Abstract
Since the advent of digitally recording magnetic variometers and the ready accessibility of computers, there has been a search for a technique to produce K-indices of geomagnetic activity by machine.The principal difficulty in the determination of K-indices is the estimation of the regular solar diurnal variation (SR) for each day, which is used as a reference from which the K-Variations are scaled. A number of methods for the machine estimation of the daily SR are reviewed. These fall into two categories: (i) by use of neighbouring quiet days, or (ii) filtering out the high frequency harmonics. Of the techniques proposed to date, those which are data adaptive are considered preferable.A data adaptive method which divides a magnetogram trace into high and low frequency harmonics is presented. It is found that there is no particular frequency below which can be considered all the SR variations and above which are all the K-variations.The latitude dependence of the tolerace to differences between estimation of the SR by machine and that manually is also discussed.
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