Geomagnetic indices

The purpose of this review is to outline the definition and method of derivation of the indices of geomagnetic activity that are most commonly used at present. The first part of the review presents the methods of derivation of Kp , ap , AE , and Dst ; the second part involves a discussion of the strengths and weaknesses of each index. It is shown that the distribution of magnetic observatories whose data are used in the production of the indices strongly influences the quantitative reliability of the indices in varying degrees. Furthermore, it is found that, in general, indices are only capable of defining the lower limit of geomagnetic activity at any given time. It is concluded that indices should only be used in statistical studies of solar‐terrestrial interactions; their use in the study of individual events is generally not advisable.