Magnetic storms bring auroras to low latitudes and perturb the magnetic field all over the Earth, disturbing navigational systems and sometimes even electrical power grids. They are caused by sudden changes in the currents flowing close to the Earth, particularly the ‘ring current’ that flows intermittently within the Earth's magnetosphere. It was thought that the ring current was fed by smaller magnetic disturbances called substorms. But it seems that substorms are instead earthquake-like releases of magnetic stresses, allowing the ring current to be built up by the larger-scale interplanetary magnetic field.