The field’s strength has waned 10 percent to 15 percent, and the deterioration has accelerated of late, increasing debate over whether it portends a reversal of the lines of magnetic force that normally envelop the Earth.

...Last month, the European Space Agency approved the world’s largest effort at tracking the field’s shifts. A trio of new satellites, called Swarm, are to monitor the collapsing field with far greater precision than before and help scientists forecast its prospective state.

“We want to get some idea of how this would evolve in the near future, just like people trying to predict the weather,” said Dr. Gauthier Hulot, a French geophysicist working on the satellite plan. “I’m personally quite convinced we should be able to work out the first predictions by the end of the mission.”