We present a new model of the magnetic › eld at the core{mantle boundary for the interval 1590{1990. The model, called gufm1, is based on a massive new compilation of historical observations of the magnetic › eld. The greater part of the new dataset originates from unpublished observations taken by mariners engaged in merchant and naval shipping. Considerable attention is given to both correction of data for possible mislocation (originating from poor knowledge of longitude) and to proper allocation of error in the data. We adopt a stochastic model for uncorrected positional errors that properly accounts for the nature of the noise process based on a Brownian motion model. The variability of navigational errors as a function of the duration of the voyages that we have analysed is consistent with this model. For the period before 1800, more than 83000 individual observations of magnetic declination were recorded at more than 64000 locations; more than 8000 new observations are for the 17th century alone. The time-dependent › eld model that we construct from the dataset is parametrized spatially in terms of spherical harmonics and temporally in B-splines, using a total of 36512 parameters. The model has improved the resolution of the core › eld, and represents the longest continuous model of the › eld available. However, full exploitation of the database may demand a new modelling methodology.

https://www.everyday-feng-shui.de/feng-shui-news/wp-content/uploads/2012/06/2000-Jackson-957-90.pdf

Four centuries of geomagnetic secular variation from historical records

Derivation, Meaning, and Use of Geomagnetic Indices

SUMMARY

A new model of the quiet-time, near-Earth magnetic field has been derived using a comprehensive approach, which includes not only POGO and Magsat satellite data, but also data from the Orsted and CHAMP satellites. The resulting model shows great improvement over its predecessors in terms of completeness of sources, time span and noise reduction in parameters. With its well separated fields and extended time domain of 1960 to mid-2002, the model is able to detect the known sequence of geomagnetic jerks within this frame and gives evidence for an event of interest around 1997. Because all sources are coestimated in a comprehensive approach, intriguing north–south features typically filtered out with other methods are being discovered in the lithospheric representation of the model, such as the S Atlantic spreading ridge and Andean subduction zone lineations. In addition, this lithospheric field exhibits significantly less noise than previous models as a result of improved data selection. The F-region currents, through which the satellites pass, are now treated as lying within meridional planes, as opposed to being purely radial. Results are consistent with those found previously for Magsat, but an analysis at Orsted altitude shows exciting evidence that the meridional currents associated with the equatorial electrojet likely close beneath the satellite. Besides the model, a new analysis technique has been developed to infer the portion of a model parameter state resolved by a particular data subset. This has proven very useful in diagnosing the cause of peculiar artefacts in the Magsat vector data, which seem to suggest the presence of a small misalignment bias in the vector magnetometer.

/pdf/extending-comprehensive-models-of-the-earth-s-magnetic-field-31uiccy3ib.pdf

Extending comprehensive models of the Earth's magnetic field with Ørsted and CHAMP data

Summary

The spatial ‘power’ spectrum of the main geomagnetic field has been estimated for harmonics up to n= 500. It is shown to consist of two components, long wavelengths being dominated by fields originating in the core, and short wavelengths by fields originating in the crust; the cross-over occurs at n≥ 11, a wavelength ≤ 3600 km.



The core field is often approximated by a set of spherical harmonic coefficients. It is shown that at present main field coefficients for n≤ 9, and secular variation coefficients for n≤ 6, are not known with significant accuracy. Estimates are made of the standard deviations of the IGRF coefficients, and the standard deviation of the IGRF field deduced. This field is known to about 0.5 per cent at the surface but only to about 10 per cent at the core. Its time variation is known only to about 20 per cent at the surface, and is very uncertain at the core.

/pdf/spatial-power-spectrum-of-the-main-geomagnetic-field-and-1je8cfieq4.pdf

Spatial Power Spectrum of the Main Geomagnetic Field, and Extrapolation to the Core

A spherical harmonic model of the earth's internal magnetic field of degree and order 23 is derived from selected Magsat data, and its power spectrum, computed with terms developed by Mauersberger (1956) and Lowes (1974), is found to exhibit a change of a slope at n = 14 which is interpreted as an indication that the core field dominates at values lower than 13 while the crust field dominates above a value of 15. The representations of the two portions of the spectrum obtained can be used to establish order-of-magnitude inaccuracies due to both crustal fields and the inability to observe core field wavelengths beyond n = 13, at which point they are obscured by the crustal field, in core field models.

