Copyright WDC C1 for STP at RAL.
The am index is derived from geomagnetic variations observed at a number of observatories (8 groups) in both hemispheres. It may be defined as the average 3-hourly range observed, in each hemisphere, within a band close to a 50-degree corrected geomagnetic latitude. Although provisional values are available during the year, even if all stations have not provided their data, final values are only published after the end of a year.
The am indices are derived from observatory K index, corrected geomagnetic latitude, angular distance of observatory to the nearest point of the auroral zone, corrected geomagnetic Longitude of that point, computed lower limit for K=9 and the actual limit used.
Two indices, an and as, are computed for the northern and southern hemispheres respectively, and the am index is the average of an and as.