Earth’s changing, erratic magnetic field causes polar navigation headaches

Earth’s liquid molten outer core, composed mostly of iron and nickel, generates an electromagnetic field emanating from the north and south poles that shields the planet from the sun’s harmful particle radiation.

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

Magnetic field models differ depending on the location of data collection – either on or near the Earth’s surface or on satellites in low Earth orbit. Past research has attributed model discrepancies to levels of space weather activity, but a recent analysis of six years of Earth and satellite magnetic field models found that model discrepancies are also caused by modeling errors rather than geophysical phenomena themselves. The results are published in Journal of Geophysical Research: Space Physics.

A University of Michigan research team evaluated differences between observations from the Swarm mission’s low-orbiting satellites and the International Geomagnetic Field Reference Thirteenth Generation, or IGRF-13, model of Earth’s magnetic field. They focused on differences during low to moderate geomagnetic conditions, which comprise 98.1% of the time between 2014 and 2020.

Satellite observations collected at various locations above Earth are sensitive to magnetic field fluctuations, while models of Earth’s magnetic field use observations to estimate Earth’s internal magnetic field without accounting for the effect of solar storms. Models of the internal magnetic field, such as 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 operating the satellite using IGRF-13 as a reference and for researching the physics of Earth’s magnetosphere, ionosphere, and thermosphere.

Model uncertainty was highest in the northern and southern polar regions, and statistical analysis revealed that the asymmetry between the northern and southern polar regions was the main factor influencing the model differences.

“We often assume a nearly symmetric magnetic field between the north and south polar regions, but in reality they are very different,” said Yining Shi, a research assistant at the University of Michigan 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 approximately 84° Magnetic Latitude (MLAT) and 169° Magnetic Longitude (MLON) and the South Pole maps approximately -74° MLAT and 19° MLON.

The polar orbiting path of the Swarm satellites creates a sampling bias with a high concentration of measurements around the geographic poles, exacerbating model differences.

“Understanding that what has been attributed to geophysical disturbances is actually caused by the asymmetry of the Earth’s magnetic field will help us to better model the geomagnetic field and also help with satellite and aerial navigation,” said Mark Moldwin, Arthur F Thurnau Professor. Climate and Space Sciences and Engineering at UM and author of the study.

Another issue of concern to the navigation community is that the polar magnetic field has been changing rapidly over the past decade or so.

“This adds another layer of complexity to creating accurate models of the magnetic field,” Moldwin said.