Definitive internal geomagnetic field models are derived for the epochs 1945, 1950, 1955, and 1960. Each model incorporates all data available to us within a five year period centered on the model epoch. For survey data, weighting was determined by examining the spread of residuals for the data, sorted by source and sorted by location, relative to the previous IGRF models. The solution included local biases for the fixed observatories. An a priori model for each epoch was derived by projecting the GSFC (12/83) model, based on Magsat data, backward in time. This projection was accomplished using a spline fit to annual secular-variation models. The spline coefficients were simultaneously fit to all spherical harmonic secular variation coefficients for the 1940 to 1980 time period. This fit included a full covariance analysis. The projected covariances were part of the a priori model for each epoch. An uncertainty model was adopted which included estimates of the effects of crustal and core fields not represented by the model. Differences between model coefficients midway in time between model epochs were compared to estimated coefficient uncertainties. Coefficient differences were within the estimated uncertainties, confirming the uncertainty model. A test for 1945 indicated that a solution without observatory biases was equal to that with such biases, within the expected uncertainties. Differences between biases from year to year are within the bounds expected based on the predicted uncertainties. The resulting models, their secular variation and their expected uncertainties are discussed in some detail.
Paleomagnetic analysis was made on 36 lava flows and dykes of different ages distributed in Hiva-Oa Island, the Marquesas Islands (9.7°S and 139.0°W) in an attempt to study the geomagnetic paleosecular variation. K-Ar ages of samples range from 2.74m.y. to 1.63m.y. The present rotation axis is contained within the 95% confidence limit of the mean VGP (88.1°N and 117.9°W). The angular dispersion of 32 poles is 13.8° with respect to the rotation axis, and its upper and lower confidence limits are 16.6° and 11.8°, respectively. These values are consistent with the values of the global trend and the values predicted by secular variation models. Low secular variation as reported in the Hawaiian Islands is not observed in the Marquesas Islands.
The average magnetization vector of Erimo Seamount is obtained by a linear least squares inversion using the topographic and magnetic anomaly data acquired during the KAIKO project. As the dipole anomaly of the seamount is superimposed on the magnetic lineations of the oceanic crust, the contribution of the lineations was estimated using a two-dimensional block model, and was subtracted from the observed data before the inversion. The VGP (Virtual Geomagnetic Pole) position, 69°N, 321°E, is here proposed as the revision of the widespread value of UYEDA and RICHARDS (1966). Combined with the recently proposed ages of the seamount, 100 to 120Ma, the VGP position provides an important clue in establishing the early Cretaceous pole position of the Pacific Plate. The inversion assuming a planar regional field, which is the conventional way of removing the anomalies caused by bodies other than the seamount, was performed for comparison. The results differ considerably from those by the former procedure. The derived structure of the seamount (a thick non-magnetic cap) and the VGP position did not agree with the submersible observation and the age of the seamount, respectively. A planar regional field is not an adequate approximation of magnetic lineations.
The effect of the rate of cooling on the magnetization of assemblies of single-domain grains is solved numerically, and analytically for the specific case of magnetite, with results that can easily be extended to other magnetic compounds. It is shown that to a good approximation the moment produced on cooling from above the transition temperature is proportional to (lnc/q)1/2 where q is the cooling rate constant, and c is a frequency on the order of 108Hz. This result is independent of the grain size and shape distribution.