The World Magnetic Model - Accuracy, limitations, magnetic poles and magnetic center

Accuracy of the WMM2010

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 (Fig. 1) and then subsequently deteriorates towards the end of the 5 year epoch (Fig. 2), when it has to be updated with revised values of the model coefficients.

Fig 1. Estimated WMM2010 declination inaccuracy for 2010.0 without considering crustal and disturbance field contributions Click image for larger version.

Limitations of the WMM2010

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.

Fig 2. Estimated WMM2010 declination inaccuracy for 2015.0 without considering crustal and disturbance field contributions. Click image for larger version.

Magnetic Poles and Magnetic Center

The geomagnetic poles, otherwise known as the dipole poles, can be computed from the first three Gauss coefficients. From the WMM2010 coefficients for 2010.0 the geomagnetic north pole is at longitude 72.21° W and latitude (geocentric and geodetic) 80.02° N and the geomagnetic south pole is at longitude 107.79° E and geodetic latitude 80.02° S. The axis of the dipole is currently inclined at about 10° to the Earth's rotation axis. In 2010.0 the north dip pole is estimated to be located at longitude 132.35° W and geodetic latitude 84.97° N and the south dip pole at longitude 137.33°E and geodetic latitude 64.42°S. The location of the center of the eccentric dipole, sometimes known as the magnetic center, computed using the first eight Gauss coefficients for 2010.0, is at approximately (r, phi, lambda) = (563 km, 22.49°N, 140.22°E).

Details of the model derivation are described in the NOAA Technical Report: The US/UK World Magnetic Model for 2010-2015 (pdf 22 MB).