TERRAPUB Earth, Planets and Space

Earth Planets Space, Vol. 62 (No. 10), pp. 815-820, 2010

On the possibility of extending the IGRF predictive secular variation model to a higher SH degree

Luís Silva1, Stefan Maus2, Gauthier Hulot1, and Erwan Thébault1

1Equipe de Géomagnétisme, Institut de Physique du Globe de Paris, Université Paris Diderot, INSU/CNRS, 4, Place Jussieu, 75252, Paris, cedex 05, France
2National Geophysical Data Center, NOAA E/GC1, 325 Broadway, Boulder, CO 80305-3328, USA

(Received January 22, 2010; Revised June 14, 2010; Accepted July 1, 2010; Online published December 31, 2010)

Abstract: The International Geomagnetic Reference Field (IGRF) is an internationally agreed global spherical harmonic model of the Earth's magnetic field of internal origin. It is currently computed every five years in the form of a model describing this field up to degree 13 at a reference epoch, plus a secular variation model up to degree 8, best estimating the linear evolution of this field over the following five years. Such a simple description of the field evolution is thought to provide a good enough prediction of the field, both for navigational and internationally agreed reference purposes (the very purpose of IGRF models). In particular, it assumes that any change in the field described by spherical harmonic degrees between 9 and 13 may be neglected over five years, given the uncertainties already involved in the determination of all other coefficients, and the practical accuracy needed for most IGRF applications. Recent progress in global field modelling based on increasingly accurate and numerous satellite data however show that all field coefficients can now be computed with much higher accuracy than possible in the past, and that higher degree secular variation coefficients could therefore also be considered for inclusion in IGRF models. The present short note intends to investigate the potential benefit of extending the IGRF predictive secular variation model to degrees higher than 8, given our current knowledge of the way the field behaves over time periods of five years.
Key words: Geomagnetism, field modelling, reference field, secular variation.

Corresponding author E-mail: lacsilva@ipgp.fr

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