Generated by the motion of molten iron in Earth's core, the magnetic field protects our planet from cosmic radiation and from the charged particles emitted by our Sun. It also provides the basis for navigation with a compass. The image on the left, based on data from Swarm (a group of satellites launched in 2013 by the European Space Agency) shows the average strength of Earth's magnetic field at the surface between January 1 and June 30, 2014. The image at right shows changes in that field over the same period. Though the colors in the righthand image are just as bright as on the left, the greatest changes were plus or minus 100 nanotesla in a field that reaches 60,000 nanotesla. Geophysicists have noted that the strength of Earth's magnetic field has decayed about 5 percent globally over the past century. However, it is not changing in a uniform way; it grows growing stronger in some places and weaker in others. The changes are a natural variation due to processes in the deep interior of the Earth. The movement of molten iron in the core creates electric currents, and electric currents create a magnetic field. Every change in the flow of the core means changes in the magnetic field.