David R. Barraclough

International geomagnetic reference field: the fourth generation

Abstract In August 1985 the International Association of Geomagnetism and Aeronomy revised the International Geomagnetism Reference Field (IGRF). This is the third revision since the first IGRF was produced in 1968. The revised IGRF now consists of 10 spherical harmonic models of the main geomagnetic field and its secular variation and covers the interval 1945–1990. For the interval 1965–1980 the constituent models are definitive (DGRFs), in the sense that it is not intended to revise them in the future. A brief description of the derivation of the revised IGRF is given, together with a brief review of basic formulae and a set of world contour maps of the geomagnetic elements for 1985.

Spatial and temporal spectra of the geomagnetic field and their scaling properties

Abstract Many natural phenomena show a relationship between their spatial and temporal Fourier spectra. This paper discusses such a connection for the geomagnetic field, when some assumptions are made about the (exponential or power-law) behaviour of the spatial power spectrum of the field itself and that of its time derivative (the spatial spectrum of the secular variation) as estimated from global geomagnetic field models. It is shown that, under either assumption, the temporal spectrum of the geomagnetic field computed at the core–mantle boundary (CMB) would have a power-law behaviour with a negative spectral exponent of about 0.5. At the Earth’s surface, although the temporal spectrum obtained from the power-law spatial model assumes a slightly more complicated form, it can be practically approximated with a power law with a negative exponent of about 3.6. Analysis of magnetic observatory data confirms these results and that the starting hypotheses are reasonable, especially in view of the possibly chaotic state of the dynamical processes underlying the generation and maintenance of the geomagnetic field.

Evidence for geomagnetic jerks from 1931 to 1971

Abstract Examination of annual mean values of the geomagnetic elements from magnetic observatories worldwide has shown that a sudden change in secular acceleration, which has been termed a jerk, took place at about 1970. Malin et al. used the set of secular variation models compiled by Hodder to provide a global description of the jerk, and attempted to quantify it by computing the mean square value of the jerk field over the Earths surface. To determine the ability of the method of Malin et al. to distinguish the 1970 jerk from noise their analysis has been repeated at 2-y intervals from 1931.5 to 1971.5, a period during which no other similar jerk is thought to have occurred. It is shown that the mean square value of the jerk field is not a reliable indicator of the existence of a jerk when taken alone as noise contributes to it additively giving rise to “apparent” jerks. This point is illustrated by examples of contour maps of the jerk field for 1949.5 which show how large mean square values relate to spurious foci in regions where there are no magnetic field observations. These maps contrast with those for the 1970 jerk which relate well to the observations and it is shown that the 1970 jerk is well-resolved from noise and is unique within the time-span considered.

Some possible evidence for a chaotic geomagnetic field from observational data

Abstract The behaviour of the geomagnetic field as observed almost continuously at three European locations over the last 130 years is investigated by means of a non-linear forecasting approach. The analysis of the data in terms of first-differences (secular variation) of the horizontal magnetic components made in phase space with the simplex technique seems to exclude the pre-eminence of any stochastic or periodic behaviour. The dimensionality of the underlying non-linear process and the corresponding largest positive Lyapunov exponent are estimated. The results give some evidence that the geomagnetic field evolves as a non-linear chaotic system with unpredictable behaviour after times greater than a few years, confirming the common practice of updating global models of the geomagnetic field every 5 years.

A Spherical Cap Harmonic Model of the Crustal Magnetic Anomaly Field in Europe Observed by Magsat

The geomagnetic field observed in current-free regions above the Earth’s surface may be expressed as the gradient of a scalar potential satisfying Laplace’s equation. Spherical cap harmonic analysis enables solution of Laplace’s equation, subject to boundary conditions appropriate to geomagnetic field analysis, in a region bounded by a spherical cap. Magsat data within a spherical cap of half-angle 35° centred on latitude 45°N, longitude 10°E have been analysed for their crustal content. The resulting estimates of the crustal vector field have been used to derive a spherical cap harmonic model of the crustal scalar potential. The model contains 256 parameters and portrays wavelengths of 1000 km and above. Vector anomaly maps derived from the model show several prominent features of which the largest is that in the Kursk region of the USSR. The model has been used to correct both the vector and total intensity data on to a 2° by 2° grid at an altitude of 400 km. Anomaly maps produced by contouring the grid averages are in good agreement with those derived from the model. The major difference is for the vertical component of the anomaly field over the Kursk region of the USSR. This is a high-amplitude short-wavelength feature which the model smooths.

Nonlinear Variability in the Geomagnetic Secular Variation of the Last 150 Years

A nonlinear forecasting analysis has been applied to the secular variation of the three-component annual means of 14 observatories, unevenly distributed over the Earths surface (12 in the northern and 2 in the southern hemisphere) and spanning the last 150 years. All results were in agreement, either in terms of possible evidence of chaos (as opposed to the hypothesis of white or colored noise), or in terms of the Kolmogorov entropy, confirming previous results obtained with only three European observatories, i.e. it is practically impossible to predict the secular variation of the geomagnetic field more than six years into the future.

BGS candidate models for the 1985 revision of the international geomagnetic reference field

Abstract The British Geological Survey (BGS) proposals for the revised International Geomagnetic Reference Field (IGRF) consisted of two main-field models (with maximum degree and order n ∗ = 10 ), for epochs 1950.0 and 1960.0, based on all available near-surface survey data collected between 1940 and 1965, and a predictive model (n ∗ = 8) of the secular variation for the interval 1985 to 1990 (mean epoch 1987.5), based on the most recent observatory annual means. This note briefly describes the data sets on which the models are based and the techniques used to derive them.

IGRF candidates for 1980 and 1985

Abstract A tenth degree and order spherical harmonic model of the main geomagnetic field for 1980 has been derived using MAGSAT data supplemented by aeromagnetic vector data collected by the Project MAGNET aircraft. A secular variation model of degree and order 8 for 1982.5 based on magnetic observatory and repeat station measurements has also been developed. The secular variation model is used to project the 1980 main-field model to 1985. The 1980 and 1985 main-field models are proposed for incorporation into the revised International Geomagnetic Reference Field.

Repeat Station Activities

A repeat station is a site whose position is accurately known and where accurate measurements of the geomagnetic field vector are made at regular intervals in order to provide information about the secular variation of the geomagnetic field. In this chapter we begin by giving a brief history of the development of repeat station networks. We then describe the instruments used to make measurements at a repeat station. These include fixing the position of the station, finding the direction of true north and measuring the components of the geomagnetic field. Emphasis is given to techniques and instruments that are in current use. We next discuss the procedures that are used to reduce the measurements to a usable form and consider the uses to which the reduced data are put. Finally, we discuss the continued importance of such data in the present era of satellite geomagnetic surveys.

Gilbert's De Magnete: An early study of magnetism and electricity

Four hundred years might seem an excessively long time to wait for a book review, but there are reasons why a prompt review would have been difficult. Not only is De Magnete written in Latin, but it is also in the form of a scientific textbook. The language problem was overcome by Paul Fleury Mottelay and Silvanus P. Thompson, who translated the book into English in 1893 and 1900, respectively. The latter was published in connection with a beautiful tercentenary limited edition by the Chiswick Press that retains much of the flavor and appearance of the original. But the notion of a scientific textbook was unfamiliar when De Magnete first appeared in 1600. Certainly, scientific subjects and even geomagnetism had been written about (for example, by Petrus Peregrinus in his Epistola De Magnete, 1269), but as reports of equipment and phenomena rather than as in-depth investigations of a subject with experimental evidence and interpretation.