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World Magnetic Model (WMM)

The World Magnetic Model (WMM) is a standard model of the core and large-scale crustal magnetic field. It is used extensively for navigation and in attitude and heading referencing systems by the UK Ministry of Defence, the US Department of Defense, the North Atlantic Treaty Organization and the International Hydrographic Organization. It is also used widely in civilian navigation and heading systems.

WMM2020 online calculator

Irregular changes in the Earth’s core field limit the lifetime of any predictive model such as the WMM. The current version (WMM2020) was released in December 2019, and is valid for the period 2020.0 to 2025.0. The model was produced by BGS jointly with the US National Oceanic and Atmospheric Administration's National Centers for Environmental Information (NOAA/NCEI), with funding from the Defence Geographic Centre in the UK and the US National Geospatial-Intelligence Agency.

 

WMM2020 consists of a degree 12 spherical harmonic model of the Earth’s main field (MF) at 2020.0, and a mean rate of change estimate (called secular variation, SV) over the period 2020.0 to 2025.0. Both parts of this model were produced collaboratively by combining models of the magnetic field derived BGS and NOAA/NCEI, as described in the technical report.

 

In order to construct the BGS contributions to WMM2020 we used data with as much coverage in space and time as possible. We took data from the three European Space Agency Swarm magnetic survey satellites (launched in November 2013) and from around 160 land-based observatories, up to October 2019. With these measurements, we constructed a complex model of the various sources of Earth's magnetic field and its time variations.

 

The WMM is a model of the core and large-scale crustal fields only. However the measured data contain unwanted signals from sources such as small-scale crustal, external ionospheric and magnetospheric and their induced counterparts. These fields would have added noise to the WMM model and could have biased its estimates.

 

BGS employed two techniques to avoid the contamination caused by external magnetic fields. Firstly we rejected those data most contaminated by these sources, as identified by a combination of local time, geomagnetic indices and solar wind data. However, this has an unwanted side effect in leaving temporal gaps in the data. This would have had a strong effect on the quality of the SV estimates. We lessened this effect by including similarly selected data from the magnetic ground observatories.

 

Our second technique was to initially model the larger unwanted sources that we could not easily reject. We therefore constructed what we called a ‘parent model’ for the WMM, including these extra sources, before removing the unwanted components to finalise the WMM. Our colleagues at NOAA/NCEI follow their own procedure of calculations to produce their own ‘parent model’, and we then validate each others' models, and combine the results to produce the final WMM2020.

 

Map of the world showing annual rate of change of declination for 2020.0 to 2025.0 from the World Magnetic Model (WMM2020).