Synonym of A-index

The A-index provides a daily average level for geomagnetic activity, based on the K-index. Because of the non-linear relationship of the K-scale to magnetometer fluctuations, it is not meaningful to take averages of a set of K indices. Instead, each K is converted into a linear scale called the "equivalent three hourly range" a-index (note the lower case). The daily A index is the average of eight "a" indices.