The Earth’s changing, irregular magnetic field gives polar navigation headaches

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Changes in the Earth’s global magnetic field over six months in 2014 as measured by the European Space Agency’s three-satellite Swarm constellation. The left map shows the average magnetic field, and the right shows the changes in magnetic field strength during this period. Credit: European Space Agency/Technical University of Denmark (ESA/DTU Space).

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Changes in the Earth’s global magnetic field over six months in 2014 as measured by the European Space Agency’s three-satellite Swarm constellation. The left map shows the average magnetic field, and the right shows the changes in magnetic field strength during this period. Credit: European Space Agency/Technical University of Denmark (ESA/DTU Space).

The Earth’s liquid molten outer core, composed mostly of iron and nickel, creates an electromagnetic field extending from the north and south poles that protects the planet from harmful solar particle radiation.

Fluctuations in the strength of Earth’s magnetic field—caused by diurnal changes in the structure of the solar wind and occasional solar storms—can affect the use of geomagnetic field models that are crucial for navigation in satellites, airplanes, ships and cars.

Magnetic field models differ depending on the location of the data collection—either on or near the Earth’s surface or a satellite in low Earth orbit. Past research attributed the differences in the models to levels of space weather activity, but a recent analysis of six-year models of the Earth’s magnetic field and satellites revealed that the differences in the models are also caused by modeling errors, not just geophysical phenomena. The results were published in Journal of Geophysical Research: Space Physics.

A University of Michigan research team evaluated the differences between observations from the Swarm mission’s low-Earth orbit satellite and a model of Earth’s magnetic field, the thirteenth generation International Geomagnetic Reference Field, or IGRF-13. They focused on differences during low to moderate geomagnetic conditions spanning 98.1% of the time between 2014 and 2020.

Satellite observations collected at various locations above Earth are sensitive to magnetic field fluctuations, while Earth magnetic field models use observations to estimate Earth’s internal magnetic field without taking into account the influence of solar storms. Internal magnetic field models like IGRF-13 are used to track changes in the Earth’s magnetic poles, such as the North Pole shifting about 45 km north-northwest each year.

Understanding these large differences is important for satellite operation when using IGRF-13 as a reference and for investigating the physics of Earth’s magnetosphere, ionosphere, and thermosphere.

Model uncertainty was highest in the North and South Polar Regions, and statistical analysis revealed that the asymmetry between the North and South Polar Regions was the main factor driving the model differences.

“We often assume a nearly symmetric magnetic field between the north and south polar regions, but they are actually very different,” said Yining Shi, an assistant research scientist at the University of Michigan’s Department of Climate and Space Science and Engineering and corresponding author of the study.

The two geographic poles map to different geomagnetic coordinates. The North Pole maps to about 84° magnetic latitude (MLAT) and 169° magnetic longitude (MLON), and the South Pole maps to about -74° MLAT and 19° MLON.

The Swarm satellite’s polar orbit path creates a sampling bias with a high concentration of measurements around the geographic poles, exacerbating model differences.

“Understanding that what is attributed to geophysical disturbances is actually due to asymmetry in the Earth’s magnetic field will help us better create models of the geomagnetic field, as well as aid in satellite and aircraft navigation,” said Mark Moldwin, the Arthur F Thurnau Professor of Climate and Space Sciences and engineering at UM and the author of the study.

Another issue of concern to the navigational community is that the polar magnetic field has been changing rapidly over the past ten years.

“This further complicates the creation of accurate models of the magnetic field,” Moldwin said.

More information:
Yining Shi et al, Non-geophysical interhemispheric asymmetries in large-scale magnetic field residuals between swarm observations and Earth magnetic field models during moderate to quiet geomagnetic conditions, Journal of Geophysical Research: Space Physics (2024). DOI: 10.1029/2023JA032092

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