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The World Magnetic Model - Accuracy, limitations, magnetic poles and error model

Accuracy of the WMM2020

Changes of the fluid flow in the Earth's outer core lead to unpredictable changes in the Earth's magnetic field. Fortunately, the system has large inertia, so that these changes take place over time scales of many years. By surveying the field for a few years, one can precisely map the present field and its rate of change and then linearly extrapolate it out into the future. Provided that suitable satellite magnetic observations are available, the prediction of the WMM is highly accurate on its release date and then subsequently deteriorates towards the end of the 5 year epoch, when it has to be updated with revised values of the model coefficients.

Limitations of the WMM2020

It is important to recognize that the WMM and the charts produced from this model characterize only the long-wavelength portion of the Earth's internal magnetic field, which is primarily generated in the Earth's fluid outer core. The portions of the geomagnetic field generated by the Earth's crust and upper mantle, and by the ionosphere and magnetosphere, are largely unrepresented in the WMM. Consequently, a magnetic sensor such as a compass or magnetometer may observe spatial and temporal magnetic anomalies when referenced to the WMM. In particular, certain local, regional, and temporal magnetic declination anomalies can exceed 10 degrees. Anomalies of this magnitude are not common but they do exist. Declination anomalies of the order of 3 or 4 degrees are not uncommon but are usually of small spatial extent. See EMM home page for a model which includes crustal fields down to 50 km wavelength.

Blackout Zone (BoZ) for the World Magnetic Model

Based on the WMM military specification, we define “Blackout Zones” (BoZ) around the north and south magnetic poles where compass accuracy is highly degraded. The BoZ are defined as regions around the north and south magnetic poles where the horizontal intensity of Earth’s magnetic field (H) is less than 2000 nT. In BoZs, WMM declination values are not accurate and compasses are unreliable.

We additionally define a “Caution Zone” (2000 nT <= H < 6000 nT) around the BoZ, where caution must be exercised while using a compass. Compass accuracy may be degraded in this region. The blackout (unreliable) and caution zones are displayed in figures 2 and 3. The WMM online/offline software display warnings when the user location is within these zones.

map of declination and blackout zone in the Arctic 2020
Fig. 1. Map of the blackout zone in the Arctic over declination contours. Click for high-resolution image.
map of declination and blackout zone in the Antarctic 2020
Fig. 2. Map of the blackout zone in the Antarctic over declination contours. Click for high-resolution image.

Error model for the WMM2020

The WMM2020 software and online calculator include an error model providing uncertainty estimates for every geomagnetic element (X, Y, Z, H, F, I and D) and every location at the Earth's surface. This model is built upon the results of the error analysis (see the WMM2020 technical report, to be published in February 2020), while taking into account the geometrical relationships between the various components. It includes both the commission error (due to inaccuracies in model coefficients) and the omission error (due to missing contributions to the total geomagnetic field such as crustal and external fields). The error values (Table 1) may be interpreted as one standard deviation difference between a hypothetical measurement and the calculator result for a location.

A global map of the magnetic declination error is provided in Fig. 3. As can be seen on this map, the error is lower at mid- to low-latitudes, while it is larger near the magnetic poles and in an area close to South Africa where the horizontal field is very low.

Component Uncertainty
X131 nT
Y94 nT
Z157 nT
H128 nT
F148 nT
D0.262+(5625/H)2 °
Table 1. Uncertainty estimates provided by the WMM2020 error model for the various field components. H is expressed in nT in the formula providing the error in D.
map of declination uncertainty 2020
Fig. 3. Global distribution of the declination error provided by the WMM2020 error model. Color scale limited to 1.2 degree. Click for high-resolution image.

Magnetic Poles

Based on the WMM2020 coefficients for 2020.0 the geomagnetic north pole is at 72.68°W longitude and 80.65°N latitude, and the geomagnetic south pole is at 107.32°E longitude and 80.65°S latitude. The axis of the dipole is currently inclined at 9.41° to the Earth's rotation axis.The WMM can also be used to calculate dip pole positions. These model dip poles are computed from all the Gauss coefficients using an iterative method. In 2020.0 the north dip pole computed from WMM2020 is located at longitude 164.04°E and latitude 86.50°N and the south dip pole at longitude 135.88°E and latitude 64.07°S. More information about geomagetic and dip poles here.

WMM Performance Assessment

Every year, the performance of the WMM is assessed by comparing it to a more recent main geomagnetic field model, and by comparing its secular variation with the actual secular variation observed at ground-based observatories and satellite-based geomagnetic virtual observatories (GVOs). The evolution of the WMM Blackout Zones and the validity of the WMM error model are also reviewed as part of this assessment. For more information, see the latest State of the Geomagnetic Field report.