J. Bartels, The technique of scaling indices K and Q of geomagnetic activity, Ann. Intern. Geophys. Year 4, 215-226, 1957;
J. Bartels,The geomagnetic measures for the time-variations of solar corpuscular radiation, described for use in correlation studies in other geophysical fields, Ann. Intern. Geophys. Year 4, 227-236, 1957;
M. Siebert, Maßzahlen der erdmagnetischen Aktivität, in Handbuch der Physik, vol. 49/3, 206-275, Springer, Berlin Heidelberg, 1971; for the description of several indices (in German);
M. Siebert and J. Meyer, Geomagnetic Activity Indices, in The Upper Atmosphere (Eds. W. Dieminger et al.), 887-911, Springer, Berlin Heidelberg, 1996; for an updated version of the German reference;
P.N. Mayaud, Derivation, Meaning and Use of Geomagnetic Indices, Geophysical Monograph 22, Am. Geophys. Union, Washington D.C., 1980; for a complete review of the present IAGA indices;
G.K. Rangarajan, Indices of magnetic activity, in Geomagnetism, edited by I.A. Jacobs, Academic, San Diego, 1989;
M. Menvielle and A. Berthelier, The K-derived planetary indices: description and availability, Rev. Geophys. 29, 3, 415-432, 1991.