E.2 Models of the internal geomagnetic field

Examples of recognised, high-degree, models of the internal field are those due to Olsen et al. [RD.138] and Maus et al. [RD.139]. These models currently cover the period (approximately) of 2000 to 2005. Another is the ‘Comprehensive Model’ of Sabaka et al (2004) [RD.140]_. This model seeks to represent all field sources under magnetically quiet conditions and models the time variation of sources by cubic splines_. The Comprehensive Model is widely used in the scientific community: it has a long history and is regarded as being amongst the most accurate and flexible for scientific purposes_. It can be expected that this model, now at revision four (CM4), continues to be occasionally updated, in the light of new scientific advances_. However the set of input parameters is relatively complex and it is not applicable under all solar and geomagnetic activity conditions_. Moreover the external field component is not as flexible as the purely internal field models such as the IGRF even though it does include a representation of the dayside Sq ionospheric magnetic field.

Models such as the International Geomagnetic Reference Field (IGRF) and World Magnetic Model (WMM) represent the internal fields of the Earth only.

See Figure E-1 and Table E-1 for more information on the current revision of the IGRF. These models are updated on a five-year basis, assuming a linear secular variation between updates. The IGRF currently extends to spherical harmonic degree 13 (equivalent to a wavelength of 3 077 km on the ground) and is produced by a collaboration of IAGA scientists. IAGA also define a definitive reference field (DGRF) for a given epoch (most recently DGRF2000), where it is agreed that no new data is likely to be forthcoming to revise and improve the existing IGRF for that epoch or earlier. Prior to 2000, the IGRF is truncated at degree 10, the increased resolution after that date being due to the improved quality of recent global satellite surveys of the main field.

The World Magnetic Model (WMM), produced jointly with the US National Geophysical Data Centre (NGDC), is the standard model in UK Ministry of Defence and US Department of Defence navigation and attitude reference systems_. It is also used widely in civilian navigation systems. The model is used on marine charts. Like the IGRF the WMM is revised according to fixed procedures every five years_. The current model, WMM2005, extends to spherical harmonic degree 13 and is valid till 2010.