The World Magnetic Model - Background
The WMM consists of a degree and order 12 spherical-harmonic main (i.e., core-generated) field
model comprised of 168 spherical-harmonic Gauss coefficients and degree and order 12 spherical-harmonic
Secular-Variation (SV) (core-generated, slow temporal variation) field model.
WMM2015 supersedes WMM2010 and should replace this model in navigation systems. Also included with the model is
computer software (available in FORTRAN or C) for computing the magnetic field components X, Y, Z, F, D, I, and H
in geodetic coordinates. The spherical-harmonic expansions used to compute the magnetic field components are
described in the NOAA Technical Report: The US/UK World Magnetic Model for 2015-2020 (pdf format).
The Earth's magnetic field, as measured by a magnetic sensor on
or above the Earth's surface, is actually the sum of
magnetic fields generated by a variety of sources. These fields
are superimposed on each other and through inductive processes
interact with each other. The most important of these
geomagnetic fields are:
- The Earth's main magnetic field generated in the conducting, fluid outer core.
- The crustal field generated in Earth's crust and upper mantle.
- The combined disturbance field from electrical currents flowing in the upper atmosphere and magnetosphere, which induce
electrical currents in the sea and ground.
Simulation of geodynamo within the Earth's liquid core by Gary Glatzmaier (UC santa Cruz).
Artist's view of the Swarm constellation (image credit ESA).
The observed magnetic field is a sum of contributions of the main field (varying in both time and space),
the crustal field (varies spatially, but considered constant in time for the time-scales of the WMM), and the disturbance fields
(varying in space and rapidly in time). Earth's main magnetic field dominates, accounting for over 95% of the field
strength at the Earth’s surface. Secular variation is the slow change in time of the main magnetic
field. The WMM represents only the main geomagnetic field.
To create an accurate magnetic field model, it is necessary to have vector component measurements with good global coverage and low noise levels. The European Space Agencie's SWARM satelite's mission is presently the most suitable magnetic observing system. Measurements of the total intensity
of the magnetic field are further available from the Danish Oersted satellite. This latter satellite stopped providing vector component measurements around 2005. Also available are ground observatory hourly mean data, although with poorer spatial coverage. The observatory data
can provide valuable constraints on the time variations of the geomagnetic field.
The World Magnetic Model is a joint product of the United States' National Geospatial-Intelligence Agency (NGA) and the United Kingdom's Defence Geographic Centre (DGC). The WMM was developed jointly by the National Geophysical Data Center (NGDC) and the British Geological Survey (BGS). The World Magnetic Model is the standard model used by the U.S. Department of Defense, the U.K. Ministry of Defence, the North Atlantic Treaty Organization (NATO) and the International Hydrographic Organization (IHO), for navigation, attitude and heading referencing systems using the geomagnetic field. It is also used widely in civilian navigation and heading systems.
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