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The 3‐hour K index was the first to provide an objective and quantitative monitoring of the irregular variations of the transient geomagnetic field observed in a given place. The use of K indices from a network of observatories to derive a planetary index of geomagnetic activity was suggested by Bartels when defining these indices. Then, Kp, am, Km, an, as , and aa were successively designed and accepted as International Association of Geomagnetism and Aeronomy indices. At present these K ‐derived planetary indices are routinely computed and circulated. They make long homogeneous data sets and are widely used for long‐term and statistical studies in geomagnetism and solar‐terrestrial physics. After a short description of the main features of transient geomagnetic activity a definition of the K index as a measure of the irregular activity is given with a summary of its basic characteristics. The derivation of K ‐derived planetary indices is described and discussed, and updated indications concerning their availability are presented. This short review provides users with the minimum required knowledge about these indices and may serve as an introduction to the Mayaud (1980) monograph.
Menvielle et al. (Thu,) studied this question.