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Abstract. The K index was developed by Bartels and is a measure of geomagnetic activity. The important feature of this index is that it is a measure of solar activity superimposed on the regular daily variation.It is the most used estimate of activity being of particular interest to researchers in ionospheric physics and telecommunications.In response to increased availability of digital data from magnetic observatories and their demand by the scientific community it becomes desireable to check whether K indices produced from magnetograms plotted from digital data are consistent with those handscaled from La Cour charts.This paper presents results of these comparisons and also reports on several methods tested to produce the K index directly from the digital data.Handscaled indices from digital plots are suitable for producing the K index. Computations using digital data do not reproduce K as defined by Bartels precisely but do provide an equivalent index which is satisfactory for most user and research purposes.

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