Copyright WDC C1 for STP at RAL.
The aa index is derived from 8 3-hourly readings taken at two antipodal stations; originally Greenwich and Melbourne but now their nearest replacements.
To calculate this index, the data for each site is standardised for latitude on each separate 3-hourly K value to 19° from the auoral zone. aa is the average of the two resulting values.
Units are nT.