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GEOMAGNETIC MODELS
On-line
Calculation
Proceed
to Run On-line Calculations
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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-2000 model
will be of most use, it being a reasonably accurate depiction of the field
everywhere on the Earth’s surface for the years 2000-2005. 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.
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