am geomagnetic index

Purpose of am index is to provide a characterization of global geomagnetic activity using a large set of stations representing all longitudes and possible hemispheric discrepancies. In each longitude sector, the K scaled for each station, are averaged, and the result is converted into amplitude using mid-class amplitudes for L9 = 500 nT (L9 being the K=9 lower limit). The weighted average of the so-obtained amplitudes give rise to the hemispheric indices an (North) and as (South), the weighting factors accounting for the differences in the longitude sector extents in each hemisphere. The planetary index am is equal to (an + as) / 2.