The performance of the World Magnetic Model 2020 (WMM2020) was assessed by comparing its predictions at 2022.0 with that of a more recent model inferred from data collected by the European Space Agency Swarm satellites until September 2021.
For all magnetic field components, the WMM2020 global root-mean-square error increased by less than 1.5% over the past two years and remained well below the maximum error allowed by the U.S. Department of Defense WMM specification.
In addition, the WMM2020 secular variation was found to still be a very good approximation of the actual secular variation observed at ground-based
observatories and Swarm-based geomagnetic virtual observatories in 2021. This suggests that nonlinear changes in the Earth’s magnetic field remained small over the past two years.
observatories and Swarm-based geomagnetic virtual observatories in 2021. This suggests that nonlinear changes in the Earth’s magnetic field remained small over the past two years.
Since 2020, the north (respectively south) magnetic dip pole has moved at an average speed of 44 km/yr (respectively 9 km/yr) without any noticeable change in direction. These movements led to minor changes in the shape and location of the WMM blackout zones, where compass accuracy is degraded.
The South Atlantic Anomaly, where the geomagnetic field intensity is smallest,
continued to deepen (by about 50 nT at sea level) and to move westwards (its center moved by about 50 km at sea level).
continued to deepen (by about 50 nT at sea level) and to move westwards (its center moved by about 50 km at sea level).
December 2021 State of the Geomagnetic Field 3
The World Magnetic Model (WMM) is a spherical harmonic model of the Earth’s main magnetic field and its slow temporal change. It is jointly developed by the National Centers for Environmental Information (NCEI) and the British Geological Survey (BGS), and is a joint product of the United States’ National Geospatial-Intelligence Agency (NGA) and the United Kingdom’s Defence Geographic Centre (DGC).
The WMM is the standard model used by the U.S. and U.K. governments (and international organizations, e.g., the North Atlantic Treaty Organization and
the International Hydrographic Organization) for navigation, attitude and heading referencing systems using the geomagnetic field. It is also used widely outside the government in navigation and heading systems.
the International Hydrographic Organization) for navigation, attitude and heading referencing systems using the geomagnetic field. It is also used widely outside the government in navigation and heading systems.
The main geomagnetic field is constantly changing due to convection flows and waves in the Earth’s core. As this change cannot entirely be predicted, the WMM uncertainty slowly increases over time.
This report reviews the performance of the latest WMM, released in December 2019 and referred to as WMM2020, verifies that it still meets specification MIL-PRF-89500B (U.S. Department of Defense, 2019) at 2022.0 and provides an assessment of its secular variation after two years.
It also includes a description of noteworthy changes in the Earth’s main magnetic field since WMM2020 was released, including continuing magnetic pole drifts and the further deepening of the South Atlantic anomaly in the geomagnetic field intensity.
https://www.ngdc.noaa.gov/geomag/WMM/data/WMMReports/WMM_Annual_Report_2021.pdf
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South Atlantic Anomaly
The South Atlantic Anomaly (SAA) is a region spanning the southern Atlantic and South America where the Earth’s magnetic field is at its weakest. In the SAA the intensity of the field is about one third of that near the magnetic poles. The SAA affects how close to the Earth energetic charged particles can reach and this has an impact for satellite radiation damage and radio propagation. Polar regions are also very much affected by energetic charged particles but the impacts there are less dependent on field intensity.
The South Atlantic Anomaly (SAA) is a region spanning the southern Atlantic and South America where the Earth’s magnetic field is at its weakest. In the SAA the intensity of the field is about one third of that near the magnetic poles. The SAA affects how close to the Earth energetic charged particles can reach and this has an impact for satellite radiation damage and radio propagation. Polar regions are also very much affected by energetic charged particles but the impacts there are less dependent on field intensity.
The SAA is deepening and moving westwards. Table 3 shows the change in the SAA from 2020.0 to 2022.0 as estimated at Earth’s surface and at 500 km by WMM2020. The area affected, as judged by the area inside the 25,000 nT contour at the Earth’s surface, has increased by about 3% over this time. This contour approximates the region where radiation damage to satellites is most likely to occur.
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