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. WMM2020 supersedes WMM2015 and WMM2015v2 and should replace these models in navigation systems. Also included with the model is computer software (available in C) for computing the magnetic field components X, Y, Z, F, D, I, and H in geodetic coordinates and their temporal variations. The spherical-harmonic expansions used to compute the magnetic field components are the same as the ones described in The US/UK World Magnetic Model for 2015-2020: Technical Report (to be updated with a new report on the WMM2020 in February 2020).

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:

1. The Earth's main magnetic field generated in the conducting, fluid outer core.
2. The crustal field generated in Earth's crust and upper mantle.
3. 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. 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 Centers for Environmental Information (NCEI) 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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