The World Magnetic Model (WMM) is a large spatial-scale representation of the Earth's magnetic field. It was developed jointly by the US
National Geophysical Data Center and the
British Geological Survey. The data and updates are issued by the US
National Geospatial Intelligence Agency and the UK
Defence Geographic Centre.
The model consists of a degree and order 12
spherical harmonic
In mathematics and physical science, spherical harmonics are special functions defined on the surface of a sphere. They are often employed in solving partial differential equations in many scientific fields.
Since the spherical harmonics form ...
expansion of the
magnetic scalar potential of the geomagnetic main field generated in the Earth's core.
Apart from the 168 spherical-harmonic "Gauss" coefficients, the model also has an equal number of spherical-harmonic Secular-Variation (SV) coefficients predicting the temporal evolution of the field over the upcoming five-year epoch.
WMM is the standard geomagnetic model of the
United States Department of Defense
The United States Department of Defense (DoD, USDOD or DOD) is an executive branch department of the federal government charged with coordinating and supervising all agencies and functions of the government directly related to national sec ...
(DoD), the
Ministry of Defence (United Kingdom)
The Ministry of Defence (MOD or MoD) is the department responsible for implementing the defence policy set by His Majesty's Government, and is the headquarters of the British Armed Forces.
The MOD states that its principal objectives are t ...
, the
North Atlantic Treaty Organization (NATO), the
World Hydrographic Office (WHO) navigation and attitude/heading reference, and the
Federal Aviation Administration
The Federal Aviation Administration (FAA) is the largest transportation agency of the U.S. government and regulates all aspects of civil aviation in the country as well as over surrounding international waters. Its powers include air traffic ...
(FAA). It is also used widely in civilian navigation systems as the magnetic model of the
World Geodetic System. For example, WMM is pre-installed in Android and iOS devices to correct for the
magnetic declination. The WMM is produced by the U.S.
National Geophysical Data Center (NGDC) in collaboration with the
British Geological Survey (BGS). The model, associated software, and documentation are distributed by the NGDC on behalf of
National Geospatial-Intelligence Agency
The National Geospatial-Intelligence Agency (NGA) is a combat support agency within the United States Department of Defense whose primary mission is collecting, analyzing, and distributing geospatial intelligence (GEOINT) in support of natio ...
(NGA).
Updated model coefficients are released at 5-year intervals, with WMM2015 (released Dec 15, 2014) supposed to last until December 31, 2019. However, due to extraordinarily large and erratic movements of the
north magnetic pole, an out-of-cycle update (WMM2015v2) was released in February 2019 (delayed by a few weeks due to the
U.S. federal government shutdown) to accurately model the magnetic field above 55° north latitude until the end of 2019. The next regular update (WMM2020) occurred in December 2019.
Enhanced Magnetic Model (EMM)
The Enhanced Magnetic Model (EMM) is a sister product of the NGDC featuring a much higher amount of data to degree and order 790, giving a wavelength of 51 km as opposed to the 3000 km of WMM. At this resolution, it is not only able to model the
Earth's magnetic field
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magneti ...
at the core-mantle boundary ("main field"), but also take into account
magnetic anomalies caused by the minerals in the Earth's crust.
See also
*
International Geomagnetic Reference Field: another model, of similar quality, which models past years as well
*
Runway numbering
References
External links
WMM model and software can be downloaded from the National Geophysical Data Center's WMM page
Geomagnetism
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