A European Space Agency satellite mission has detected a weakening of our planet's magnetic field that protects us from an unceasing bombardment of charged particles and cosmic rays, scientists report.

Six months of measurements by instruments on the ESA's Swarm constellation of three satellites have yielded evidence the field is weakening continuously, with declines at their most dramatic over the Earth's Western Hemisphere, they said.

Amidst a general trend of weakening there were a few spots, such as over the south of the Indian Ocean, that actually got stronger, they said, noting also a gradual shifting of Earth's magnetic north pole toward Siberia.

The findings suggest the possibility the magnetic field is about to undergo one of its periodic reversals, when south becomes north and vice-versa, though it may need thousands of years for the "flip-flop" to occur, experts said.

The Swarm satellites take their measurements primarily from the magnetic signature created by our planet's iron-nickel core.

In the future, the researchers say, they intend to study changes in the magnetic field from additional sources including the Earth's mantle, its crust and ocean, as well as above us in the magnetosphere and ionosphere.

"These initial results demonstrate the excellent performance of Swarm," said Rune Floberghagen, ESA's Swarm Mission Manager. "With unprecedented resolution, the data also exhibit Swarm's capability to map fine-scale features of the magnetic field."

One practical benefit of the research will be the ability to improve the accuracy in satellite navigation systems that are sensitive to electrical and magnetic conditions in the upper atmosphere, the researchers said.

The Swarm mission represents a significant step forward in our ability to study and understand the Earth's magnetic field, said Volker Leibig, ESA director of Earth observation.

"I started my career in magnetometry and the accuracy we had then in the laboratories was less than what we can fly in space now," he said.

"So what we have on Swarm is fantastic, but we need long-time series to understand fully the Earth's magnetic field, and we will get that from this mission."

Such understanding is vital, the researchers emphasize, because without our magnetic field in place cosmic rays and charged particles would eventually strip away all of Earth's atmosphere, as they have done on Mars, leaving our planet looking naked and dead.

The initial results from the magnetic field studies were presented June 19 in Copenhagen, Denmark, at a "Swarm Science Meeting."