Association Internationale de Géomagnétisme et d’Aéronomie

Although the science of geomagnetism is old, many fundamental questions still remain unanswered concerning fluid motions in the earth's core, the dynamo producing and sustaining the field, interpretation of results of surveys, drawing of charts, a reference field, the secular change, and magnetic anomalies and their geologic meaning. The World Magnetic Survey (WMS) was designed to overcome the principal deficiencies in surveys and to stimulate the use of the data in studies of the fundamental problems in geomagnetism.

Abstract The International Geomagnetic Reference Field (IGRF) is a series of mathematical models of the main geomagnetic field and its secular variation, the models consisting of sets of spherical harmonic (or Gauss) coefficients. The IGRF has become a widely used means of deriving values of geomagnetic field components in, for example, studies of magnetic anomalies and investigations of charged particle motions in the ionosphere and the magnetosphere.

The International Geomagnetic Reference Field (IGRF) recently underwent a major revision.In August 1981, at its Fourth Scientific Assembly at Edinburgh, the International Association of Geomagnetism and Aeronomy adopted a resolution revising the IGRF.The revision has three components: (1) it extends the present series of IGRF models up to 1985, (2) it introduces a new series of definitive models for 1965-1975, and (3) it defines a provisional reference field for 1975-1980.The IGRF is now considerably more complex than it was, but also more useful.This issue is devoted to papers related to the new revision of the IGRF.The first paper gives a history of the IGRF, describes its latest revision, and shows how the revision was determined.The next three papers are reports of the derivations of the three sets of models that were proposed as revisions.The next six papers are reports of assessments that were made of the proposed models.The last paper is a report of a study of some of the spectral characteristics of the new IGRF models.The successful continuation of the IGRF depends on the continual acquisition of measurements of the Earth's magnetic field.Without the fresh set of data provided by the recent MAGSAT satellite survey, the new IGRF model for 1980 would undoubtedly have been distinctly inferior ; and without the continual flow of data from the worldwide magnetic observatories, the tasks of reducing data to epoch, analyzing secular variation, and developing forecast models, would be practically impossible.Data-gathering programs such as these must not only be continued, but must be expanded if the science of geomagnetism, including the development of the IGRF, is to be advanced.

The International Geomagnetic Reference Field (IGRF) underwent its fifth revision at the 20th General Assembly if the International Union of Geodesy and Geophysics (IUGG) in Vienna during August of 1991.With this revision the IGRF consists of nine definitive models (DGRFs) spanning the interval 1945.0 to 1985; an IGRF for the interval 1990.0 to 1995.0 (IGRF 1990); and a provisional IGRF (PGRF) defined by the linear interpolation between the coefficients of DGRF 1985 and IGRF 1990.This issue contains papers describing the new revision to the IGRF.The first paper describes the new revision, discusses the history of the IGRF, and reviews the nature, accuracy and proper use of the IGRF.The second through fifth papers describe the candidate models from which the revision was constructed and the remaining nine papers contain assessments of the candidate models verses various regional and global data sets.It is important to note that the quality and, therefore, usefulness, of the IGRF is only as good as the data used in its derivation.One measure of this is that the degree and order of the models is maintained at 10.Even when satellite data are available, modelers are dependent upon observatories, repeat stations and surveys on land, sea and air.No modeler has time to scour the world to be sure he has all the key data.We rely on the transmittal of all available data to the World Data Centers, and we heartily thank the administrators of and workers at those observatories and other institutions who have made their data available.At the same time it is useful to point out that it does the community no good if data is withheld from the data center, for whatever reason.