... Now and then it experiences a magnetic storm , a disturbance of the magnetic field observable all around the globe, lasting a few days and adding appreciably to the Earth's trapped plasma. The storm is accompanied by large bright auroral displays, often extending well beyond the auroral zone, a rare spectacle for those not used to it. ... After analyzing them with a network of four stations, he concluded that they came from large electric currents flowing along the auroral arcs. ...