anniversary, the motions of the earth's magnetic dip poles and geomagnetic poles over the past 400 years are investigated. The predominantly dipolar geomagnetic field is approximated by an axial dipole whose axis is coincident with the earth's rotational axis. In a spherical harmonic representation of the field, this simple model corresponds to the first term. This geocentric model, whose poles coincide with the geographic poles, is successfully used by paleomagne- ticians. A better approximation is that of a geocentric dipole inclined at 11.20 to the earth's axis of rotation. Its axis presently intersects the earth's surface at 78.80N and 70.90W, in northwest Greenland, and 78.80S and 109.10E in Antarctica. These points are called Contribution of the Earth Physics Branch No. 1013. 225 226 E. DAWSON and L. R. NEWITT geomagnetic poles. This model is represented by the first three terms of a spherical harmonic expression of the field. The observed main field of the earth is comprised of a nondipole portion superimposed on the dipole field. The nondipole field is represented by the higher order harmonics in a spherical harmonic representation. The two magnetic poles of the observed field, the magnetic dip poles, are located where magnetic dip or inclination is +900. In 1980 the north magnetic dip pole was located at 77.30N and 101.80W in the Canadian Archipelago, and the south magnetic dip pole at 65.60S and 139.40E, just off mainland Antarctica. Archeomagnetic and paleomagnetic studies use the concept of a virtual geomagnetic pole (VGP). Geologically speaking, it is calculated for an instant in time and is related to the inclined geocentric dipole, (IRVING, 1964). By applying paleomagnetic techniques for determining magnetic declination (D) and magnetic inclination (I) to archeological artifacts such as pottery, bricks and kilns, VGP's have been determined for historic and pre- historic times. In addition to investigating the motions of the poles, relations will be sought between geomagnetic poles and virtual geomagnetic poles over historic times. It is hoped that a study of these past motions may lead to better predictions of future movements and assist in unravelling the complexities of the magnetic field in polar regions. 2. The Field over the Two Polar Areas MCDONALD and GUNST (1968) have shown that the geomagnetic field is asymmetric with the southern hemisphere predominating. This hemispherical asymmetry appears to be increasing as the geomagnetic moment decreases. This asymmetry is readily displayed by the main field configuration in the two polar areas. In the Antarctic, the location of the total force (F) maximum, 70,000 nT, coincides almost exactly with the south magnetic dip pole. Here, as noted by NAGATA(1964), the dipole field is regionally intensified by almost 10 %. In the Arctic, the north magnetic dip pole is over 2,000km north of the total force maximum. At this dip pole, F, 58,000 nT, is about 10% less than the dipole field value. The regional field is enhanced in central Canada and Siberia where the two northern total force maxima, N 62,000 nT, are located. This nondipolar behaviour of the Arctic main field is reflected in the unique elongated pattern of the horizontal force (H) and in the restricted secular path of the north magnetic dip pole shown in Fig. 1. Although there is a general tendency for F to decrease world-wide, as represented by the north dip pole area. He interpreted this as due to a northward shift of the earth's decreasing 25 nT per year in the south dip pole area, it was increasing 50 nT per year in the north dip pole area. He interpreted this as due to a northward shift of the earth's magnetic dipole. Examination of recent secular change in F for 1980 at Resolute Bay magnetic observatory, 290 km south of the north dip pole, and at Dumont d'Urville, 120km south of the south dip pole, shows that while F is now slightly decreasing in the north, and decreasing as much as 80 nT per year at the south dip pole, the overall secular change difference between the two remains about the same. There is some evidence from plots of isoporic foci affecting the Canadian secular variation pattern (DAWSON and NEWITT, 1978a) that at least part of the change in the north is due to changes in the nondipole field. The Magnetic Poles of the Earth 227 Fig. 1. (a) The secular path of the north magnetic dip pole. (b) The secular path of the south magnetic dip pole. Numbered triangles denote observed positions listed in Table 1. Dots denote positions calculated from a least squares fit to positions listed in Table 2. Dashed lines depict periods of uncertain pole positions. 3. Motion of the Magnetic Dip Poles 3.1 Observed coordinates of the dip poles KNAPP (1969)lists 47 published positions for the Antarctic dip pole from 1642 to 1965. Many of these are re-determinations of original positions by other authors, or positions scaled from maps, or positions based on spherical harmonic models of the field. Table 1 shows a compilation of 15 positions for both dip poles based on the original observations of the observers and published by them or their immediate scientific authority. 3.2 Computed coordinates of the dip poles For the period 1550 to 1970, BARRACLOUGH (1978) lists the coefficients of 252 spherical harmonic models normalized, where necessary, to the Schmidt quasi-normalized form, expressed in nT. By adding 12 recent world magnetic models, we extended this time period to 1980. To reduce objectively the number of models to a manageable size and permit the computation of reasonably accurate pole positions, coefficients were meaned to obtain an average spherical harmonic model (ASHM) for each epoch. Only models with at least a maximum degree (NMAX) of 4 were used. In a few cases, NMAX was arbitrarily restricted to 12 on averaging. This assumes that since the field contributions of the higher terms are small, no significant change is made in a computed dip pole position by ignoring these terms. Coefficients differing from the mean value by at least 2 standard deviations were rejected and the mean re-computed. Altogether 27 spherical harmonic models were 228 E. DAWSON and L. R. NEWITT Table 1(a). Coordinates of the north magnetic dip pole. Table 1(b). Coordinates of the south magnetic dip pole. rejected. The earth shape adopted for each ASHM, oblate or spherical, was based on the shape of the majority of the models determining the average. Dip pole positions were determined iteratively from an approximate pole position To minimize computing time, a search was made, not for the location of H=0 nT but for H<10 nT in the north and H<20 nT in the south. A larger allowance was made for the south owing to the larger field gradients in the area of the south dip pole. Tests show that these assumptions introduce positional errors of less than 12km in the computed pole positions. Table 2 lists the coordinates of the dip poles computed from: ASHM's, where n is the number of models used to determine the average and NMAX is the maximum degree of the ASHM. A comparison between the observed and computed dip pole coordinates shows that there is a tendency for model values to be north and west of the observed coordinates by, in general, 10, and at least double this in the south dip pole longitude. 3.3 The secular motion of the dip poles SV causes a slow drift with time in the position of the dip poles.
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