Models
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The magnetic-field models used in the on-line calculator are succinct mathematical descriptions of the Earth’s surficial magnetic field. They are constructed by fitting a set of basis functions, usually spherical harmonics or spherical caps, to magnetic data, such as those collected at USGS observatories, and, as such, models are useful interpolators for estimating the field between measurement locations and between measurement times. Magnetic models are often used for investigative research, and they are routinely used for navigation and survey orientation. The magnetic-field direction that is sometimes plotted on geological maps is based on a magnetic-field model. The models are expressed in two ways, in the form of charts, showing the various magnetic-field components at the surface, and in form of calculator which gives the field components at a chosen time and geographical location. Readers having curiosities about geomagnetism, including its vectorial nature at the Earth's surface, are referred to our Introduction to Geomagnetism and Further Reading pages.
Model Types
For most users interested in the present geomagnetic field the IGRF-2005 model will be of most use, it being a reasonably accurate depiction of the field everywhere on the Earth’s surface for the years 2005-2010. IGRF (International Geomagnetic Reference Field) models are ‘pre-dictive’, in that they are based on data collected in the recent past, preceding their construction, and are intended to represent the field in the near future, following their construction. On the other hand, the DGRF (Definitive Geomagnetic Reference Field) models are ‘post-dictive’, in that they are based on data collected in the past, over a certain duration of time, and are intended to represent the field in the past, over the same duration of time. Obviously, since it takes time to produce a field model, any model intended to represent the present must be ‘pre-dictive’. The other models presented here, the WMM and the BGS, are more specialized. The World Magnetic Models (WMM) were constructed in collaboration with the British Geological Survey (BGS) under contract for the British Hydrographic Office, the US National Imagery and Mapping Agency, each of whom have specialized needs and who require delivery of the WMM field model on strict schedule, demands that are not normally met by the many international agencies involved in the construction of IGRF and DGRF models. The BGS historical models are intended to represent the field in the more distant past.
References
- Barraclough, D. R., 1974. Spherical Harmonic Analysis of the Geomagnetic Field for Eight Epochs between 1600 and 1910, Geophys. J. R. Astr. Soc., 36, 497-513.
- IAGA Division V, Working Group 8, [Including Program member Quinn, J. M.], 2000. International Geomagnetic Reference Field, 2000, Phys. Earth. Planet. Inter., 120, 39-42, Pure Appl. Geophys., 157 , 1797-1802, Geophys. J. Int., 141, 259-262.
- Macmillan, S. & Quinn, J. M., 2000. The 2000 revision of the Joint UK/US Geomagnetic Field Models and the IGRF candidate model , Earth Planets Space, 52, 1149-1162.
- Macmillan, S. & Quinn, J. M., 2000. The derivation of the World Magnetic Model 2000 , Brit. Geol. Surv. Tech. Rept., WM/00/17R.
Limitations
The user of magnetic-field models and charts must be aware of their limitations. Since the magnetic field is extremely complicated, in both space and time, magnetic-field models are, by practical necessity, something of an approximation of the actual magnetic field. For example, global models of the field, such as the IGRF, do not account for very local magnetization. Indeed, there is no way that they could, since many geological formations, and for that matter, many rocks, are magnetized, if only partially. Moreover, the models do not fully account for magnetic-field ingredients generated by ionospheric and magnetospheric electric currents, since these can create essentially unpredictable, localized and transient perturbations to the main field, particularly at high latitudes. Despite these qualifiers, because magnetic-field models and charts are of such enormous practical utility, they are provided here as a service to the public.
