Notes on the auroral electrojet indices

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Abstract
The auroral electrojet indices (AU, AL, and AE) have served well for more than two decades as measures of magnetospheric substorm activity. However, as substorm studies have progressed considerably during the last several years, the accuracy of the present electrojet indices has become an important issue. Thus it is opportune to reexamine and evaluate the accuracy of the present electrojet indices and improve them if necessary. For a better use of the present indices and for future improvement we examine the limitations of the auroral electrojet indices as an accurate quantitative measure of the auroral electrojets and of magnetospheric substorms. Such limitations should be kept in mind in studying individual substorms, the correlation with solar wind parameters, etc., particularly because the accuracy of the AE index decreases for AE < ∼250 nT. Some of the limitations arise from the data availability and also from the present simplified scheme in deriving them, but some of them originate in the definition itself. A few suggestions are made to improve the present indices, which can be implemented efficiently when digital outputs become available from all the observatories contributing to the electrojet indices.

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