Observations of Birkeland currents at auroral latitudes

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Abstract
After many years of languishing interest, field‐aligned currents (Birkeland currents) are now regarded as an important link between the magnetosphere and the ionosphere. We describe techniques that are used to measure them and then discuss in some detail measurements that have been interpreted as indicating the presence of Birkeland currents. We conclude that measurements substantiate the theory of large‐scale flow from the magnetosphere to the polar caps in a dawn‐dusk direction with opposite flow equatorward of the auroral zone connecting to the Alfvén layer. Sheet current flow in premidnight discrete aurora seems experimentally established also, but its connection with auroral acceleration and with the larger‐scale flow is not yet understood. The currents flowing during a substorm are not yet fully defined either.