Radial dipoles as the sources of the Earth's main magnetic field

Abstract

It is assumed that magnetic dipoles are useful as a first approximation to the electrical currents in the core that produce the earth's main magnetic field. For simplicity the model is restricted to a central dipole and several additional radial dipoles at equal distances from the center of the earth. A least‐squares method is used to adjust the amplitude, latitude, and longitude of each dipole for a best fit to the observed field components on the earth's surface. In the first of four studies the observed field was the field of the United States 1945 world charts. Originally 11 dipoles, 10 of them at the core‐mantle interface at 0.54 earth radii, were used. Progressively better fits were obtained as the dipoles were placed deeper, and two of the dipoles were eliminated at greater depths. The 29‐parameter, 9‐dipole model, with the radial dipoles at 0.28 earth radii, produced nearly as good a fit to the 1945 field as Vestine's 48 spherical harmonic coefficients. Models were also fitted to the United States 1955 world chart field, to the British Admiralty 1955 world chart field, and to the field synthesized from the Finch‐Leaton spherical harmonic coefficients for 1955. The last model produced the best fit. In all cases the radial dipoles are surprisingly deep and the central dipole is considerably stronger than the centered dipole given by the first three spherical harmonic coefficients. The great depth of the radial dipoles is qualitatively explained by a shielding effect from currents in the mantle and core. The spherical harmonic coefficients from the analyses of Vestine and of Finch and Leaton are compared with the spherical harmonic coefficients computed from the dipole parameters.

Interpreting regional geomagnetic anomalies and their secular variation with the aid of radial dipoles and current loops.

“…It can be noted here that the value of 0.25 for qo obtained by simple methods here is roughly the same as was found e.g, by ALLDREDGE and HURWITZ (1964) from a model of nine radial dipoles.…”

“…This version of the TWOD is comparable to the dipole models of HURWITZ (1964) andSTEARNS (1969): it contains a model (parameters P;) for the observed anomaly field and a model for the secular variation field (parameters Ps). The P/s obtained (see Table 6) are roughly the same as in Table 4, thus confirming conclusions made in Chapter 3.1.…”

“…MCNISH (1940), LOWES and RUNCORN (1951), LOWES (1955), ALLDREDGE and HURWITZ (1964), ALLDREDGE and STEARNS (1969). Model in which the dipoles are not necessarily radial have also been studied by ZIDAROV (1969) and ZIDAROV and BOCHEV (1969).…”

Circular Current Loops, Magnetic Dipoles and Spherical Harmonic Analyses

“…Some moderately successful efforts have been made to represent the field in terms of dipoles placed in the core (MCNISH, 1940;ALLDREDGE and HURWITZ, 1964;ALLDREDGE and STEARNS, 1969;ZIDAROV and BOCHEV, 1969) in an effort to have the model come closer to representing real sources. Dipole sources are only a first approximation to the real sources which are thought to be a very complicated network of electrical currents in the core of the Earth.…”

Areal coverage of spurious reversals of the earth's magnetic field.

“…However, LOWES (1955) stated that the directions of the dipoles necessary to produce the non-dipole field were not so certain. ALLDREDGE and HURWITZ (1964) and Cox (1968) both used vertical dipoles to model the non-dipole field.…”