A weak spot in Earth's protective magnetic field is growing larger and exposing orbiting satellites and astronauts to more solar radiation, according to more than a decade of measurements by three orbiting observatories.

The observations by the European Space Agency's Swarm trio of satellites found that Earth's already weak magnetic field over the South Atlantic Ocean — a region known as the South Atlantic Anomaly (SAA) — is getting worse and that it has grown by an area half the size of continental Europe since 2014. At the same time, a region over Canada where the field is particularly strong has shrunk, while another strong field region in Siberia has grown, the measurements show.

"The region of weak magnetic field in the South Atlantic has continued to increase in size over the past 11 years since the launch of the Swarm satellite constellation," explained Chris Finlay, a geomagnetism researcher at the Danmarks Tekniske Universitet. "Although its growth was expected based on early observations, it is important to confirm this change in Earth's magnetic field is continuing." Finlay is the lead author of a new study published in the journal Physics of the Earth and Planetary Interiors that analyzes data from the Swarm satellites.