Copyright WDC C1 for STP at RAL.
A network of sites round the globe each measures its local geomagnetic activity of the atmosphere. The different locations means the range of values for each station varies. Each site's readings is scaled onto the Kp Scale of 0 - 9. These scales vary from site to site, but 9 on two different sites is always a high activity level. Higher latitudes (eg Norway) are more active than lower ones (eg Sahara). From these data a number of other useful indices are calculated, including aa, am and Ap.