The electron current that produces the aurora, also produces its own magnetic field. This magnetic field modifies the Earth's own field, so that on the ground near an aurora the Earth's magnetic field changes irratically. It isn't the aurora itself that affects the Earth, but the process that has produced the aurora. This can upset radio communication, or even produce brown-outs and black-outs in our electricity distribution network as current surges are induced by the moving magnetic fields associated with the 'magnetic storms' in space.
