UK Solar System Data Centre
Copyright WDC C1 for STP at RAL.
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.