Some properties of the Svalgaard A/C Index

Several properties of the A/C index of polar cap variations introduced by L. Svalgaard in 1972 have been found to vary with time. In the presatellite era, C days, as measured by the Ap index, are almost twice as geomagnetically active as A days, while in the modern epoch they have essentially identical activity. Prior to 1962 there were over 40 percent more A days than C days per year, while during the modern epoch there are essentially equal numbers of A days and C days. In view of this strong bias to assigning a ''toward'' classification on geomagnetically active days we recommend against using the Svalgaard A/C classification in studies of geomagnetic activity. We further recommend that a full and thorough documentation of the index be prepared and/or that others undertake to compile such a classification separately. (auth)