Spherical harmonic analysis of the earth's magnetic field is limited in the resolution that can be obtained. This limitation is caused by inadequacies of computers and of available data sets. The fundamental wavelength in spherical harmonic analysis is the circumference of the earth. To resolve wavelengths as short as 100 km would require a spherical harmonic analysis carried out to a degree and order 400 involving 160,800 coefficients. This is impractical even with modern computers. This limitation of spherical harmonic analysis can be overcome by using rectangular harmonic analysis in successively smaller areas so that the data are more fully utilized. Rectangular harmonic analysis is illustrated for data from Europe and then again for a subset of the data from a smaller area of Europe. The magnetic field at 15 observatories for the smaller area can be computed to an rms residual of only 7 nT for all three components using two sets of rectangular harmonic coefficients and the AWC/75 world chart model. Rectangular harmonic analysis and spherical harmonic analysis are complementary.

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.

French and United Kingdom workers have published reports describing a sudden change in the secular acceleration, called an impulse or a jerk, which took place in 1969. They claim that this change took place in a period of a year or two and that the sources for the alleged jerk are internal. This paper questions their method of analysis, pointing out that their method of piecemeal fitting of parabolas to the data will always create a discontinuity in the secular acceleration where the parabolas join and that the place where the parabolas join is an a priori assumption and not a result of the analysis. It is also shown that the jerk defined as an approximation to the third time derivative is mostly of external origin. A short review of the uses of the terms impulses and jerks is given in the introduction.

Fourier analyses were made of the geomagnetic horizontal annual means from 20 observatories and the vertical component annual means from 17 observatories from 1900 to 1972 for most of the observatories. Curves synthesized from the Fourier coefficients for periods from approximately 4 to 13 yr have patterns easily explained in terms of a westward-flowing ring current that generally increases as the sunspot numbers increase but lags behind the sunspot numbers by a year or two. External spherical harmonic coefficients should be included in models of the earth's magnetic field. By using the horizontal component curves, synthesized as described above, for seven selected observatories and the vertical component curves for six selected observatories, values of both internal and external first-order zonal spherical harmonic coefficients were determined for a 22-yr span. The amplitude and phase of the ratio of these coefficients are consistent with an earth having a conductivity of 33 mhos/m from the center of the earth to a radius of 0.9 RE surrounded by a nonconducting shell to the surface. [1952] indicated that geomagnetic secular change impulses were evident in most observatory records. An impulse was defined as the apparent sudden change in the slope of a curve of annual means of a geomagnetic component versus time. The impulses for all observatories tended to occur simultaneously (impulse epochs). They found the impulse concept useful in the development of isoporic charts. Weber and Roberts [1951] and Walker and O'DeaAlldredge [1975] has shown that the impulse concept can be adequately explained in terms of solar activity and the resulting buildup and decay of the ring current and related effects.In this paper, magnetic data are processed in a direct way to show effects which are clearly related to the solar cycle. Because only the long-term average effects of the solar cycle activity on the geomagnetic field are being investigated, the annual means of the magnetic components for all days are used. [1974] have made similar studies, using different analysis techniques. Although the results of this paper are in general agreement with theirs, there are, as we will see, important differences. Table 1. When two observatories are shown in the first column, the period of operation of each is indicated on the next line. In each case these were nearby stations (only a few tens of kilometers apart). In most cases, in the analysis for these pairs of stations the base line of the first was shifted to match that of the second, and then the data for both stations were treated as though they were one station. The two observatories are close enough that the geomagnetic latitude given for the second observatory can adequately represent the entire data set. This study was limited to H and Z because they should reveal the effects of external sources more directly than would other components, such as Yukutake [1965], Bhargava and Yacob [1969], and Rivin D^T^ PROCESSING TO SHOW SOLAR EFFECTS This study utilizes the annual means of t...

Total‐intensity aeromagnetic surveys of the Aleutian Marshall, and Bermuda Islands were completed in 1948. The anomalies associated with the Aleutian volcanoes are attributed mainly to topographic relief and are not an indication of the degree of volcanic activity. Eniwetok presents a magnetic pattern that would be produced by an irregular‐shaped rimmed depression in the basement, modified by the two adjoining seamounts, and differs from Bikini, whose magnetic features would be produced by a broad seamount with irregular surface relief. The Bermuda survey demonstrated magnetic features typical of volcanic rocks. Comparison of an observed and a theoretical profile computed by Press and Ewing indicates that their assumptions are reasonably correct. The Aleutian Trench survey shows anomalies that are attributed to susceptibility contrasts but none that can be correlated with the trench. A traverse from Adak, Aleutian Islands, to Kwajalein, Marshall Islands, exhibited several large anomalies that are presumed to be caused by susceptibility contrasts but may be indications of uncharted seamounts. Two traverses, one from Cape May, N.J., to Bermuda and the other from Bermuda to Long Island, N.Y., reveal a change in the magnetic field approximately 300 miles from the Atlantic Coast that indicates a possible thinning of the sial and an exposure of sima.