A-Index Definition

The A-index was invented because there was a need to derive some kind of daily average level for geomagnetic activity. 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. What is done instead is to convert each K back into a linear scale called the "equivalent three hourly range" a-index (note the lower case). The daily A index is merely the average of eight "a" indices. The following table illustrates the conversion between K and "a":

K	A
0	0
1	3
2	7
3	15
4	27
5	48
6	80
7	140
8	240
9	400

Formula

Thus, for example, if the K indices for the day were 3 4 6 5 3 2 2 1, the daily A index is the average of the equivalent amplitudes:

A = (15 + 27 + 80 + 48 + 15 + 7 + 7 + 3) /8 = 25.25

More Geomagnetic A-Indice Information and data. http://www.swpc.noaa.gov/alerts/a-index.html