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aa Derivation and Representation

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.