World Magnetic Model
The World Magnetic Model (WMM) is a large spatial-scale representation of the Earth's magnetic field. It consists of a degree and order 12 spherical harmonic expansion of the magnetic 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 (DoD), the Ministry of Defence (United Kingdom), the North Atlantic Treaty Organization (NATO), and the World Hydrographic Office (WHO) navigation and attitude/heading reference. It is also used widely in civilian navigation systems. 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 National Geophysical Data Center (NGDC) on behalf of National Geospatial-Intelligence Agency (NGA). Updated model coefficients are released at 5-year intervals, with the current model (WMM2015) expiring on December 31, 2019. The current World Magnetic Model (WMM2015) has been released on NGDC website on Dec 15, 2014.
Because of extraordinarily large and erratic movements of the north magnetic pole, an out-of-cycle update to the WMM was due to be released on January 15, 2019. Due to the ongoing US federal government shutdown, this update has been delayed to January 30.
- International Geomagnetic Reference Field: another model, of similar quality, which models past years as well
- Witze, Alexandra (January 9, 2019). "Earth's magnetic field is acting up and geologists don't know why". Nature. 565: 143–144. doi:10.1038/d41586-019-00007-1.
- WMM model and software can be downloaded from the National Geophysical Data Center's WMM page
- World Magnetic Model method in the Android operating system
- World Magnetic Model 2015 NOAA press release
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