/pdf/a-geomagnetic-field-spectrum-5ua01foab2.pdf

A geomagnetic field spectrum

Annual mean data from worldwide magnetic observatories show that there was a jerk1 (step-change in the second time derivative) in the geomagnetic field, which took place over an interval of <2 yr around 1970. If such a short-lived phenomenon originated in the Earth's core, and were still detectable after passing through the mantle to the surface, this would imply a much lower conductivity for the lower mantle than had been believed previously. Here we show by an objective test that most of the jerk has an internal origin.

Was the 1970 geomagnetic jerk of internal or external origin

The observations of magnetic declination and dip made in London over the past 400 years comprise one of the longest and most complete series of such data for anywhere in the world. They give an indication of secular change on a time scale intermediate between that of recent observatory measurements and of archaeomagnetic measurements. We have attempted to catalogue all direct measurements of declination and dip, paying particular attention to completeness in the earlier years, and tracing the data to their original source wherever possible. Sufficient details of the observations are given for an assessment of their reliability and accuracy. The data are then corrected to a single site, fitted with a continuous smooth curve, and discussed.

The Direction of the Earth's Magnetic Field at London, 1570-1975

Summary. Horizontal and vertical intensity data, obtained between 1957.0 and 1961.0 at 69 observatories, are analysed to determine the worldwide distribution of the annual variation of the geomagnetic field. Only data observed near local midnight are used, to avoid the small, but significant contamination from Sq. Over most of the world the variation is found to be small, with a clear dependence on latitude, but near the poles it is larger and more erratic. The non-polar variation is subjected to spherical harmonic analysis and separated into parts of internal and external origin. The polar variations are shown to be consistent with a north—south oscillation of the mean position of the auroral electrojets during the year. It is suggested that, with the exception of the polar effect, the annual variation is not due to ionospheric currents (as was hitherto believed), but results from an annual variation in the latitude of the ring current.

/pdf/annual-variation-of-the-geomagnetic-field-c4laiv9zkv.pdf

Annual variation of the geomagnetic field

HALLEY1 first noticed that the magnetic declination at a number of sites changed with time in a manner that was consistent with a steady westward drift of the magnetic field relative to the surface of the Earth. After long neglect, interest in westward drift was revived by the work of Bullard et al.2, who examined the westward drift of the non-dipole part of the field, which they considered to originate less deep within the core than the slower moving dipole field. Since then, there have been numerous other studies (for example refs 3–5) introducing various refinements, such as separation of the field into drifting and standing parts, variation of drift rate with latitude and separate rates of drift for each harmonic component. A common feature of all such studies is that they consider the drift of the field to be a rotation about the geographical axis. Here we remove this constraint and examine the possibility that the secular changes in the magnetic field might be more closely represented by rotation about an axis other than the geographical axis. This should not be confused with the “northward drift”6,7 which is merely the change in the latitude coordinate of the eccentric dipole position.

Rotation of the Earth's Magnetic Field

Summary. A spherical harmonic model of the second time-derivative of the geomagnetic field is determined, for the first time, directly from measures of the secular acceleration based on observatory annual mean data. The data span the interval 1964.5-1975.5, and 165 observatories are included. The model comprises the 32 coefficients of degree and order up to 6 that are significant at the 5 per cent level. Its primary purpose is to aid in the reduction of data to epoch for the 1980 series of navigational charts. The model is compared with earlier estimates of secular acceleration, derived by less direct methods.

/pdf/geomagnetic-secular-acceleration-1t23rkvr6w.pdf

S. R. C. Malin

Papers

Was the 1970 geomagnetic jerk of internal or external origin

The Direction of the Earth's Magnetic Field at London, 1570-1975

Annual variation of the geomagnetic field

Rotation of the Earth's Magnetic Field

Geomagnetic secular acceleration