When a bright aurora is seen in the auroral zone, a strong magnetic disturbance is usually also observed there. The disturbing magnetic field can be much stronger than that of a magnetic storm but it is strictly local, fading away quickly as one moves equatorward.
This limited extent suggested that the currents which disturbed the field flowed somewhere nearby, probably near the auroral arcs. The Norwegian Kristian Birkeland, who carefully observed auroral disturbances around the turn of the century, concluded that those currents flowed parallel to the ground, along the auroral formation.
Any electric current, however, must flow in a closed circuit, and since it seemed to be caused (like that of the aurora) by processes taking place in distant space, Birkeland proposed that it came down from space at one end of the arc and returned to space at the other end. The drawing here illustrates the idea and was taken from Birkeland's 1903 report on his expeditions to the auroral zone:.
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