Copyright WDC C1 for STP at RAL.
The aa data's average aa data indices have been derived by two antipodal sites, originally Greenwich and Melbourne but now their nearest replacements. The daily values are formed from an average of the 8 three-hourly values.
The aa index comes from a request made by the Royal Society of London at the Madrid IAGA in 1969 by S. Chapman for Ci before 1884. Early records from Greenwich and Melbourne give a long series of data. Where early individual data records are missing other stations provide approximate information instead. Current data coverage runs from 1868 through to 1996.